Ch 18 Reading/Concept, Astro Chapters 15-19, Astro Final, 16, Pre-Lecture Chapter 17 + Visual Quiz, ASTR FINAL, Astronomy Ch. 17, ASTR=Chapter 18 Visual Quiz, Module 13: Exploring Our Galaxy: The Milky Way, Chapter 18 : Astronomy Reading Quiz

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Galaxy's Bulge Mass

The larger a galaxy's bulge mass, the larger the supermassive black hole is at its center.

Which two factors are critical to the existence of the carbon dioxide (CO2) cycle on Earth?

active volcanism and active tectonics.

What is the approximate temperature of the universe (as a whole) today?

What is the significance of the Planck time?

Before it, conditions were so extreme that our current understanding of physics is insufficient to predict what might have occurred.

MWG X-ray Background

Reveal very bright X-ray sources, mostly from black holes and neutron stars. Most of the X-rays in our galaxy, however, are represented by the thin, white, wavy lines.

The elevation angle of Polaris above the horizon can approximately indicate your

latitude

Consider a spinning disk of pizza dough, as shown here. What would the rotation curve for the spinning dough look like?

line is going straight from bottom left to upper right.

Based on this diagram, "red sequence" galaxies are __________ than "blue cloud" galaxies.

redder and more luminous

The surface of Mercury is most similar to which of the following objects?

the Moon

On this graph, the time corresponding to the point at the far lower right of the diagonal line represents __________.

the age of the universe today, in seconds

On this graph, the time corresponding to the point at the far lower right of the diagonal line represents __________. (graph declines linearly from left to right)

the age of the universe today, in seconds

Which of the following observations cannot be explained by the Big Bang theory unless we assume that an episode of inflation occurred?

the fact that the temperature of the cosmic microwave background is almost the same everywhere

This sequence of paintings represents the formation of the Milky Way Galaxy. What law of nature explains why the galaxy began to rotate rapidly and flatten out as it shrunk in size?

the law of conservation of angular momentum

ch 16

Approximately what is the current age of the universe in seconds?

4 X 10^17 s

3) If you had something the size of a sugar cube that was made of white dwarf matter, it would weigh A) as much as a truck. B) about 5 pounds. C) as much as the entire Earth. D) as much as an average person.

Bulge Mass Linked to Black Hole Mass

A comparison of the cores of four elliptical galaxies (center) shows that the more massive a galaxy's central bulge of stars, the heftier its supermassive black hole (right). Regardless of their size, the bulges always turn out to be 500 times as massive as the giant black holes at the hub of their galaxies.

A standard candle is _________.

A light source of known luminosity standard candles known luminosities

Most abundant molecule in gas clouds

Hot Bubbles

High-speed gas ejected into space by winds from supergiants and supernovae sweeps up surrounding interstellar material, excavating a bubble of hot, ionized gas. Hot bubbles fill about 20%-50% of the MWG's disk Nebula N44F is a bubble—a 35-light-year-diameter gas cavity carved by the stellar wind and intense ultraviolet radiation from a young hot star. N44F is part of a larger emission nebula (N44) with superbubble structure located in the Large Magellanic Cloud (in the constellation Dorado). N44 is approximately 160,000-170,000 l-y distant. The super bubble structure of N44 itself is shaped by the radiation pressure of a 40-star group located near its center; the stars are blue-white, very luminous, and powerful.

Which statement about the cosmic microwave background is not true?

It is the result of a mixture of radiation from many independent sources, such as stars and galaxies.

What is the most important feature of a scientific hypothesis?

It makes one or more predictions that are testable

How does the interstellar medium affect our view of most of the galaxy?

It prevents us from seeing most of the galactic disk with visible and ultraviolet light.

Who is considered the founder of modern astronomy?

Nicholas Copernicus

Halo

No star formation.

Dusty Gas Clouds

Obscure our view in visible light to the center of our galaxy

Milky Way has a double halo

Observations with the Sloan Digital Sky Survey show that the outer Milky Way is a really a mixture of two distinct components rotating in opposite directions. The inner component of our galaxy's halo spins clockwise with the galaxy's rotation at about 22 km/s (50,000 mph). The outer component rotates counter-clockwise to the galaxy at about 45 km/s (100,000 mph).

Radio mapping

Of neutral and ionized hydrogen regions give us a picture of our galaxy's structure.

Population II stars

Old; low concentrations of heavy elements; in halo.

A neutral atom of normal hydrogen consists of what?

One proton and one electron

Suppose that Star X and Star Y both have redshifted spectra, but Star X has a larger redshift than Star Y. What can you conclude?

Star X is moving away from us faster than Star Y.

Sun orbiting the MWG

Sun orbits the MWG once every 230 million years

Suppose that the Sun were suddenly to shrink in size but its mass remained the same. According to the law of conservation of angular momentum, what would happen?

The Sun would rotate faster than it does now.

What do we mean by the term "ecliptic"?

The apparent path of the Sun through the zodiacal constellations

(As viewed form Earth) Mars, Jupiter, Saturn, Uranus, and Neptune exhibit retrograde motion _______

When they are un the same direction of the sky as the Sun

Olbers's paradox is an apparently simple question, but its resolution suggests that the universe is finite in age. What is the question?

Why is the sky dark at night?

Workings of SMBH

Working with the NRAO's VLBA radio telescope, astronomers have been able to probe the supermassive black hole in BL Lacertae (BL Lac), some 950 million light-years (291 Mpc) from Earth. BL Lac is a blazar, the most energetic type of black-hole-powered galactic core. Material pulled inward toward the black hole forms an accretion disk. Theory says as the material moves from the outer edge of the disk inward, magnetic field lines perpendicular to the disk are twisted, forming a tightly-coiled bundle that propels and confines the ejected particles. Closer to the black hole, space itself, including the magnetic fields, is twisted by the strong gravitational pull and rotation of the black hole. This theoretical view was confirmed by the VLBA observations.

X-Ray Flare from the MWG's Central Black Hole

X-ray flares from the galactic center suggest that tidal forces of suspected black holes occasionally tear apart chunks of matter about to fall in.

What is this a picture of?

a cluster of galaxies

The current temperature of the universe as a whole is

a few K

Which kind of star is most likely to be part of the spheroidal population?

an M star

In the past, the temperature of the universe was _____

hotter than it is today

According to current understanding, the key requirement for life is

liquid water.

The Orion Nebula is a star forming region visible to the unaided eye. How might we describe the gas?

low density gas excited to emit

The H-R Diagram is a plot of

luminosities of stars in L vs. their photospheric temperatures

Spiral arms appear bright because ________.

they contain more hot young stars than other parts of the disk

Most of the mass of the Galaxy is in what form?

Dark Matter

Galactic Center

Most likely contains supermassive black hole.

The Big Bang theory is supported by two major lines of evidence that alternative models have not successfully explained. What are they?

(1) The theory predicts the existence of and the specific characteristics of the observed cosmic microwave background; (2) the theory correctly predicts the observed overall chemical composition of the primeval universe.

The Big Bang theory is widely accepted today because it successfully predicts and explains several key observed features of the universe. Two of its important predictions that have been verified are:

(1) the existence and specific characteristics of the observed cosmic microwave background; (2) the observed overall chemical composition of the universe.

Atomic & Molecular Hydrogen

(A) H (atomic hydrogen) is the most common hydrogen isotope with an abundance of more than 99.98%. Because the nucleus of this isotope consists of only a single proton, it is given the descriptive but rarely used formal name protium. (B) In H2 (molecular hydrogen), two hydrogen atoms share the electrons of the valance orbital of the individual hydrogen atoms so that each hydrogen atom is now sharing two electrons.

Star Motion Around MWG's Central Supermassive Black Hole

(A) Infrared NACO image of S2. NACO is the combination of NAOS (Nasmyth Adaptive Optics System that is equipped with both visible and infrared sensors) and CONICA (a Near-Infrared Imager and Spectrograph). (B) The orbit of star S2 as observed between 1992 and 2002, relative to SgrA* (marked with a circle). The solid curve is the best-fitting elliptical orbit—one of the foci is at the position of SgrA*. The positions of S2 at the different epochs are indicated by crosses with the dates (expressed in fractions of the year) shown at each point. The size of the crosses indicates the measurement errors.

Maps of H I Regions (Neutral H)

(A) Neutral atomic hydrogen from radio surveys of the 21-cm transition of hydrogen. This all-sky image is a composite of several surveys with ground-based telescopes in the northern and southern hemispheres. (NASA/GSFC) (B) A map of the Milky Way Galaxy at 21-cm shows that the distribution of neutral hydrogen (H) is concentrated in the spiral arms in H I regions. The Sun is marked by the yellow arrow, and the galactic center is a blue dot. The green dotted lines outline what is known as the "cone of avoidance" found behind the galactic center and is due to confusion in the H I signal.

Herschel's Milky Way

(A) The Milky Way map of William and Caroline Herschel (1785). Herschel saw a flattened Milky Way with the Sun at the center. Herschel actually counted stars to create his map. (B) William Herschel (1738-1822), portrait by James Sharples, c. 1805. (C) Caroline Herschel (1750-1848) in an 1847 lithograph.

The Sun's orbital moon

(Radius and velocity) tells us mass of the galaxy within Sun's orbit: 1.0 x 10<11M(sun symbol)

Which of the following represent observations or measurements that have confirmed key predictions of the Big Bang model?

-Characteristics of the cosmic microwave background. -The abundance of helium in the universe.

Complete each of the following sentences about the early universe

-Most of the helium in the universe formed during the ERA OF NUCLEOSYNTHESIS. -The GUT ERA refers to a time period in which the strong, weak, and electromagnetic forces were all unified. -The PLANCK ERA refers to a time period in which all four forces are thought to have been unified. -The COSMIC MICROWAVE BACKGROUND is our name for the photons we see that were first released when the universe was about 380,000 years old. -The BIG BANG is the name we give to the moment when the expansion of the universe began. -A very short period of extremely rapid expansion, called INFLATION, is thought to have occurred when the universe was about 10^-38 second old.

What is the range of mass for main sequence stars?

0.08 to 80 M

This diagram represents an edge-on view of our Milky Way Galaxy. Of the four labeled stars, which one could represent the Sun's position in the galaxy?

1 (far left of the ring)

The mass of a supermassive black hole thought to power a typical bright active galactic nucleus is typically ______.

1 billion solar masses

What fraction of the mass of the interstellar medium is made up of interstellar dust?

1 percent

What is the length of the main sequence lifetime of a star with a mass of 2 solar masses?

1.7 billions years

A star is at a distance of 12 parsecs. What is its trigonometric parallax?

1/12 arc seconds

A star has a distance of 25 parsecs. What is its stellar parallax, as measured from the Earth?

1/25 of an arc second.

Based on the number of galaxies visible in the Hubble eXtreme Deep Field (Figure 16.1 in your text), the estimated number of galaxies in our observable universe is at least ______. 100 million 50,000 100 billion infinity

100 billion This estimate is found by counting galaxies in this small piece of the sky, and multiplying by the number of such pieces it would take to fill the entire sky.

How luminous are white dwarf stars, compared to the Sun?

100 to 10,000 times fainter

Prior to the Industrial Revolution what was the natural time scale of the variation of the carbon dioxide level in the Earth's atmosphere?

100,000 years

A spectrum of star reveals that it is a main sequence star of spectral type F0. This means that an estimate of its absolute magnitude is +2.7. Photometry of this star indicates that its apparent magnitude is 12.7. How far away is the star?

1000 parsecs

What was the approximate temperature of the universe when the universe was just 1 second old?

10^10 K

Current estimates place the age of the universe at about _________.

14 billion years More precisely, the current estimate is 13.7 billion years, give or take a few hundred million years.

Say a comet has a highly elliptical orbit and that the semi-major axis of its orbit is 10 AU. What is its maximum distance from the Sun?

19 AU

At which position in this diagram is it the first day of summer in the northern hemisphere?

2 (The one to the left of the Sun)

This diagram represents an edge-on view of our Milky Way Galaxy. Of the four labeled stars, which one is located close to a place where evidence suggests we would find a several million solar mass black hole?

2 deadass center

According to the Big Bang theory, how many forces - and which ones - operated in the universe during the GUT era?

2 forces: gravity and a single force that later became the strong, weak, and electromagnetic forces

Approximately how many stars are there in the Milky Way galaxy?

200 billion

Say you're outside at night on a. clear, moonless night in west Texas. Approximately how many stars could you see with your unaided eye?

2000

21-cm Radio Line

21-cm line: Atomic hydrogen emits a spectral line with a wavelength of 21 cm in the radio (microwave) portion of the EM spectrum and is called a hydrogen spin-flip transition Atomic hydrogen (H) has is found in two distinct forms: large, low-density (1 atom/cm<3) warm clouds (10,000K) smaller, higher-density (100 atoms /cm<3) cool clouds (100K) Emission of 21-cm photon from neutral hydrogen. The 21-cm wavelength is equal to a frequency of 1420.4 MHz. An electron orbiting a proton with parallel spins (top) has higher energy than if the spins were anti-parallel (bottom). The microwaves of the hydrogen line come from the atomic transition between the two hyperfine levels of the hydrogen 1s ground state (aka hyperfine splitting).

The trigonometric parallax of a star is 1/22 of an arc second. How far away is it?

22 parsecs

How much lower in the sky is the Sun at local noontime on the first day of winter compared to the day of the autumnal equinox?

23.5 degrees.

How long does it take for the Moon to go through one cycle of its phases?

29.5 days

How many accelerators are there in your car?

3 (the gas pedal, brakes, and steering wheel)

If the main mass of this F0 main sequence is 1.6 solar masses, how long will this star be a main sequence star?

3 billion years

In the laboratory the wavelength of the hydrogen beta line is 4861 A and the wavelength of the hydrogen alpha line is 6563 A. If we take a spectrum of a spectrum of a galaxy and find that these two lines are observed at 4910 and 6629 A, repetitively, how fast is the galaxy receding from us?

3000 km/sec

This diagram represents an edge-on view of our Milky Way Galaxy. Of the four labeled stars, which one is located in what we call the halo of the galaxy?

4 (top right dot)

Say we discover an object whose orbital period around the Sun is 8 years. What is its mean distance form the Sun?

4 AU

Say the Moon's orbit became much. more elliptical, such that its maximum distance from the Earth were two times as far as the minimum distance. If F1 is the minimum gravitational force between the Moon and the Earth (at apogee), what would be the maximum gravitational force between the Moon and the Earth?

4 times F1

Say the Sun was formed 4.6 billion years ago and its initial composition was 75 percent hydrogen, 23 percent helium, and 2 percent everything else. What is the present composition of the Sun in its core?

40% H, 58% He, and 2% everything else

Overall composition of the MWG

70% H, 28% He, 2% heavy elements

Consider two reflecting telescopes of similar design. One has an objective mirror of diameter 8 inches. The other has an objective mirror of diameter 24 inches. If we take 60 second exposures through each with the same digital camera, how much fainter are the faintest stars detected with the larger telescope?

9 times fainter

Say you own a reflecting telescope of diameter 12 inches and focal length 48 inches. You can have eyepieces of focal length 2 inches, 1 inch, and 1/2 of an inch. What is the highest magnification you can achieve with this equipment?

12) How is an X-ray burst (in an X-ray binary system) similar to a nova? A) Both involve explosions on the surface of stellar corpse. B) Both typically recur every few hours to every few days. C) Both are thought to involve fusion of hydrogen into helium. D) Both result in the complete destruction of their host stars.

13) Which statement concerning black hole masses and Schwarzschild radii is not true? A) In a binary system with a black hole, the Schwarzschild radius depends on the distance from the black hole to the companion star. B) The more massive the black hole, the larger the Schwarzschild radius. C) Even an object as small as you could become a black hole if there were some way to compress you to a size smaller than your Schwarzschild radius. D) For black holes produced in massive star supernovae, Schwarzschild radii are typically a few to a few tens of kilometers.

16) When we see X rays from an accretion disk in a binary system, we can't immediately tell whether the accretion disk surrounds a neutron star or a black hole. Suppose we then observe each of the following phenomena in this system. Which one would rule out the possibility of a black hole? A) intense X-ray bursts B) spectral lines from the companion star that alternately shift to shorter and longer wavelengths C) visible and ultraviolet light from the companion star D) bright X-ray emission that varies on a time scale of a few hours

17) What do we mean by the singularity of a black hole? A) It is the center of the black hole, a place of infinite density where the known laws of physics cannot describe the conditions. B) It is the "point of no return" of the black hole; anything closer than this point will not be able to escape the gravitational force of the black hole. C) It is the edge of the black hole, where one could leave the observable universe. D) The term is intended to emphasize the fact that an object can become a black hole only once, and a black hole cannot evolve into anything else.

18) What makes us think that the star system Cygnus X-1 contains a black hole? A) It emits X rays characteristic of an accretion disk, but the unseen star in the system is too massive to be a neutron star. B) No light is emitted from this star system, so it must contain a black hole. C) The fact that we see strong X-ray emission tells us that the system must contain a black hole. D) Cygnus X-1 is a powerful X-ray burster, so it must contain a black hole.

19) Imagine an advanced civilization living on a planet orbiting at a distance of 10 AU (1.5 billion kilometers) from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole. The black hole is surrounded by an accretion disk. Sometime within the next million years or so, the civilization's planet is likely to be doomed because A) the red giant will probably supernova within the next million years. B) jets of material shot out of the accretion disk will shoot down their planet. C) the red giant star, which provides most of energy the civilization needs to exist, will soon be destroyed in the accretion disk. D) tidal forces from the black hole will rip the planet apart.

21) Which of the following statements about electron degeneracy pressure and neutron degeneracy pressure is true? A) Electron degeneracy pressure is the main source of pressure in white dwarfs, while neutron degeneracy pressure is the main source of pressure in neutron stars. B) Both electron degeneracy pressure and neutron degeneracy pressure help govern the internal structure of a main-sequence star. C) The life of a white dwarf is an ongoing battle between electron degeneracy pressure and neutron degeneracy pressure. D) In a black hole, the pressure coming from neutron degeneracy pressure is slightly greater than that coming from electron degeneracy pressure.

3) Which of the following best describes why a white dwarf cannot have a mass greater than the 1.4-solar-mass limit? A) Electron degeneracy pressure depends on the speeds of electrons, which approach the speed of light as a white dwarf's mass approaches the 1.4-solar-mass limit. B) White dwarfs get hotter with increasing mass, and above the 1.4-solar-mass limit they would be so hot that even their electrons would melt. C) White dwarfs are made only from stars that have masses less than the 1.4-solar-mass limit. D) The upper limit to a white dwarf's mass is something we have learned from observations, but no one knows why this limit exists.

6) According to our modern understanding, what is a nova? A) an explosion on the surface of a white dwarf in a close binary system B) the explosion of a massive star at the end of its life C) the sudden formation of a new star in the sky D) a rapidly spinning neutron star

6) According to present understanding, a nova is caused by A) hydrogen fusion on the surface of a white dwarf. B) carbon fusion in the core of a white dwarf. C) hydrogen fusion on the surface of a neutron star. D) a white dwarf that gains enough mass to exceed the 1.4-solar-mass limit.

9) A typical neutron star is more massive than our Sun and about the size (radius) of A) a small asteroid (10 km in diameter). B) Earth. C) the Moon. D) Jupiter.

9) Which of the following best describes what would happen if a 1.5-solar-mass neutron star, with a diameter of a few kilometers, were suddenly (for unexplained reasons) to appear in your home town? A) The entire Earth would end up as a thin layer, about 1 cm thick, over the surface of the neutron star. B) It would rapidly sink to the center of Earth. C) The combined mass of Earth and the neutron star would cause the neutron star to collapse into a black hole. D) It would crash into Earth, throwing vast amounts of dust into the atmosphere that, in turn, would cool Earth; this is probably what caused the extinction of the dinosaurs.

Cepheid Variable? A bright source of variable X-ray emission, thought to harbor a supermassive black hole A type of galaxy that varies in its light output A type of very luminous star that makes an excellent standard candle A main-sequence star of spectral type B5

A Type of very luminous star that makes an excellent standard candle Cepheids are pulsating variable stars for which we can infer the luminosity from the time between peaks of brightness, which makes them valuable as standard candles.

Galaxy Disk Evolution

A computer simulation showing the development and evolution of the disk of a galaxy such as the Milky Way. The simulation begins with conditions about 9 billion years ago, after material for the disk of our galaxy had largely come together but the actual disk formation had not yet started. The simulation shows that stars like the Sun, though maintaining circular orbits around the galaxy's core, could migrate large distances from where they were born.

We can deduce that a star is part of an eclipsing binary pair or has a planet that transits across its disk by obtaining what kind of data?

A light curve.

What does this diagram represent?

A particle and antiparticle colliding and converting all their mass into photons.

What does this diagram represent?(anitelectron + and electron - merge into center to two gamma ray photons)

A particle and antiparticle colliding and converting all their mass into photons.

According to Kepler's Third Law, how does a planet's mass affect its orbit around the Sun?

A planet's mass has no significant effect on its orbit around the Sun.

Which of the following best describes how the greenhouse effect works?

A planet's surface absorbs visible sunlight and returns this absorbed energy to space as infrared light. Greenhouse gases slow the escape of this infrared radiation, which thereby heats the lower atmosphere.

Nebulae

A region of active star formation is characterized by hot, massive stars. Hot blobs of gas-ionization nebulae (aka emission nebulae or H II regions)-are often found near these hot stars The Orion Nebula is the most famous example Ionized hydrogen glows red in visible light; we see ionized oxygen as green Reflection nebulae-caused by light reflected off dust grains-appear blue because the red light is scattered by the dust. Nebulosity around the hot stars of the Pleiades is an example. Black regions in nebulae are dark, dusty gas clouds that block our view in visible light of stars beyond them. The Pelican Nebula is an ionization nebula associated with the North America Nebula in the constel-lation Cygnus about 1,800 l-y (0.55 kpc) distant. The Pelican Nebula is located near first magnitude star Deneb and is divided from its more prominent neighbor, the North America Nebula, by a molecular cloud filled with dark dust.

Supernova Aftermath

A supernova remnant (SNR) cools and begins to emit visible light as it expands. New elements made by the supernova mix into the interstellar medium. The Cygnus Loop supernova remnant (SNR) is 1,470 l-y (0.45 kpc) distant and 130 l-y (39.8 pc) across and is the result of a massive stellar explosion that occurred 5,000-8,000 years ago. (A) Cyg-nus Loop in the visible. (J. Hester-ASU/Davide De Martin-ESA) (B) Ultraviolet image of the Cygnus Loop Nebula, taken by NASA's Galaxy Evolution Explorer.

(A) Where do stars tend to form in our galaxy (the general location)? (Hint: It is one of the main components. (B) Active star-forming regions contain what items?

A. Spiral Arms B. Molecular clouds, hot stars, and ionization nebulae.

What are the 5 major components of a spiral galaxy like the Milky Way?

A.Spiral arms B.Flat disk C.Central bulge D.Dim halo E.Globular clusters circling the galaxy center

(You are situated in the northern hemisphere.) Say you go outside at 1 AM and the Moon is high in the south. You set up a telescope on a tripod and put the Moon in the field of view and lock the telescope in place. If you come out at 1AM the following night, approximately where is the Moon?

About 13 degrees to the left of where the telescope is pointing.

This figure shows an all-sky map of the cosmic microwave background recorded by the Planck telescope. How long did this light travel through space before it reached the telescope?

About 14 billion years.

About 14.5 degrees to the right where it was a 1 AM, and a little lower in the sky.

Disk

Active regions of star formation.

Which kind of light moves the fastest in a vacuum?

All light is electromagnetic radiation and moves at the same speed in a vacuum.

If a comet has an elliptical orbit that brings it close to the Sun when it is at perihelion, what eventually happens to the comet?

All the ices holding together the nucleus boil off (sublimate), leaving only rocks and dust.

What are we seeing in this photograph? (Hubble space photograph filled with galaxies)

Almost every object in the photo is a galaxy, and these galaxies are at many different distances from us.

Which of the following statements correctly describes current understanding of the eventual fate of the universe?

Although the accelerating expansion suggests that the universe will continue to expand forever, it is possible that future discoveries will indicate a different fate.

H II Region

An H II region is a volume of space where hydrogen in the interstellar medium is ionized rather than in a neutral state. These are regions where hot, blue O and B stars are pouring large amounts of ultraviolet radiation into the surrounding cloud from which they recently formed. O and B stars can ionize all H (and other atoms) for dozens or even hundreds of light-years in every direction, producing a Stromgren sphere.

Question 22: According to current understanding, what is a quasar?

An active galactic nucleus that is particularly bright right answer feedback: Observations show that quasars lie in the centers of distant galaxies, indicating that they are very luminous active galactic nuclei.

When we look at an object that is 2,000 light years away we see it

As it was 2,000 years ago.

Which of the following is the best answer to the question, "Why does the Sun shine?"

As the Sun was forming, gravitational contraction increased the Sun's temperature until the core become hot enough for nuclear fusion, which ever since has generated the heat that makes the Sun shine.

Interstellar Dust

Astronomers have yet to capture a true interstellar dust grain. However, interplanetary dust grains, such as this porous chondrite, have been captured and studied. Interplanetary dust particles are thought to be similar in composition but larger in size than interstellar dust grains. Interstellar dust particles are extremely small at a micron (μ = 10−6 m) or less across, which is approximately the wavelength of blue light waves. Dust grains are irregularly shaped and are composed of silicates, carbon, ice, and/or iron compounds.

Local Arm Update

Astronomers once thought that our solar system's location in the Milky Way Galaxy placed it in a small structure called the Local Arm (also referred to as the Orion Spur). Recent research indicates that the Local Arm is far larger than previously thought and is more like the adjacent major arms rather than a small spur. This image shows the Local Arm as a probable major branch of the Perseus Arm. Note that this rendering of the Milky Way has been flipped both vertically and horizontally with respect to earlier similar renderings that show details of our galaxy's structure.

What was Aristotle's proof that the Earth must be very nearly spherical?

At the start and end of a lunar eclipse, the shadow of the Earth is always observed to be circular.

Molecular Cloud Formation

Atomic hydrogen gas forms as hot gas cools, allowing electrons to join with protons Molecular clouds form next, after gas cools enough to allow to atoms to combine into molecules Molecular clouds in Orion are composed of Mostly H2 About 28% He About 1% CO Many other molecules

11) Pulsars are thought to be A) accreting white dwarfs. B) rapidly rotating neutron stars. C) unstable high-mass stars. D) accreting black holes.

11) Which statement about pulsars is not thought to be true? A) All pulsars are neutron stars, but not all neutron stars are pulsars. B) Pulsars can form only in close binary systems. C) A pulsar must have a very strong magnetic field and rotate quite rapidly. D) Pulsars are kept from collapsing by neutron degeneracy pressure.

17) Which of the following observatories is most likely to discover a black hole in a binary system? A) the Hubble Space Telescope B) the Chandra X-Ray Observatory C) the SOFIA airborne infrared observatory D) the Arecibo Radio Observatory

2) A typical white dwarf is A) as large in diameter as the Sun but only about as massive as Earth. B) as massive as the Sun but only about as large in size as Earth. C) about the same size and mass as the Sun but much hotter. D) as massive as the Sun but only about as large in size as Jupiter.

2) The more massive a white dwarf, the A) higher its temperature. B) smaller its radius. C) larger its radius. D) higher its luminosity.

20) Consider again the civilization described in the previous question. (They live on a planet orbiting 10 AU from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole surrounded by an accretion disk.) One foolhardy day, a daring individual in their space force (let's call him Major Tom) decides to become the first of his species to cross the event horizon of the black hole. To add to the drama, he decides to go in wearing only a thin space suit, which offers no shielding against radiation, no cushioning against any forces, and so on. Which of the following is most likely to kill him first (or at least to start the process of killing him first)? A) tidal forces due to the black hole B) X rays from the accretion disk C) the crush of gravity at the singularity embedded within the black hole D) the sucking force from the black hole, which will cause his head to explode

20) Scientists have detected thousands of gamma ray bursts. The evidence suggests that most or all of these bursts A) have occurred in the central regions of the Milky Way. B) have occurred in distant galaxies. C) come from the same types of close binary systems that produce X-ray bursts. D) come from the Oort cloud surrounding the Sun.

5) What is an accretion disk? A) any flattened disk in space, such as the disk of the Milky Way Galaxy B) a disk of hot gas swirling rapidly around a white dwarf, neutron star, or black hole C) a stream of gas flowing from one star to its binary companion star D) a disk of material found around every white dwarf in the Milky Way Galaxy

7) Which of the following is not true about differences between novae and supernovae? A) Novae are much less luminous than supernovae. B) Supernovae eject gas into space but novae do not. C) Novae occur only in binary star systems, while supernovae can occur both among single stars and among binary star systems. D) The same star can undergo novae explosions more than once, but can undergo only a single supernova.

CO Acts as a Tracer for H2

Because CO is asymmetrical and relatively massive (28 vs. 2 amu for molecular hydrogen), it produces a much stronger spectral line. Radiation is emitted at millimeter wave-lengths when the CO molecule slows either its vibration or rotation. For every CO molecule there are about 10,000 H2 molecules; thus, emissions from CO molecules can be used as a tracer to map molecular hydrogen clouds in the galaxy.

Why do virtually all the galaxies in the universe appear to be moving away from our own?

Because expansion causes all galaxies to move away from nearly all others.

CO Reveals Milky Way Arm

Because the Milky Way contains large amounts of dust that blocks our optical views, it is extremely difficult to study the galaxy from our vantage point within the disk. In 2011, researchers used CO (carbon monoxide) emission to search for evidence of spiral arms in the most distant parts of the galaxy and discovered a large new spiral arm peppered with dense concentrations of molecular gas.

In principle, if we could see all the way to the cosmological horizon we could see the Big Bang taking place. However, our view is blocked for times prior to about 380,000 years after the Big Bang. Why?

Before that time, the gas in the universe was dense and ionized and therefore did not allow light to travel freely.

Black Hole Accretion Disk

Black hole accretion disks are compact halos created as dust, gas and other debris are pulled toward a black hole event horizon. Accretion disks radiate electromagnetic radiation, the frequency of which depends on the mass of the black hole. The more massive it is, the higher the energy of radiation emitted into space. As accretion disk matter falls toward the event horizon, approximately 10% of the mass is converted into energy and ejected as X-rays. This is a far more efficient energy conversion rate than the most efficient nuclear fusion reaction (approximately 0.5%). This X-ray emission can then be observed, creating a quasar, signifying a SMBH is driving the active galaxy.

In stars, helium can sometimes be fused into carbon and heavier elements (in their final stages of life). Why didn't the same fusion processes produce carbon and heavier elements in the early universe?

By the time stable helium nuclei had formed, the temperature and density had already dropped too low for helium fusion to occur.

1) A white dwarf is A) a precursor to a black hole. B) an early stage of a neutron star. C) what most stars become when they die. D) a brown dwarf that has exhausted its fuel for nuclear fusion.

12) How does an accretion disk around a neutron star differ from an accretion disk around a white dwarf? A) The accretion disk around a neutron star is made mostly of helium while the accretion disk around a white dwarf is made mostly of hydrogen. B) The accretion disk around a neutron star is more likely to give birth to planets. C) The accretion disk around a neutron star is much hotter and emits higher-energy radiation. D) The accretion disk around a neutron star always contains much more mass.

13) What is the basic definition of a black hole? A) a dead star that has faded from view B) any object made from dark matter C) an object with gravity so strong that not even light can escape D) a compact mass that emits no visible light

14) Based on current understanding, the minimum mass of a black hole that forms during a massive star supernova is roughly A) 0.5 solar masses. B) 1.4 solar masses. C) 3 solar masses. D) 10 solar masses.

15) Which of statement below about black holes is not true? A) Although we are not 100% certain that black holes exist, we have strong observational evidence in favor of their existence. B) If you watch someone else fall into a black hole, you will never see him (or her) cross the event horizon; you'll only see him fade from view as the light he emits or reflects becomes more and more redshifted. C) A spaceship passing near a 10-solar-mass black hole is much more likely to be destroyed than a spaceship passing at the same distance from the center of a 10-solar-mass main-sequence star. D) If you fell into a black hole, you would experience time to be running normally as you plunged rapidly across the event horizon.

19) The Schwarzschild radius of a black hole depends on A) the observationally measured radius of the black hole. B) the way in which the black hole formed. C) only the mass of the black hole. D) both the mass and chemical composition of the black hole.

4) The white dwarf that remains when our Sun dies will be mostly made of A) hydrogen. B) helium. C) carbon. D) neutrons.

5) Which statement about accretion disks is not true? A) The gas in the inner parts of the disk travels faster than the gas in the outer parts of the disk. B) The gas in the inner parts of the disk is hotter than the gas in the outer parts of the disk. C) The primary factor determining whether a white dwarf has an accretion disk is the white dwarf's mass. D) Accretion disks are made primarily of hydrogen and helium gas.

7) Suppose that a white dwarf is gaining mass through accretion in a binary system. What happens if the mass someday reaches the 1.4 solar mass limit? A) The white dwarf will collapse in size, becoming a neutron star. B) The white dwarf will undergo a nova explosion. C) The white dwarf will explode completely as a white dwarf supernova. D) The white dwarf will collapse to become a black hole.

8) A neutron star is A) the remains of a star that died by expelling its outer layers in a planetary nebula. B) a star made mostly of elements with high atomic mass numbers, so that they have lots of neutrons. C) the remains of a star that died in a massive star supernova (if no black hole was created). D) an object that will ultimately become a black hole.

8) Will our Sun ever undergo a white dwarf supernova explosion? Why or why not? A) Yes, right at the end of its double-shell burning stage of life. B) Yes, about a million years after it becomes a white dwarf. C) No, because it is not orbited by another star. D) No, because the Sun's core will never be hot enough to fuse carbon and other heavier elements into iron.

Interstellar medium

Clouds of gas and dust

Which of the following statements about large-scale structure is probably not true?

Clusters and superclusters appear to be randomly scattered about the universe, like dots sprinkled randomly on a wall.

What are the dark blobs in this photograph from the Hubble Space Telescope?

Cold, dense molecular clouds in which stars are forming.

Cosmic Rays

Cosmic rays from space hit Earth's atmosphere all the time. When a high-energy cosmic ray enters the atmosphere, it can cause an "air shower." The cosmic ray hits a molecule in the atmosphere and "breaks up," producing many sub-atomic particles. A real air shower can make millions of particles. The cosmic ray (in red, at the top) makes many other particles. The sub-atomic particles shown here include protons (green), neutrons (orange), pions (yellow), muons (purple), photons (black), and electrons and positrons (pink).

Dimensions of the MWG

Current estimates put the diameter of the MWG over 100,000 1-y.

Future of Our Galaxy

Current measurements suggest that, in about three billion years, the Milky Way and Andromeda galaxies may collide. This movie shows a supercomputer simulation of one possible collision scenario between the Milky Way and Andromeda. Each spiral galaxy is represented by a disk of stars surrounded by a spherical "dark matter" halo. The Milky Way is shown face-on and is initially at the bottom of the frame while the Andromeda moves from the top of the frame down and is tilted from this perspective. The movie's field of view is about one million light years (10 billion billion km) across, and the total elapsed time of the movie is about 1 billion years.

Local Bubble and Galactic Neighborhood

Currently, the Sun is passing through a Local Interstellar Cloud (LIC), shown in violet, which is flowing away from the Scorpius-Centaurus Association of young stars. The LIC resides in a low-density hole in the interstellar medium (ISM) called the Local Bubble, shown in black. Nearby, high-density molecular clouds including the Aquila Rift surround star forming regions, each shown in orange.

1) Which of the following statements about degeneracy pressure is not true? A) Degeneracy pressure can continue to support an object against gravitational collapse even if the object becomes extremely cold. B) Degeneracy pressure arises from a quantum mechanical effect that we don't notice in our daily lives. C) Black holes form when gravity overcomes neutron degeneracy pressure. D) Degeneracy pressure can arise only from interactions among electrons.

10) Each Voyager spacecraft carries a "postcard" designed to be understandable to any aliens that might someday encounter it. On the "postcard," scientists pinpointed the location of Earth by triangulating it between pulsars. Why did the scientists choose pulsars rather than some other type of star? A) Pulsars are very bright and therefore easy to find. B) Several pulsars are located within a dozen light-years of our solar system, making them useful for finding our solar system. C) We're pretty sure that aliens will have only radio telescopes and not optical telescopes, so they'll have a better chance of seeing pulsars than ordinary stars. D) Pulsars are easy to identify by their almost perfectly steady periods of pulsation.

10) If you had something the size of a sugar cube that was made of neutron star matter, it would weigh A) about 50 pounds. B) as much as the entire Earth. C) about as much as a truck. D) about as much as a large mountain.

14) Suppose you drop a clock toward a black hole. As you look at the clock from a high orbit, what will you notice? A) Time on the clock will run faster as it approaches the black hole, and light from the clock will be increasingly blueshifted. B) The clock will fall toward the black hole at a steady rate, so that you'll see it plunge through the event horizon within just a few minutes. C) The clock will fall faster and faster, reaching the speed of light as it crosses the event horizon. D) Time on the clock will run slower as it approaches the black hole, and light from the clock will be increasingly redshifted.

15) What do we mean by the event horizon of a black hole? A) It is the very center of the black hole. B) It is the distance from the black hole at which stable orbits are possible. C) It is the place where X rays are emitted from black holes. D) It is the point beyond which neither light nor anything else can escape.

16) Imagine that our Sun were magically and suddenly replaced by a black hole of the same mass (1 solar mass). What would happen to Earth in its orbit? A) Earth would almost instantly be sucked into oblivion in the black hole. B) Earth would orbit faster, but at the same distance. C) Earth would slowly spiral inward until it settled into an orbit about the size of Mercury's current orbit. D) Nothing—Earth's orbit would remain the same.

18) Which of the following statements about gamma ray bursts is not true? A) Gamma ray bursts are among the most luminous events that ever occur in the universe. B) The events responsible for gamma ray bursts apparently produce only gamma rays, and no other light that we can hope to detect. C) Gamma ray bursts were originally discovered by satellites designed to look for signs of nuclear bomb tests on Earth. D) Based on their distribution in the sky, we can rule out a connection between gamma ray bursts and X-ray binaries in the Milky Way Galaxy.

4) The maximum mass of a white dwarf is A) about the mass of our Sun. B) limitless; there is no theoretical limit to the maximum mass of a white dwarf. C) about 3 times the mass of our Sun. D) about 1.4 times the mass of our Sun.

Some people wish that we lived in a recollapsing universe that would eventually stop expanding and start contracting. Based on current understanding, which of the following would have to be true for this to be the case?

Dark energy does not exist and there is much more matter than current evidence suggests.

Density Wave Theory

Density wave theory, proposed by C. C. Lin and Frank Shu in the mid-1960s, explains the spiral arm structure observed in spiral galaxies. The theory introduces the idea of long-lived density waves, which are sections of the galactic disk that have a greater mass density (about 10%-20% greater).The density wave collects gas and dust as it sweeps across the galaxy disk. The gas is compressed (becomes denser), forms stars that develop first as H II regions and then young clusters. In the galaxy, stars, gas, and dust move through the density waves, are compressed, and then move out of them. Simulation of a galaxy with a simple spiral arm pattern. Although the spiral arms do not rotate, the galaxy does. Because the spiral pattern moves slower than the rotation of the stars and gas, stars move in and out of the spiral arms as time progresses. The spiral arm structure is thus long-lived.

According to the Big Bang theory, why do we live in a universe that is made of almost entirely of matter rather than antimatter?

During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated except the very slight excess of matter particles.

According to the Big Bang theory, why do we live in a universe that is made of almost entirely of matter rather than antimatter?

During the first 0.001 second after the Big Bang, particles and antiparticles were made in almost but not perfectly equal numbers. Everything annihilated expect the very slight excess of matter particles

21) Consider again the civilization described in the previous question. (They live on a planet orbiting 10 AU from a close binary star system that consists of a 15 MSun red giant star and a 10 MSun black hole surrounded by an accretion disk.) Through a bizarre (and scientifically unexplainable) fluctuation in the space-time continuum, a copy of a book from that civilization arrives on your desk; it is entitled Iguoonos: How We Evolved. In the first chapter, you learn that these beings evolved from organisms that lived 5 billion years ago. Which of the following statements should you expect to find as you continue to read this book? A) As a result of traumatic experiences to their evolutionary ancestors, they dislike television. B) Their immediate ancestors were chimpanzees. C) They believe that the presence of two stars in their system was critical to their evolution. D) They evolved from primitive wormlike creatures that had 13 legs, 4 eyes, and bald heads, thus explaining why such critters are now considered a spectacular delicacy. E) They evolved on a different planet in a different star system, and moved to their current location.

Which analogy best explains why inflation predicts that the overall geometry of the observable universe should appear to be flat?

Earth is so big that it the part explored by an ant appears flat.

Satellites in low-Earth orbits are more likely to crash to Earth when the sunspot cycle is near solar maximum because

Earth's upper atmosphere tends to expand during solar maximum, exerting extra drag on satellites in low orbits.

It is more difficult to determine the amount of dark matter in an elliptical galaxy than in a spiral galaxy. Why?

Elliptical galaxies lack the atomic hydrogen gas that we use to determine orbital speeds at great distances from the centers of spiral galaxies.

Components of the MWG

Flat disk bright central bulge spiral arms dimmer round halo surrounding everything; few hundred globular clusters circle the galaxy's center

MWG Formation

Formed from several smaller galactic gas clouds.

Ionizaton Nebulae

Found around short-lived high mass stars whose strong ultraviolet radiation ionizes the nebulae's gas molecules. Such nebulae are regions of active star formation. (A) The Orion Nebula (M42) is about 1,350 l-y (0.4 kpc) distant and is largely an ionization nebula. (NASA) (B) The Cone Nebula is an H II region in the constellation Monoceros. It was discovered by William Herschel on December 26, 1785. The nebula is located about 2,700 l-y (830 pc) distant. The Cone Nebula forms part of the nebulosity surrounding the Christmas Tree Cluster. The designation of NGC 2264 refers to both objects and not the nebula alone.

History of Milky Way Disk Population

Found in disk Often called Population I Contains both young and old stars All of the stars have heavy element proportions of 2%, like our Sun The Pleiades (M45) and their reflection nebulae are young, hot, Population I stars that often gather in loose associations called open clusters.

History of the Milky Way Spheroidal Population

Found in halo and bulge Often called population II Most stars are old, red, dim, and smaller in mass than the Sun Heavy elements concentrations are 100X less (0.02% vs 2%) Region is nearly gas-free compared to disk Stars formed early in the galaxy's history

MWG Look-Alikes

Four galaxies that look like the Milky Way Galaxy. (A) NGC 6744 is at a distance of 31 million l-y (9.5 Mpc), but it is larger with a diameter of 175,000 l-y (53.6 kpc). (B) NGC 3953 is 55 million l-y (16.9 Mpc) distant with a diameter of 95,000 l-y (29.1 kpc). (C) NGC 7723 is 80 million l-y (24.5 Mpc) distant with a diameter of 90,000 l-y (27.6 kpc). (D) NGC 5970 is 105 million l-y (32.2 Mpc) distant with a diameter of 85,000 l-y (26.0 kpc).

This photo shows the Large Magellanic Cloud, or LMC for short. Based on what you have learned in about the LMC from your textbook, which of the following statements about the LMC is not true?

Galaxies similar to the LMC are very rare.

Why should galaxy collisions have been more common in the past than they are today?

Galaxies were closer together in the past because the universe was smaller. As the universe expands, the average distance between galaxies increases, making collisions less likely (on average) as time passes. (Note that this does not affect the likelihood of collisions within clusters, since clusters are gravitationally bound and are not expanding with time.)

Larger the Bulge Mass, Larger the Black Hole Mass

Galaxies with large bulges have large black holes. Galaxies with smaller bulges have small black holes at their centers. A plot of bulge mass vs. central black hole mass shows a linear relationship. As the bulge mass increases in larger and larger galaxies, the mass of the corresponding central supermassive black hole also increases in direct proportion.

A Milky Way Lookalike

Galaxy Messier 83 lies roughly 15 million l-y (4.6 Mpc) away towards the southern constellation of Hydra (the sea serpent). It stretches over 40,000 l-y (12.3 kpc), making it roughly 2.5 times smaller than our own Milky Way. However, in some respects, Messier 83 is quite similar to our own galaxy. Both the Milky Way and Messier 83 possess a bar across their respective galactic nucleus, the dense spherical conglomeration of stars seen at the center of spiral galaxies.

Milky Way Revealed

Galileo showed that the band of light in the night sky was composed of thousands of stars. Milky Way Galaxy (MWG) holds over 100 billion stars and is one among 125 billion galaxies in the observable universe. (Recent estimates place the number of stars in the MWG at upwards of 200-400 billion.) Components of the MWG: Flat disk Bright central bulge Spiral arms Dimmer, round halo surrounding everything Few hundred globular clusters circle the galaxy's center. The Milky Way arch emerging from the Cerro Paranal, Chile, on the left, and sinking into the Antofagasta's night lights. The bright object in the center, above the Milky Way, is Jupiter, and the Magellanic Clouds are visible on the left side.

Which of the following photons have the shortest wavelengths and the highest energies?

Gamma rays

Gas Clouds and Spiral Structure

Gas clouds follow a similar motion to stars, but outgoing gas is obstructed when it meets a spiral arm. When clouds of atomic hydrogen with spiral arm motions meet with clouds crossing the arm, higher densities occur and greater turbulence is created. These higher density H I regions can generate molecular gas clouds from which protostars form.

If you could watch a time-lapse movie of the interstellar medium over hundreds of millions of years, what would you see?

Gas that is often moving at high speed, particularly after one or more supernovae, and constantly changing form between molecular clouds, atomic hydrogen, and hot, ionized bubbles and superbubbles.

Which of the following is not one of the three major categories of galaxies? Spiral galaxies Globular galaxies Elliptical galaxies Irregular galaxies

Globular Galaxies

M4 Finder Chart

Globular cluster M4 is fairly easy to find in binoculars and small telescopes under dark skies. M4 lies in Scorpius, a short distance SW of the bright star α Scorpii, better known as Antares ("Rival of Mars"). M4 is best viewed from June 1 to September 1.

M4, Globular Cluster

Globular clusters contain hundreds of thousands of stars and form a halo around the nuclear bulge of our galaxy. The stars in globular clusters are very old, between 9 and 12 billion years old. M4 is perhaps the closest globular cluster to us at 7,000 l-y (2.1 kpc). (A) M4 and its relative position to Earth in the Milky Way Galaxy. (NASA and A. Feild/ STScI) (B) M4 imaged by a professional 2.2-m telescope (ESO). M4 is found near Antares in the constellation Scorpius and is visible in the southern sky at 10:00 pm from June 1 to September 1.

A spectrum of a normal star would look like which spectrum above?

Graph with notches going down

Saw you have put a bowling ball on a shelf in the closet, and some slight vibration causes the bowling ball to roll off the shelf and fall to the floor. As the bowling ball is falling

Gravitational Potential Energy is converted into Kinetic Energy

Molecular Clouds

Gravitational forces in molecular clouds gather molecules into the compact core that eventually become protostars. Once a few stars begin to form, UV radiation from high-mass stars ionize and heat the gas in the molecular cloud, preventing much of the gas in the cloud from turning into stars. This process is called molecular cloud erosion. Because more gas is locked up in brown dwarfs and stellar corpses with each star-gas-star cycle, star formation in the MWG will taper off over the next billion years or so. A 50 light-year-wide (15.3 pc) view of the central region of the Carina Nebula where star birth and molecular cloud erosion is taking place.

Stars form in molecular clouds

Gravity forms stars out of the gas in molecular clouds, completing the star-gas-star cycle. Arrows point to where stars are emerging from their cocoons of dust and gas. Gas pillars in M16, the Eagle Nebula, which is 6,500 l-y (2.0 kpc) distant in the constellation Serpens. The tallest pillar (far left) is about 4 l-y (1.2 pc) long from base to tip. This is a higher resolution image taken in 2014 as a tribute to the original photograph.

Why do orbits of disk stars bob up and down?

Gravity of disk stars pulls them toward disk.

Galactic Habitable Zone

Habitable zone of the Milky Way (green) excludes the dangerous inner regions and the metal poor outer regions of our galaxy. It is analogous to the habitable zone on the much smaller scale of our solar system (inset). Neither zone has sharp boundaries.

Halo Stars vs. Disk Stars

Halo and disk stars in the Milky Way show significant differences in age and chemical composition. Halo stars are older and have far less heavy elements.

Which internal heat source still generates heat within the terrestrial planets today?

Heat from radioactive decay of unstable elements such as thorium and uranium

Hot Interstellar Bubble

High-mass stars have a strong stellar winds that blow bubbles of hot gas. The Bubble Nebula, NGC 7635 in Cassiopeia, is 6 l-y (1.8 pc) across and about 7,100 l-y (2.1 kpc) distant. The "bubble" is created by the stellar wind from a massive hot, 8.7 magnitude young central star (the 15 ± 5 M SAO 20575). The nebula is near a giant molecular cloud which contains the expansion of the bubble nebula while itself being excited by the hot central star, causing it to glow. It was discovered in 1787 by William Herschel.

The data points in this diagram represent the measured speeds and distances of various galaxies, and the solid line represents a best fit to these data. The trend indicated by the solid line is known as:

Hubble's Law

Spiral Arms and the Winding Problem

If the spiral arms were rigid mass concentration, the galaxy must rotate as a whole around its center in order to maintain its spiral structure. Observation shows that this is not the case. However, differential rotation in spiral galaxies has been observed and would dissolve spiral arms in a short period of time (several revolutions) where they composed of fixed mass concentrations. This is known as the winding problem. Orbits predicted by the density wave theory allows the existence of stable spiral arms. Stars move in and out of the spiral arms as they orbit the galaxy.

Milky Way from Above

If we could view the Milky Way from above the disk, we would see its spiral arms.

Star Orbiting SMBH at Galaxy's Center

In 2002, the star known as S2 approached the SMBH at our galaxy's center to within 17 light-hours (only three times the distance between the Sun and Pluto) while traveling at no less than 5,000 km/s. S2 is one of the closest observable stars to the compact radio source and supermassive black hole SgrA* at the very center of the Milky Way. The star's orbital period is just over 15 years.

Supernova-Induced Star Formation

In galaxies, we often find clusters of young stars near other young stars. This phenomenon is called supernova-induced star formation. The very massive stars form first and explode into supernova. These explosions send shock waves into the molecular cloud, causing nearby gas to compress and form more stars. This allows a type of stellar coherence (young stars are found near other young stars) to build up and is responsible for the pinwheel patterns we see in galaxies

Deducing the Galactic Center

In the 1920s, Harlow Shapley demonstrated that the Milky Way's globular clusters are centered on a point thousands of light-years from our Sun. He correctly deduced that this point is the center of our galaxy, not our Sun. He used the period-luminosity relation of RR Lyrae stars to find the distances to the globular clusters. Our Sun lies in the outer part of the galactic disk, about 27,000 light-years (8.3 kpc) from its center. A study published in 2014 of 100 massive young stars in the galactic core has yielded the most accurate distance yet measured from Earth to the Milky Way's center: 27,200 ± 520 light-years (8.33 kpc). A study published in November 2016 comparing the speeds of 200,000 stars orbiting the Milky Way led to a refined distance estimate of 24,788 to 26,745 light-years (7.6 to 8.2 kpc). Studying our galaxy is difficult because dust obscures our view and we are inside. Harlow Shapley (1885-1972) also showed that Cepheid variables cannot be eclipsing binaries; he was the first to propose that they are pulsating stars. In 1953, he proposed his "liquid water belt" theory, now known as the concept of a habitable zone for planets.

Star Formation: Halo vs. Disk

In the disks of spiral galaxies like the Milky Way, we see many ionization nebulae (red from the ionized hydrogen) and young, blue stars. Conversely, in the halo of spiral galaxies we observe no ionization nebula (as well as dust) and no young, blue stars, only older orange and red stars. Stars in the halo are old stars. Young stars are absent because of the lack of gas and dust to build new ones. Stars in the disk are typically younger as there exists the gas clouds required for star birth.

Galaxy Formation Over Tike

In the early universe, matter in the form of gas is almost uniformly distributed. As time passes, gravity draws gas into denser regions of space. Eventually, the densest regions go on to become galaxies. Structure formation in the gaseous component of the universe, in a simulation box 100 Mpc/h on a side. From left to right: z = 6, z = 2, and z = 0. Formed stellar material is shown in yellow.

How does the hypothesis of inflation account for the existence of the "seeds" of density from which galaxies and other large structures formed?

Inflation would have caused random, microscopic quantum fluctuations to grow so large in size that they became the seeds of structure.

How does the idea of inflation account for the existence of the "seeds" of density from which galaxies and other large structures formed?

Inflation would have caused random, microscopic quantum fluctuations to grow so large in size that they became the seeds of structure.

The top panorama shows our view of the Milky Way in all directions as it appears in visible light. The bottom panorama shows the same view, but in a different wavelength of light. What wavelength band are we seeing in the bottom photo, and how do you know?

Infrared light, because the dust that appears dark in the visible light photo glows in infrared light.

MWG at Infrared Wavelengths

Infrared wavelengths (microns) permit astronomers to see through the dust that obscures large parts of the Galaxy when viewed at visible wavelengths. This technology was developed following WWII. Infrared: 100-12 microns (0.1-0.012 mm)/ f= 3.0 x 10<3 to 25.0 x 10<3 GHz Mid-infrared: 11-6.8 microns (0.011-0.0068 mm)/ f- 27.3 x 10<3 to 44.1 x 10<3 GHz Near Infrared: 3.5-1.25 microns (0.0035-0.00125 mm)/ f= 85.7 x 10<3 to 240.0 x 10<3 GHz

Atomic Hydrogen Clouds:

Interstellar space is filled with extremely tenuous clouds of gas which are composed mostly of atomic hydrogen (H). The neutral hydrogen atom (known as H I) consists of 1 proton and 1 electron. The proton and electron spin like tops but only have two orientations: their spin axes are either parallel or anti-parallel. It is a rare event for hydrogen atoms in the IM to switch from the parallel to the anti-parallel configuration, but when they do they emit radio waves with a wavelength of 21 centimeters (~8 inches) and a corresponding frequency of exactly 1420 MHz. Radio telescopes tuned to this frequency have mapped the neutral hydrogen in the sky. The image shows an all-sky H I survey with the plane of our Milky Way Galaxy running horizontally through the center. In this false color image, no stars are visible, just diffuse clouds of gas tens to hundreds of light years across which cluster near the plane. The gas clouds seem to form arching, looping structures, stirred up by stellar activity in the galactic disk.

A circular orbit

Is a special case of an elliptical orbit

Which of the following is not an observed characteristic of the cosmic microwave background?

It contains prominent spectral lines of hydrogen, the primary chemical ingredient of the universe.

The Earth's distance from the Sun varies by +- 1.7 percent. How does Earth's varying distance from the Sun affect our seasons?

It doesn't -- Earth's orbital distance plays no significant role in the seasons.

Why do we call dark matter "dark"?

It emits no radiation that we have been able to detect.

What happens to gas as it freely expands?

It gets less dense and cools

Which of the following is not a requirement for an object to be considered a planet?

It has one or more Moons.

This figure is a spacetime diagram, which means it shows space (distance in this case) on the horizontal axis and time on the vertical axis. Study it carefully to be sure you understand it. Now, look at the square around one of the galaxy images. What does the position of this image mean on this diagram?

It indicates that the galaxy was closer to the Milky Way when the supernova exploded 400 million years ago.

Is this a photo of a spiral galaxy or an elliptical galaxy, and how do you know?

It is a spiral galaxy, because we can see a dusty disk going across the center.

Which of the following statements is not true of the object known as Sgr A* in the center of our galaxy?

It is by far the brightest source of visible light lying in the direction of the galactic center.

How does the habitable zone around a star of spectral type G compare to that around a star of spectral type M?

It is larger.

Most nearby stars move relative to the Sun at speeds below about 30 km/s. Suppose you observe a nearby star that is moving much faster than this (say, 300 km/s). Which of the following is a likely explanation for its high speed?

It is probably a halo star that is currently passing through the disk.

Which statement about the cosmic microwave background is NOT true?

It is the result of a mixture of radiation from many independent sources, such as stars and galaxies.

This diagram represents what we call a saddle-shaped geometry. In the context of this chapter, what is its significance?

It represents a two-dimensional analogy to one possible geometry for our universe.

This diagram represents what we call a saddle-shaped geometry. In the context of this chapter, what is its significance?.

It represents a two-dimensional analogy to one possible geometry for our universe.

How would you expect a star that formed recently in the disk of the galaxy to differ from one that formed early in the history of the disk?

It should have a higher fraction of elements heavier than hydrogen and helium.

What does this graph show?

It shows calculations indicating that the temperature of the universe began quite high but is now quite low.

What does this graph show? (graph declines linearly from left to right)

It shows calculations indicating that the temperature of the universe began quite high but is now quite low.

The data points on this graph represent the measured spectrum of the cosmic microwave background, while the solid curve represents a theoretically calculated thermal radiation spectrum for a temperature of 2.73 K. What is the significance of the near-perfect match between the data and the theoretical spectrum?

It shows that the cosmic microwave background has a thermal radiation spectrum, just as the Big Bang theory predicts it should.

Each dot on this graph represents an individual Cepheid variable star. Which of the following is true for a Cepheid that is 10,000 times as luminous as the Sun?

It varies gradually in brightness, with a peak in brightness about every 30 days.

We have talked about vectors and scalars. Which of the following is a scalar?

Kinetic Energy.

According to the model in which active galactic nuclei are powered by supermassive black holes, the high luminosity of an active galactic nucleus primarily consists of ______. light emitted by hot gas in an accretion disk that swirls around the black hole radio waves emitted from radio lobes found on either side of the galaxy we see in visible light intense radiation emitted by the black hole itself the combined light of thousands of young, high-mass stars that orbit the black hole

Light emitted by hot gas in an accretion disk that swirls around the black hole

Which of the following best describes why the Sun emits more of its energy in the form of visible light?

Like all objects, the Sun emits thermal radiation with a spectrum that depends on its temperature, and the Sun's surface temperature is just right for emitting mostly visible light.

Spiral density waves

Likely caused the formation of spiral arms.

In 1 trillion (10<12) years, where will most of the gas currently in stars and clouds be found?

Locked into white dwarfs and very low-mass stars.

Planetary Nebula

Lower mass stars return gas to interstellar space through stellar winds and planetary nebulae Planetary nebula IC 4406 ("Retina Nebula"), located in Lupus about 1,900 l-y (0.58 kpc) distant, is likely a hollow cylinder, with its square appearance the result of our vantage point in viewing the cylinder from the side.

Galileo and the Milky Way

Made the first telescopic observation of the Milky Way Galaxy and saw more stars than he count count. About 175 years later, William and Caroline Herschel did count stars in an attempt to map the extent of the Milky Way. (slide 2).

Harlow Shapley

Maps the positions of globular clusters and correctly deduces that they center on a point 26,000 l-y from us, which must be the galactic center.

Mass of the MWG within the Sun's orbit

Mass of the MWG within the Sun's orbit: 1.0 x 10<11 M (Sun symbol) Estimated total mass of the MWG: ~10<12M (Sun symbol) Mr=r x v<2/G (A) The orbital speed (v) and radius (r) of an object on a circular orbit around the galaxy tells us the mass (Mr) within that orbit. This relationship is known as the orbital velocity law. (B) Schematic of the Sun's 230-million-year orbit about the center of the Milky Way Galaxy.

Which of the following offers at least some evidence that inflation really occurred in the early universe?

Measurements of brightness variations in the cosmic microwave background.

This graph is made from data measurements of the cosmic microwave background (dots) and a model based on the hypothesis of inflation. What does it show?

Measurements of the separation between regions of different temperature (in the cosmic microwave background) agree with predictions made by the hypothesis of inflation

This graph is made from measurements of the cosmic microwave background. What does it show? (graph has tons of peaks and troughs)

Measurements of the separation between regions of different temperature (in the cosmic microwave background) agree with predictions made by the theory of inflation

Population I stars

Mix of old and new; heavy element concentrations like that found in Sun; in disk.

Detecting Molecules

Molecular hydrogen (H2) is hard to detect, so most of what we know about molecular clouds comes from observing spectral lines of molecules that make up a tiny fraction of a cloud's mass. Most abundant of these molecules is CO (carbon monoxide), which produces strong radio emission lines at the 10K-30K temperatures of molecular clouds. Astronomers have detected over 120 different kinds of molecules, including water (H20), ammonia (NH3), and ethyl alcohol (C2H5OH)

The Sun's Neighborhood

More active than the halo; typical of galactic disk Over 300 stars within 33 l-y (10.1 pc) Most are dim, red M star A few are young, hot stars No massive O or B stars No star clusters have formed recently Exist inside of a hot bubble (local bubble) Map of the local galactic neighborhood showing the Sun located near the edge of our local interstellar cloud (LIC). Alpha-Centauri is located just over 4 l-y (1.2 pc) away in the neighboring G-cloud complex. Outside these clouds, the density may be lower than 0.001 atoms/cm3. Our Sun and the LIC have a relative velocity of 26 km/sec.

If an asteroid a few miles in diameter collided with the Earth, the destructive power would be comparable to

More than 100,000 times the power of the largest nuclear explosion humans have ever made

The MWG

Most likely a barred spiral 120,000 l-y across, containing 200-400 billion stars

Which of the following statements comparing halo stars to our Sun is not true?

Most stars in the halo have either died or are in their final stages of life, while the Sun is only in about the middle of its lifetime.

X-Ray Flare at Galaxy Center

NASA's Nuclear Spectroscopic Telescope Array, or NuSTAR, captured these first, focused views of the supermassive black hole at the center of our galaxy in high-energy X-rays. The background image, taken in infrared light, shows the location of our Milky Way's SMBH, called Sagittarius A* (Sgr A*). In the main image, the brightest white dot is the hottest material located closest to the black hole, and the surrounding pinkish blob is hot gas, likely belonging to a nearby supernova remnant. The time series at right shows a flare caught by NuSTAR over an observing period of two days. The middle panel shows the peak of the flare, when the black hole was consuming and heating matter to temperatures up to the peak of the flare, up to 100 million (108) K. The main image is composed of light seen at four different X-ray energies. Blue light represents energies of 10-30 keV; green is 7-10 keV; and red is 3-7 keV. The time series shows light with energies of 3-30 keV.

Is space expanding within clusters of galaxies?

No, because their gravity is strong enough to hold them together even while the universe as a whole expands.

Galileo's observations of the moons of Jupiter most significantly showed that

Not all objects have to orbit the Earth.

Interstellar Material

Note that all interstellar material is actually 70% h, 28% He, and 2% heavy elements by mass. Some of the heavy elements are in the form of tiny, solid dust grains composed of carbon and silicon minerals. Although dust grains make up 1% of the mass of atomic H clouds, they are responsible for the absorption of visible light that prevents us from seeing through the galaxy's disk. As the atomic H cloud cools, H atoms can combine to form molecular hydrogen (H2) in molecular clouds Approximately 99% of the interstellar medium is composed of interstellar gas, and of its mass, about 70% is hydrogen (either molecular or atomic), with the remaining 28% helium. About 2% is composed of heavier elements. Of this 2%, 1% by mass is in the form of solid grains called dust. The interstellar gas consists of neutral atoms and molecules, as well as charged particles (ions and electrons). This gas is extremely dilute, with an average density of about 1 atom/cm3. Interstellar gas is typically found in two forms: cold clouds of neutral atomic or molecular hydrogen; and hot ionized hydrogen near hot young stars.

The principal spectral types of stars, from hot to cool, are represented by what sequence of letters?

OBAFGKM

HST Finds Extrasolar Planets at Center of MWG

On October 4, 2006, NASA announced the discovery of 16 extrasolar planet candidates orbiting a variety of distant stars in the central region of our Milky Way galaxy, 27,000 l-y (8.3 kpc) away. Using the Hubble Space Telescope, astronomers measured the slight dimming of a star due to the passage of a planet in front of it, an event called a transit. The suspected planets are at least the size of Jupiter.

Stars in disk orbit

Orbit in the same direction with a little up-and-down motion

Star Motions in the Galactic Bulge and Halo

Orbits of stars in the bulge and halo of our galaxy have random orientations to the galactic plane

Formation of the Milky Way

Our galaxy began as a giant protogalactic cloud containing all the H and He that finally became stars Stars of the spheroidal population formed first Heavy elements analysis suggests that the MWG formed from a few smaller clouds

Galactic Recylcing

Overall composition of the MWG: 70% H, 28% He, 2% heavy elements (elements heavier than helium). Chemical enrichment: process of adding to the abundance of heavy elements. Matter expelled from supernovae have enough velocity to escape our galaxy but interaction with the interstellar medium keeps this matter enriched with heavy elements within the galaxy. A supernova explosions enriches the surrounding interstellar medium with heavy elements.

Shown following are the names of several eras that are used to describe the history of the universe. Rank the eras from left to right in the order in which they occurred, from first to last.

Planck, GUT, electroweak, particle, nucleosynthesis, nuclei, atoms, galaxies

Globular Clusters in the MWG

Plots of 152 globular clusters onto a side view of the Milky Way Galaxy, revealing how globular clusters form a spherical halo around our galaxy. Currently, there are 157 known globulars in the MWG, with several more likely to be discovered.

Which of the following statements can NOT be tested by science today?

Prior to the Planck time, our universe emerged from another universe.

Which of the following statements cannot be tested by science today?

Prior to the Planck time, our universe sprouted from another universe.

Which is the correct order for the stages of the Sun's life?

Protostar stage, main sequence, giant, planetary nebula phase, white dwarf.

The figure above illustrates the apparent position of Jupiter (in ecliptic coordinates, as viewed from the Earth) over a 14 year span of time. The loops in its apparent motion are an illustration of ________________

Ptolemy's model

Erosion of Molecular Clouds

Radiation from newly formed stars is eroding these star-forming molecular clouds. Gas pillars within the Carina Nebula, which is 7,500 l-y (2.3 kpc) distant.

SN Remnants & Cosmic Rays

Radio emission in SN remnants is from particles accelerated to near light speed. Cosmic rays probably come from supernovae. (A) False color radio view of supernova remnant Cassiopeia A as seen by the VLA. Cas A is 11,000 l-y (3.4 kpc) distant and is about 10 l-y (3 pc) across. (National Radio Astronomy Observatory) (B) False color image of supernova remnant Cassiopeia A composited of data from three sources. Red is infrared data from the Spitzer Space Telescope, orange is visible data from the Hubble Space Telescope, and blue and green are data from the Chandra X-ray Observatory.

Measuring No-Return Point of a Supermassive Black Hole

Radio telescopes in Hawaii, Arizona, and California were linked together to create a virtual telescope called the Event Horizon Telescope (EHT). Using the EHT, astronomers have measured the black hole's "point of no return"—the closest distance that matter can approach before being irretrievably pulled into the black hole. The supermassive black hole lies at the center of giant elliptical galaxy M87, which is 53.5 million l-y (16.4 Mpc) distant. Even at this distance, this black hole is so big that its apparent size in the sky is about the same as the black hole at the center of the Milky Way galaxy. According to general relativity theory, a black hole's mass and spin determine how close material can orbit before becoming unstable and falling in toward the event horizon. The innermost stable orbit was found to be only 5.5 times the size of the black hole's event horizon. This size suggests that the accretion disk is spinning in the same direction as the black hole.

MWG at Radio Wavelenghts

Radio wavelengths (cm to mm) are effective for studying clouds of atomic and molecular hydrogen gas that permeate our galaxy. Radio: 73.5 cm / f = 408 MHz Atomic Hydrogen (H): 21.4 cm/ f= 1.4 GHz Radio: 12.5-11.1 cm/ f= 2.4-2.7 GHz Molecular hydrogen (H2): 2.6 mm (2,600 microns)/ f=115GHz Radio emission from carbon monoxide reveals molecular clouds

Local Interstellar Medium

Recent observations show that our Sun is moving through a Local Interstellar Cloud as this cloud flows outwards from the Scorpius-Centaurus Association star forming region. Our Sun may exit the Local Interstellar Cloud during the next 10,000 years.

Reflection Nebulae

Reflection nebulae scatter the light from stars. Look bluer than nearby stars for the same reason our sky looks blue. Shorter wavelengths (bluer) rays are scattered by the gas atoms.

This figure shows an all-sky map of the cosmic microwave background recorded by the Planck telescope. What do the dark and bright variations represent?

Regions of the sky with very slightly different density at the time the radiation was emitted.

This figure shows an all-sky map of the cosmic microwave background recorded by the WMAP telescope. What do the dark and bright variations represent?

Regions of the sky with very slightly different density at the time the radiation was emitted.

Evidence for Dark Matter comes from

Rotation curves of large spiral galaxies

Say we compress the whole history of the universe (13.7 billion years) into a calendar of 365 days. January 1st at 1 second after Midnight represents the Big Bang and December 31 at almost represents "now". We believe that the Earth was formed about 4.6 billion years ago. What date on our compressed calendar corresponds to the formation of the Earth?

September 2

Shapley's Globular Cluster Distribution

Shapley estimated the distance from the Sun to the center of the Milky Way at 28,000 l-y (8.5 kpc). Recent refinement of the distance puts the value at 27,000 l-y (8.3 kpc). (A) Shapley's distribution of the globular clusters compared to Hershel's version of the Milky Way. (B) The center of the Milky Way inferred by Shapley is marked by the blue X.

Superbubbles

Shockwaves from several closely positioned supernovae can create a superbubble that can escape from the galaxy's disk through a blowout. The galactic fountain model states that fountains of hot gas rise from the disk into the halo through elongated bubbles carved by blowouts. This model is difficult to verify at present. A 30-million-year-old superbubble located about 22.8 kl-y (7 kpc) from the Sun and 13.0 kl-y (4 kpc) from the galactic center. Green color is neutral hydrogen detected by its 21-cm radio emission, purple is ionized hydrogen detected by its optical emission, white is where there are matching amounts of both neutral and ionized hydrogen. Such superbubbles are known to be blown by powerful stellar winds and supernovae occurring in star clusters in arms of both our and other spiral galaxies.

What is the brightest star in the night sky?

Sirius

Which of the following is an important starting assumption in models of galaxy formation? Black holes form first, seeding the formation of galaxies. Some regions in the universe start out denser than others. All galaxies start out as spiral galaxies. Galaxies form first, then black holes.

Some regions in the universe start out denser than others. These small density enhancements are the seeds around which galaxies form; without these "seeds," models indicate that galaxies could not yet have formed in a 14-billion-year-old universe.

Spiral Arms

Spiral arms are most likely caused by huge spiral density waves that cause gas, dust, and stars to bunch up into long-term patterns (the spiral arms) Remember that these waves, like ocean waves, move through matter without carrying matter with it. Spiral density waves are caused by a gravitational disturbance, most likely from a passing galaxy. (A) A spiral density wave can explain the existence of galactic spiral arms. (B) Spiral arms are visible in this image of galaxy M81 that combines data from the Hubble, Spitzer, and GALEX space telescopes. M81 is 11.8 million l-y (3.6 Mpc) distant in Ursa Major.

Spiral Density Wave Analogy

Spiral density waves are like traffic jams. Clouds and stars speed up to the density wave (are accelerated toward it) and are tugged backward as they leave, so they accumulate in the density wave (like cars bunching up behind a slower-moving vehicle). Clouds compress and form stars in the density wave, but only the fainter stars live long enough to make it out of the wave.

General Shape of Spiral Galaxies

Spiral galaxies are shaped a bit like a fried egg. The disk is thin like the white of the egg. The galaxy's nucleus or bulge is like the yolk. (A) Fried eggs are an analog of the shapes of spiral galaxies. (B) NGC 4565 (the Needle Galaxy), is an edge-on spiral galaxy in the Constellation Coma Berenices, 31 million l-y distant (9.5 MPC), with a length of 125,000 l-y (38.3)

We can confirm that hot, bright main sequence stars evolve more rapidly than cool, dimmer main sequence stars by studying

Star clusters

Star-Gas-Star Cycle

Star-gas-star cycle: involves the following process: Hot, ionized gas from exploding stars cools first to clouds of atomic hydrogen (H) and then cools further to clouds of molecular hydrogen (2). These molecular clouds then can contract to form new stars more highly enriched in heavy elements. Massive versions of these new stars will age and explode, beginning the process once again. All Stars return much of their mass to the interstellar medium via: Stellar winds that blow throughout their lives. "Death events" of planetary nebulae or supernovae. The Helix Nebula (NGC 7293) is a planetary nebula in the constellation Aquarius about 690 l-y (211.4 pc) distant. The nebula is about 5.6 l-y (1.7 pc) in diameter.

Stars Orbit the Galaxy

Stars in the disk all orbit in the same direction with a little-up-and-down motion Stars orbit in the bulge and halo and halo have random orientations The Sun's orbital motion (radius and velocity) tells us mass of the galaxy within the Sun's orbit: 1.0 x 10<11M (sun symbol) The Sun is 27,000 l-y from the center of the galaxy and takes 230 million years to complete one orbit. Sun orbits the galactic center in a clockwise motion

Summary of Galactic Recycling

Stars make new elements by fusion. Dying stars expel gas and new elements, producing hot bubbles (~10<6K) Hot gas cools, allowing atomic hydrogen clouds to form (~100-10,000K) Further cooling permits molecules to form, making molecular clouds (~30K) Gravity forms new stars (and planets in molecular clouds)

Galaxy's Core

Stars seen swooping toward a small central region supports conclusion that a supermassive black hole exists at our galaxy's core.

If we could watch spiral arms from a telescope situated above the Milky Way over 500 million years, what would we see happen?

Stars will move through the spiral arms, bunching up closer as they pass through. Young hot stars will form and die within the arms before having a chance to move out.

M87's Prominent Jet

Streaming out from the center of galaxy M87 like a cosmic searchlight is a black-hole-powered jet of electrons and other sub-atomic particles traveling at nearly the speed of light. In this Hubble image, the blue jet contrasts with the yellow glow from the combined light of billions of unseen stars that make up this galaxy. Lying at the center of M87, the supermassive black hole has a mass of 6.6 billion solar masses. M87 is 53.5 million l-y (16.4 Mpc) distant in the constellation Virgo. This jet of energetic plasma originates at the core and extends outward at least 4,900 l-y (1,500 pc), traveling at relativistic speed

Supernova Remnants (SNR)

Supernovae generate shock waves. Observing a supernova remnant we see the aftermath of its shock wave. The supernova shock wave can also accelerate electrons near to the speed of light, creating a radio emission called synchrotron radiation. Supernovae can also generate cosmic rays, which are composed of electrons, protons, and atomic nuclei that are accelerated nearly to the speed of light. The Pencil Nebula supernova shockwave is part of the Vela supernova remnant.

Suppose a scientist holds a press conference at which he claims that 10% of the matter in the Milky Way is in the form of dust grains. Does his claim seem reasonable? Why or why not?

The 10% figure is too high because there are not enough heavy elements to make that much dust.

Suppose you want to observe and study the radiation from gas inside an interstellar bubble created by a supernova. Which of the following observatories will be most useful?

The Chandra X-ray Observatory

Although the entire universe may be much larger than our observable universe, we can see only within our observable universe. The "boundary" of our observable universe is called _________. the Big Bang the cosmological horizon the Hubble Deep Field the lookback time

The Cosmological Horizon It is a horizon in time, not in space. That is, there is no physical boundary at the cosmological horizon, but we cannot see beyond it because we'd be trying to look to a time before the universe was born. (See Figure 1.3 of The Essential Cosmic Perspective.)

The Galactic Center

The ESO 3.6-m telescope at La Silla, during observations, with the telescope's dome lit by the Moon. Across the sky is the plane of the Milky Way. Above the telescope dome, and partially hidden behind dark interstellar dust clouds, is the prominent yellowish central bulge of the Milky Way. By following the dark lane which seems to grow from the center of the Galaxy toward the top, we find the reddish nebula around Antares (Alpha Scorpii). The Galactic Center itself lies in the constellation of Sagittarius and reaches its maximum visibility during the southern winter season.

Which of the following best expresses the "Copernican principle"?

The Earth is not in a special place; it is just one of a number of planets orbiting the Sun.

A computer accessory salesman attempts to convince you to purchase a "solar neutrino shield" for your new computer. Why do you turn down this offer?

The Earth's natural magnetic field already offered excellent protection against the onslaught of solar neutrinos.

What would happen to the orbit of the Earth if the Sun somehow became a black hole

The Earth's orbit would be unchanged

Dark Nebula

The Horsehead Nebula (IC 434 or Barnard 33) in Orion is a dark nebula backlighted by an emission nebula. The shape was first noticed in 1888 by Williamina Fleming on a photographic plate. The nebula is 1,500 l-y (0.46 kpc) distant. Part of the vast Orion complex.

Horsehead in the Infrared

The Horsehead Nebula (also known at Bernard 33) imaged in the infrared by the Hubble Space Telescope in 2013. The backlit wisps along the Horsehead's upper ridge are being illuminated by Sigma Orionis, a young five-star system just off the top of the image. A harsh ultraviolet glare from one of these bright stars is slowly evaporating the nebula. Along the nebula's top ridge, two fledgling stars peek out from their now-exposed nurseries. Gas clouds surrounding the Horsehead have already dissipated, but the tip of the jutting pillar contains a slightly higher density of hydrogen and helium, laced with dust. This casts a shadow that protects material behind it from being photo-evaporated, and a pillar structure forms. Astronomers estimate that the Horsehead formation has about five million years left before it too disintegrates.

Milky Way Part 2

The LMC and SMC are companion galaxies that orbit the MWG at distances of ≈150,000 l-y (46 kpc) Another small galaxy—Sagittarius dwarf elliptical (SgrDEG)—lies even closer but is obscured from view by the MW's galactic plane. The interstellar medium (IM)—clouds of gas and dust—fill the galactic disk, obscuring our view in visible light. Because of the IM, astronomers were long fooled into thinking that our solar system was located near our galaxy's center. (A) The Sagittarius Dwarf Elliptical Galaxy is slowly being consumed by the Milky Way Galaxy. (Patrick Cseresnjes, l'Observatory de Paris) (B) The SgrDEG is being ripped by tidal forces into long streams of stars that will eventually be merged into the Milky Way Galaxy.

Close-up Local Bubble

The Local Bubble is a cavity in the interstellar medium (ISM) of the Orion Arm of the Milky Way. It is at least 300 light-years (91.9 pc) across. The Solar System has been traveling through the Local Bubble for the last 3 million years. Most astronomers believe that the Local Bubble was formed by nearby supernovae that pushed aside gas and dust in the local ISM, leaving behind hot, low-density material. This is an artist's rendering.

Milky Way's Disk is Warped

The Magellanic Clouds, the Milky Way's closet satellites, appear to be interacting with our galaxy's dark matter to create a mysterious warp in the galactic disk. The warp is seen most clearly in the thin disk of hydrogen gas permeating the galaxy. The motion of the Magellanic Clouds through the dark matter creates a wake that enhances their gravitational influence on the disk. (A) This computer simulation video shows how the Magellanic Clouds (yellow bead) produce the warp observed in the hydrogen gas layer of the Milky Way Galaxy, which is in the middle. The position of the Sun is about half way out in the picture of the galaxy along the line marked X. The cross-hatched area represents the warped hydrogen layer at the present time. The looping line is the orbit of the Magellanic Clouds and the position of the bead on the line represents the location of the clouds at the present time. The orbital period is about 1.5 billion years. (Martin Weinberg-UMass/Leo Blitz-UC Berkeley) (B) The warp of spiral galaxy ESO 510-13's disk is likely similar to that discovered in the Milky Way Galaxy.

The Milky Way Galaxy

The Milky Way galaxy appears in our sky as a faint band of light. Dusty gas clouds obscure our view because they absorb visible light. This is the interstellar medium that makes new star systems.

MWG Look-Alike Systems

The Milky Way is a fairly typical galaxy on its own, but when paired with its close neighbors—the Magellanic Clouds—it is very rare, and could have been one of a kind, until a survey found another two examples just like it. Researchers working with the International Center for Radio Astronomy Research (ICRAR) searched for groups of galaxies similar to the MWG in the most detailed map of the local universe yet, the Galaxy and Mass Assembly survey (GAMA). This image shows one of the two "exact matches" to the Milky Way system found in the survey. The larger galaxy, denoted GAMA202627, which is similar to the Milky Way, clearly has two large companions off to the bottom left of the image. In this image, bluer colors indicate hotter, younger, stars like many of those that are found in our galaxy. The survey found about 3% of galaxies similar to the Milky Way have companion galaxies like the Magellanic Clouds, so our system is quite rare.

What do we mean when we say that the Moon's phase is "waxing"?

The Moon is becoming more illuminated from day to day

Milky Way has 4 Spiral Arms

The Sun is near the inner rim of the Orion Arm, within the Local Fluff of the Local Bubble, and in the Gould Belt, at a distance of 26.4 ± 1.0 kl-y (8.09 ± 0.31 kpc) from the galactic center. The Sun is currently 5-30 pc (16-98 l-y) from the central plane of the galactic disk. The distance between the local arm and the next arm out, the Perseus Arm, is about 2,000 pc (6,500 l-y). The Sun, and thus the solar system, is located in the Milky Way's galactic habitable zone. In late 2013, a 12-year study published in the Monthly Notices of the Royal Astronomical Society has confirmed that the Milky Way Galaxy has four main spiral arms, following years of debate that it has only two arms. This survey of ~1,650 young, massive stars (red dots) throughout our galaxy reveals that the Milky Way has four prominent arms after all—not just two (Perseus and Scutum-Centaurus), as earlier observations had suggested. In a follow-up study published in 2015, a Brazilian team used star clusters embedded in their natal clouds to trace our galaxy's structure. Their results indicate a four-armed spiral galaxy that includes the Sagittarius-Carina, Perseus, and Outer arms. Though this second study reinforces the 4-arm model, further study will follow on this matter. Galactic distribution of massive young stars and compact and ultra-compact H II regions with luminosities greater than 104 times that of the Sun. The map shows the positions of the complexes and individual sources as red and blue circles, respectively. The sizes of the markers give an indication of their luminosity, as depicted in the upper-right corner. The position of the Sun is shown by the small circle above the Galactic Center. The two solid lines enclose the Galactic Center region that was excluded from survey due to problems with source confusion and distance determination. The smaller of the two dot-dashed circles represents the locus of tangent points, while the larger circle shows the radius of the solar circle.

Based on current evidence, how does the actual density of matter in the universe compare to the critical density?

The actual average density of matter, even with dark matter included, is only about a quarter of the critical density.

What do we mean by the term "absolute visual magnitude"?

The apparent visual magnitude a star would have if its distance were 10 parsecs

What was the significance of the end of the era of nucleosynthesis, when the universe was about 5 minutes old?

The basic chemical composition of the universe had been determined.

This image shows a colliding pair of galaxy clusters known together as the Bullet Cluster. The blue region represents a map of the cluster's dark matter. How was this blue map made?

The blue region was inferred from studies of how the cluster causes gravitational lensing of objects located behind it.

This photograph shows an interstellar bubble about 10 light-years in diameter. If you could photograph this same region about 100 years from now, how would you expect it to look different?

The bubble will be slightly larger.

What happens when a particle of matter meets its corresponding antiparticle of antimatter?

The combined mass of the two particles is completely transformed into energy (photons).

Active Galaxy M87

The core of the active galaxy M87 is seen to have a disk of hot gas moving very quickly around the center. Doppler shifts of the disk material close to the center show that the gas is moving at speeds of hundreds of kilometers per second. Blueshifted lines are produced from one part of the disk and redshifted lines are produced from the opposite part of the disk. This is clear proof that the disk is rotating. The speed and distance the gas is from the center show that the central object must have a mass of 2.5 billion solar masses. Only a black hole could be this massive and compact. The jet coming from the nucleus (visible in the wider-field view at right in the next slide) is also seen to be perpendicular to the plane of the disk. Visible light image of active galaxy M87. This large elliptical galaxy is 53.5 million light-years (16.4 Mpc) from Earth in Virgo .

This graph shows how the average distances between galaxies changes with time in the four models for the universe, and also shows data points measured from white dwarf supernovae. Which of the following statements best describes what the data are telling us?

The data indicate that we live in an accelerating universe.

Suppose that Hubble's constant were 11 km/s/million light years instead of 22 km/s/million light years. How would this graph look different?

The data would follow a more shallow line, closer to horizontal than the current line.

The Big Bang theory seems to explain how elements were formed during the first few minutes after the Big Bang. Which hypothetical observation below (these are not real observations) would call our current theory into question?

The discovery of a galaxy with a helium abundance of only 10% by mass.

The Big Bang theory seems to explain how elements were formed during the first few minutes after the Big Bang. Which hypothetical observation that follows (these are not real observations) would call our current theory into question?

The discovery of a galaxy with a helium abundance of only 10% by mass.

All-Sky View of the Milky Way Galaxy

The dust lanes can obscure some features. Infrared imaging reaches into these regions, and radio astronomy can look all the way through with less detail. The very center, shows a window to the farther side. In the center, stars are mostly very old, and this causes more of a yellow color.

Study this diagram that summarizes the eras of the universe. Which of the following statements is not true?

The electroweak era was the longest lasting of all the eras in the history of the universe.

Why do scientists assume that the early universe was much hotter and denser than the universe of today?

The fact that the universe is expanding implies that objects were closer together in the past, and compressing material makes it hotter and denser.

If our year were twice as long (that is, if Earth took twice as many days to complete each orbit around the Sun), but Earth's rotation period and axis tilt were unchanged, then

The four seasons would be twice as long as they were now

Imagine that it turns out that dark matter (not dark energy) is made up of an unstable form of matter and that all of it suddenly decays tomorrow into photons or other forms of energy. Based on current understanding, which of the following would begin to occur?

The galaxies in clusters would begin to fly apart.

If we took a spectrum of glowing gas in between the stars of the Orion Nebula, what kind of spectrum would we obtain?

The graph with the vertical lines

When we use an analogy that represents the expanding universe with the surface of an expanding balloon, what does the inside of the balloon represent

The inside of the balloon does not represent any part of our universe. right answer feedback: The surface of the balloon represents all three dimensions of space in our universe.

Spiral Arms and Star Formation

The majority of star formation in the disk happens in spiral arms. Spiral arms are waves of star formation. M51, the Whirlpool Galaxy, is located in the constellation Canes Venatici (the Hunting Dogs), about 23 million l-y (7.0 Mpc) distant. This extremely detailed image shows the presence of star-forming clouds, nebula, and young stars in its spiral arms.

Study this graph, focusing on the red curve and reddish horizontal swath. Which statement below correctly interprets what the graph shows?

The measured abundance of deuterium agrees with the theoretically predicted abundance only if we make the prediction with a model of the universe in which ordinary matter makes up 4% of the critical density.

What does Hubble's law tell us? he faster a spiral galaxy's rotation speed, the more luminous it is. The longer the period of a Cepheid variable, the greater its luminosity. For every force, there is an equal and opposite reaction force. The more distant a galaxy, the faster it is moving away from us.

The more distant a galaxy, the faster it is moving away from us. right answer feedback: From this fact, we infer that we live in an expanding universe. Question

Star Motions in the MWG

The motions of 14,000 stars studied during a 15-year monitoring program are shown making their most recent orbital revolution around the galactic center before converging into the small volume where they were observed. The duration of the animation corresponds to about 250 million years. The yellow dot and white curve show how the Sun moved during this last of its approximately 20 laps around our galaxy since its birth.

Which of the following best explains why the night sky is dark?

The observable universe is not infinite.

This photo shows the central region of a cluster of galaxies. Based on what you have learned from your textbook, which of the following statements about this photo is not true?

The oldest stars in these galaxies are much older than the oldest stars in our own Milky Way.

Orbital Speed Inside and Outside of a Mass Distribution

The orbital speed of an object-such as a star orbiting the central core of a galaxy- is a measure of the amount and distribution of the mass that is attracting it gravitationally. Assume the galaxy is a sphere of uniform density, p, which is diffuse enough to permit the orbiting of a mass, m, within the radius, of the total mass distribution. In this idealized case, the inverse-square law of gravity and the centripetal force relationship can be used to calculate the orbital velocity for a circular orbit within the galaxy. Conceptually, this calculation works because the orbiting mass experiences net attraction only by that mass inside its orbit (ri); the mass outside its orbit (r circle) exerts a zero net force on the orbiting object. (slide 42)

What evidence suggests that most of the mass of the Milky Way is in the form of dark matter?

The orbital speeds of stars far from the galactic center are surprisingly high.

Spiral galaxy rotation curves are generally fairly flat out to large distances. Suppose that spiral galaxies did not contain dark matter. How would their rotation curves be different?

The orbital speeds would fall off sharply with increasing distance from the galactic center.

What is the best evidence for an extremely massive black hole in the center of the Milky Way?

The orbits of stars in the center of the galaxy indicate that the presence of an approximately 4-million-solar-mass object in a region no larger than our solar system.

Star Motions at Galaxy Center

The orbits of stars within the central 1.0 X 1.0 arcseconds of our galaxy. In the background, the central portion of a diffraction-limited image taken in 2004 is displayed. While every star in this image has been seen to move over the past 9 years, estimates of orbital parameters are only possible for the seven stars that have had significant curvature detected. The annual average positions for these seven stars are plotted as colored dots, which have increasing color saturation with time. Also plotted are the best fitting simultaneous orbital solutions. These orbits provide the best evidence yet for a supermassive black hole, which has a mass of 3.7 million times the mass of the Sun.

Added Star Motions Confirm SMBH at Galaxy Center

The orbits of stars within the central arcsecond of our galaxy. In the background, the central portion of a diffraction-limited image taken in 2012 is displayed. The orbits have been inferred from images taken with the primitive technique of speckle imaging (1995-2005) and with the more sophisticated adaptive optics (2005-2012). While several stars can be seen in their motions through this region, only two stars—S0-2 and the newly discovered S0-102—have been traced through a complete orbit. They are the most tightly bound to the black hole and therefore comprise the most information about it. S0-2, which has an orbital period of 16 years, proved the existence of a black hole. The addition of S0-102, with a period of 11.5 years, will for the first time allow us to test the warping of space and time this close to a black hole. Stars that have been observed through at least one turning point in their orbit are shown in blue.

What two observable properties of a Cepheid variable are directly related to one another? Its mass and its distance Its luminosity and its mass The period between its peaks of brightness and its distance The period between its peaks of brightness and its luminosity

The period between its peaks of brightness and its luminosity right answer feedback: The period-luminosity relation allows us to determine its luminosity from the period between its peaks of brightness.

Which of the following is at a maximum when a planet is closet to the Sun?

The planet's kinetic energy.

Which of the following phenomena is probably not related to the presence of a supermassive black hole? Quasars The presence of globular clusters in the halos of galaxies The radio emission from radio galaxies The huge jets seen emerging from the centers of some galaxies

The presence of globular clusters in the halos of galaxies

Notice the well-defined spiral arms in this photograph of the galaxy M51. What makes the spiral arms so much brighter than regions between the arms?

The presence of many massive young stars in the spiral arms.

This a radio image of the galaxy Cygnus A, with the visible image of the galaxy shown in the inset. The likely explanation for the two huge clumps of radio emission far to either side of the visible galaxy is:

The radio emission comes from gas shot out into space by jets powered by a supermassive black hole in the galactic center.

Sagittarius A

The size of Sagittarius A*, a region of radio emissions around the Milky Way's central black hole, is shown here compared to the orbit of Earth. The expected shadow of the black hole, which researchers hope to spot one day, is also shown for size comparison. The intrinsic size of Sagittarius A* as measured with the VLBA at 43 GHz and compared to the expected visible size of the event horizon (the black hole's shadow). With a diameter of 2 AU, the radio source would just fit inside the Earth's orbit. For an observer on Earth, the event horizon would look 40 times bigger than the Sun in the sky if the black hole were at the Sun's position.

OB Star-Forming Region

The star cluster Cygnus OB2 contains more than 60 O-type stars and about 1,000 B-type stars. At a relatively nearby distance to Earth of about 4,700 l-y (1.44 kpc), Cygnus OB2 is the closest massive cluster. The region shown in the image spans about 16 l-y (4.9 pc). Young stars ranging in age from 1-7 million years were detected. The infrared data indicate that a very low fraction of the stars have circumstellar disks of dust and gas because the intense radiation of these stars corrodes and quickly destroys their disks. In this image, X-rays from Chandra (blue) have been combined with infrared data from NASA's Spitzer Space Telescope (red) and optical data from the Isaac Newton Telescope (orange).

Star-Gas-Star Cycle Layout

The star-gas-star cycle (sometimes gas-star-gas) recycles gas from old stars into new star systems. 1. H II Regions: hot bubbles of ionized H (H+) 2. H I Regions: atomic hydrogen (H) clouds 3. Cool molecular clouds of H2 4. Stars form in molecular clouds 5.Stars live and create heavier elements via nuclear fusion 6. Stars die and gas returns via supernovae and stellar winds

How did star formation likely proceed in the protogalactic cloud that formed the Milky Way?

The stars that formed first could orbit the center of the galaxy in any direction at any inclination.

Why does the temperature of the gas between galaxies in galaxy clusters tell us about the mass of the cluster?

The temperature tells us the average speeds of the gas particles, which are held in the cluster by gravity, so we can use these speeds to determine the cluster mass.

If all the planets lined up along the Earth-Sun line

The tides on Earth would be almost unchanged, as a planet's tidal force on the Earth is tens of thousands or hundreds of thousands of times weaker than the Sun's tidal force.

How should we expect the Milky Way's interstellar medium to be different in 50 billion years than it is today?

The total amount of gas will be much less than it is today.

Which of the following statements explains why the night sky is dark?

The universe has a finite age.

Heart of the Milky Way

The very colorful Rho Ophiuchi and Antares region features prominently to the right, as well as much darker areas, such as the Pipe and Snake Nebulae. The dusty lane of our Milky Way runs obliquely through the image, dotted with remarkable bright, reddish nebulae, such as the Lagoon and the Trifid Nebulae, as well as NGC 6357 and NGC 6334. This dark lane also hosts the very center of our galaxy, where a supermassive black hole is lurking.

This graph shows the rotation curves of four different spiral galaxies. Based on these curves, what do all four galaxies have in common?

Their most distant stars all orbit at about the same speed as stars located about 30,000 light-years from their centers.

Which of the following is not implied by this diagram?

There are six distinct forces known to operate in the universe, and a seventh force called the "super force" might also exist.

Scientists do not know what dark energy is, yet they still claim that it is the dominant constituent of the mass-energy of the universe. What (if any) evidence supports this claim?

There are two lines of evidence: measurements of the accelerating expansion and the universal geometry inferred from the cosmic microwave background.

Although most astronomers assume dark matter really exists, there is at least one other possible explanation for the phenomena attributed to dark matter. What is it?

There could be something wrong or incomplete with our understanding of how gravity operates on galaxy-size scales.

Which of the following statements best explains what we mean when we say that the electroweak and strong forces 'froze out' at 10^-38 second after the Big Bang?

These two forces first became distinct at this time.

Given that white dwarf supernovae are such good standard candles, why don't we use them to measure the distance to all galaxies? We would, but we don't have enough telescopes. They are rare events, so we have observed them in only a tiny fraction of all galaxies. They can occur only in spiral galaxies, not elliptical galaxies. We cannot see them beyond a distance of about 100 million light-years

They are rare events, so we have observed them in only a tiny fraction of all galaxies. right answer feedback: A white dwarf supernova may occur on average only every few centuries in a particular galaxy, so we have to be "lucky" to have caught one in action in that galaxy during the time we have been monitoring galaxies with large telescopes. Of course, by monitoring thousands or millions of galaxies, we'll catch a number of them every year.

If WIMPs really exist and make up most of the dark matter in galaxies, which of the following is not one of their characteristics?

They travel at speeds close to the speed of light.

The ancient Greeks get a lot of attention for their contributions to science because

They were the first people known to try to explain nature with models based on reason and mathematics, without resorting to the supernatural.

Flares and Falling Asteroids

This Chandra image shows the center of our galaxy, with a supermassive black hole known as Sagittarius A* (Sgr A*) in the center. Chandra has detected X-ray flares about once a day from Sgr A*. A possible explanation for the mysterious flares is that there is a cloud around Sgr A* that contains hundreds of trillions of asteroids and comets, which have been stripped from their parent stars. The panel on the left is a composite image of the region around the black hole, with red representing low-energy X-rays, green as medium-energy X-rays, and blue the highest. An asteroid that undergoes a close encounter with another object, such as a star or planet, can be thrown into an orbit headed towards Sgr A*. (See artist's illustrations at right.) If the asteroid passes within about 160 million km (100 million mi) of the black hole, it would be torn into pieces by the black hole's tidal forces (middle-right panel). These fragments would then be vaporized by friction as they pass through the hot, thin gas flowing onto Sgr A*, similar to a meteor burning up as it falls through Earth's atmosphere. A flare is produced (bottom-right panel), and eventually the remains of the asteroid are swallowed by the black hole.

Supermassive Black Hole in M31

This artist's concept shows a view across a mysterious disk of young, blue stars encircling a supermassive black hole at the core of the Andromeda Galaxy (M31). The region around the black hole is barely visible at the center of the disk. The background stars are the typical older, redder population of stars that inhabit the cores of most galaxies. Spectroscopic observations by the Hubble Space Telescope reveal that the blue light consists of more than 400 stars that formed in a burst of activity about 200 million years ago. The stars are tightly packed in a disk that is only a light-year across. Under the black hole's gravitational grip, the stars are traveling very fast: 3.6 million kilometers an hour or 1,000 km/s (2.2 million miles an hour or 611 mi/s).

The Mysterious Galactic Center

Though the center is obscured from us in visible light by dust and gas, we can probe the galactic center using infrared, X-Ray, and radio telescopes About 1,000 l-y (0.31 kpc) from the center, we detect a turbulent region of gas clouds and a cluster of several million closely packed stars In 2002, astronomers saw stars swooping toward a small central region containing 3-4 million solar masses. Most probably this region-no bigger than our solar system- is a supermassive black hole.

Why are radio telescopes (or arrays of radio telescopes) extremely large compared to optical telescopes?

To achieve comparable resolution to the best optical images.

True or False these following features are apart of centeal dominant galaxies They're found in clusters of galaxies. They are thought to form by the merger of several smaller galaxies. They often have multiple galactic nuclei near their centers.

True

According to the Big Bang theory, how many forces -- and which ones -- operated in the universe during the GUT era?

Two forces: gravity and a single force that later became the strong, weak, and electromagnetic forces.

This diagram shows several stages in a computer simulation of a collision between two galaxies. What happens?

Two spiral galaxies merge to become an elliptical galaxy surrounded by debris.

Supermassive Black Holes

Two supermassive black holes spiral towards each other in galaxy cluster Abell 400, which is 326 million l-y (100 Mpc) distant in Cetus.

Cloud Erosion by Radiation

Ultraviolet radiation from O-type stars can disrupt and erode dense knots of dust and gas known as Bok globules. IC 2944, also known as the Running Chicken Nebula or the Lambda Cen Nebula, is an open cluster with an associated emission nebula found in the constellation Centaurus, near the star Lambda Centauri. 6,500 l-y (2 kpc) distant. It features Bok globules, which are frequently a site of active star formation. This Hubble Space Telescope image is a close up of a set of Bok globules discovered in IC 2944 by South African astronomer A. David Thackeray in 1950. These globules are now known as Thackeray's Globules. It is likely that the globules are dense clumps of gas and dust that existed before the massive O-stars were born. But once these luminous stars began to irradiate and destroy their surroundings, the clumps became visible when their less dense surroundings were eroded away, thus exposing them to the full brunt of the ultraviolet radiation and the expanding H II region. Had the appearance of the luminous O-stars been a bit delayed, it is likely that the clumps would actually have collapsed to form several more low-mass stars like the Sun. Instead they are now being toasted and torn apart.

How does gravitational lensing tell us about the mass of a galaxy cluster?

Using Einstein's general theory of relativity, we can calculate the cluster's mass from the precise way in which it distorts the light of galaxies behind it.

Updated Mass Limit for MWG

Using the motions of distant stars, astronomers have made a new determination of the Milky Way's mass. They measured the motions of 2,400 "blue horizontal branch" stars in the extended stellar halo that surrounds the disk of the galaxy. An earlier study using a much smaller data set gave an implied mass of the galaxy up to 2 trillion solar masses. In contrast, when the new measurement within 180,000 l-y (55.2 kpc) is corrected to a total-mass measurement, it yields a value slightly under 1 trillion solar masses.

This photograph shows X-ray emission from a supernova remnant. What is the source of the X-rays?

Very hot gas expanding outward from the site of a supernova.

MWG at Visible Wavelengths

Visible wavelengths (nm) continue to play an important role in astronomy 400 years after Galileo first pointed a telescope at the night sky. Imaging at optical wavelengths began in the late 19th century when photographic cameras were attached to telescopes. Optical (visible)= 652 nm (0.000652 mm or 6.52 x 10<-7 m) / f= 460 x 10<3 GHz. Visible light emitted by stars is scattered and absorbed by dust.

Name the 3 important greenhouse gases?

Water Vapor, Nitrous Oxide, Methane

How do we know that galaxy clusters contain a lot of mass in the form of hot gas that fills spaces between individual galaxies?

We detect this gas with X-ray telescopes.

Why can't current theories describe what happened during the Planck era?

We do not yet have a theory that links quantum mechanics and general relativity.

Schematic of the MWG

We see our galaxy edge-on Primary features: disk, bulge, halo, globular clusters Note that the Sun (and the Solar System) are a considerable distance from the galactic center The galactic north pole (GNP) and south pole (GSP) are defined by the Sun's position.

How do we determine the conditions that existed in the very early universe?

We work backward from current conditions to calculate what temperatures and densities must have been when the observable universe was much smaller in size.

Evidence for the Shape of the MWG

When neutral and ionized hydrogen regions of the Milky Way are plotted, we get a picture of the spiral structure. New information also indicates that the MWG is a barred spiral. Milky Way's spiral arms marked in red. The color shading indicates the reconstructed gas density that researchers used to map the galaxy's spiral arms. In addition to the two main spiral arms in the inner galaxy, two weaker arms exist. These arms end about 10,000 light-years (3.06 kpc) from the galaxy's center.

MWG at Short Wavelengths

X-ray and gamma-ray wavelengths (nm) permit astronomers to study the most energetic processes in the universe. Such short wavelengths can only be observed from above the Earth's atmosphere. Ultraviolet: 200-115 nm (0.2-0.115 micron or 2 x 10<-7 to 1.15 x 10<-7 m)/ f= 1.5 x10<6 to 2.6 x 10<6 GHz Come from massive (hot) stars, active galactic nuclei (AGN), and supernova X-Ray: 8.3-5nm (0.0083-0.005 micron or 8.3 x 10<-9 to 5 x 10<-9m)/ f=36 x 10<6 to 60 x 10<6 GHz) X-Ray emissions come from hot gas bubbles (diffuse blobs) and X-Ray binaries (point-like sources) Gamma-Ray: <4.16 nm (<4.16 x 10<-3 micron or 4.16 x 10<-6mm or 4.16 x 10<-9 m)/ f= >72 x 10<6 GHz Gamma-ray emissions come from collisions of cosmic rays with atomic nuclei in interstellar clouds.

The flat rotation curves of spiral galaxies tell us that they contain a lot of dark matter. Do they tell us anything about where the dark matter is located within the galaxy?

Yes, they tell us that dark matter is spread throughout the galaxy, with most located at large distances form the galactic center.

A photograph of a cluster of galaxies shows distorted images of galaxies that lie behind it at greater distances. This is an example of what astronomers call:

a gravitational lens.

This figure shows a "slice of the universe" from the Sloan Digital Sky Survey. What is the Sloan Great Wall (indicated by the arrow)?

a huge collection of galaxies extending a billion light-years in length

Which of these galaxies would you most likely find at the center of a large cluster of galaxies?

a large elliptical galaxy

The distinguishing feature of a starburst galaxy is _________ a very large luminosity compared to the total luminosity of the Milky Way a rate of star formation that may be 100 or more times greater than that in the Milky Way the presence of an unusually large number of binary star systems containing X-ray bursters strong radio emission from "lobes" of material well outside the visible boundaries of the galaxy

a rate of star formation that may be 100 or more times greater than that in the Milky Way The term starburst refers to a burst of star formation

Which of the following possible types of universe would not expand forever?

a recollapsing universe

What do we mean by the term "scientific paradigm"?

a scientific idea accepted as true by a large majority of members of a scientific community.

The theory of evolution is

a scientific theory, meaning that it is backed by extensive evidence.

What type of star makes a Type II supernova?

a single massive star

What do we mean by inflation?

a sudden and extremely rapid expansion of the universe that occurred in a tiny fraction of a second during the universe's first second of existence

The white arrow points to an object that lies within the disk of the galaxy shown. What is this object?

a supernova

Which kind of object is the best standard candle for measuring distances to extremely distant galaxies?

a white dwarf supernova

When we say that a cluster of galaxies is acting as a gravitational lens, what do we mean? a. It bends or distorts the light coming from galaxies located behind it. b. It is an unusually large cluster that has a lot of gravity. c. It magnifies the effects of gravity that we see in the cluster. d. The overall shape of the cluster is that of a lens.

a. It bends or distorts the light coming from galaxies located behind it.

Which of the following best summarizes what we mean by dark energy? a. It is a name given to whatever is causing the expansion of the universe to accelerate with time. b. It is a type of energy that is associated with the "dark side" of The Force that rules the cosmos. c. It is the energy contained in dark matter. d. It is the energy of black holes.

a. It is a name given to whatever is causing the expansion of the universe to accelerate with time.

This figure shows an all-sky map of the cosmic microwave background recorded by the WMAP telescope. How long did this light travel through space before it reached the telescope?

about 14 billion years

If you looked at the field of view seen in this Hubble Space telescope with your naked eye, about how big would it appear in the sky?

about the size of a grain of sand held at arm's length against the sky

The bright object in this photo is a quasar in the center of a distant galaxy. According to current understanding, about how big is the source of the bright light?

about the size of our solar system

These diagrams represent four possible models for the universe. Each model shows how the size of the observable universe changes with time. Of the four models, which one gives the universe the oldest age at present?

accelerating universe

These diagrams represent four possible models for the universe. Which model presumes the existence of some type of dark energy in the universe?

accelerating universe

The unusually bright centers found in some galaxies are called _________.

active galactic nuclei The power source for active galactic nuclei is thought to be accretion onto a supermassive black hole

Models of the Big Bang that include inflation predict that the overall geometry of the universe should be "flat" (in spacetime). This prediction __________.

agrees with data found by studying the cosmic microwave background

If we could see our own galaxy from 2 million light-years away, it would appear _________.

as a flattened disk with a central bulge and spiral arms, spanning a few degrees across the sky

If you build a radio telescope and tune its receiver to a wavelength of 21 centimeters, what might you detect in interstellar space?

atomic hydrogen about 50 degrees about zero.

The most common form of gas in the disk of the Milky Way galaxy is _________.

atomic hydrogen gas

Which of the following best sums up current scientific thinking about the nature of dark energy? a. Dark energy is most likely made up of weakly interacting particles that do not interact with light. b. Dark energy probably exists, but we have little (if any) idea what it is. c. Dark energy most likely consists of a form of photons that we can't see or detect. d. Dark energy is the source of the mind weapon used by Sith Lords in Star Wars.

b. Dark energy probably exists, but we have little (if any) idea what it is.

The text states that luminous matter in the Milky Way seems to be much like the tip of an iceberg. This refers to the idea that __________. a. luminous matter emits white light, much like the light reflected from icebergs b. dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way c. the luminous matter of the Milky Way is essentially floating on the surface of a great sea of dark matter d. black holes are much more interesting than ordinary stars that give off light

b. dark matter represents much more mass and extends much further from the galactic center than the visible stars of the Milky Way

According to current understanding, if the universe continues to expand forever, the last major source of light will come from __________. a. collisions between galaxies b. evaporation of black holes c. the last supernovas d. gamma-ray bursts

b. evaporation of black holes

Measuring the amount of deuterium in the universe allows us to set a limit on __________. a. the current age of the universe b. the density of ordinary (baryonic) matter the universe c. the acceleration of the universe d. the total amount of mass in the universe

b. the density of ordinary (baryonic) matter the universe

Although we know less about dark matter in elliptical galaxies than in spiral galaxies, what does current evidence suggest? a. Unlike the broad distribution of dark matter in spiral galaxies, elliptical galaxies probably contain dark matter only near their centers. b. Elliptical galaxies probably contain far less dark matter than spiral galaxies. c. Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies. d. Elliptical galaxies probably contain far more dark matter than spiral galaxies.

c. Elliptical galaxies probably contain about the same proportion of their mass in the form of dark matter as do spiral galaxies.

What is the primary way in which we determine the mass distribution of a spiral galaxy? a. We count the number of stars we can see at different distances from the galaxy's center. b. We apply Newton's version of Kepler's third law to the orbits of globular clusters in the galaxy's halo. c. We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center. d. We calculate its mass-to-light ratio.

c. We construct its rotation curve by measuring Doppler shifts from gas clouds at different distances from the galaxy's center.

What is a rotation curve? a. a precise description of the shape of a star's orbit around the center of the Milky Way Galaxy b. a curve used to decide whether a star's orbit places it in the disk or the halo of a spiral galaxy c. a graph showing how orbital velocity depends on distance from the center for a spiral galaxy d. a graph that shows a galaxy's mass on the vertical axis and size on the horizontal axis

c. a graph showing how orbital velocity depends on distance from the center for a spiral galaxy

Which of the following is not one of the three main strategies used to measure the mass of a galaxy clusters? a. observing how the cluster bends light from galaxies located behind it b. studying X-ray emission from hot gas inside the cluster c. measuring the temperatures of stars in the halos of the galaxies d. measuring the speeds of galaxies orbiting the cluster's center

c. measuring the temperatures of stars in the halos of the galaxies

The primary evidence that has led astronomers to conclude that the expansion of the universe is accelerating comes from __________. a. observations of the speeds of individual galaxies in clusters b. measurements of the rotation curve for the universe c. observations of white dwarf supernovae d. measurements of how galaxy speeds away from the Milky Way have increased during the past century

c. observations of white dwarf supernovae

When we speak of the large-scale structure of the universe, we mean __________. a. the overall shape of the observable universe b. the structure of any individual cluster of galaxies c. the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe d. the structure of any large galaxy

c. the overall arrangement of galaxies, clusters of galaxies, and superclusters in the universe

When a proton and an antiproton collide, they

convert into two photons.

During the main sequence of a star's life it is

converting protons into helium nuclei in its core, releasing energy and neutrinos

Based on evidence both from measurements of the acceleration of the expansion rate and from careful study of the cosmic microwave background, about what percentage of the universe's total mass and energy takes the form of ordinary atomic matter (protons, neutrons, and electrons)? a. 68% b. 27% c. 0.5% d. 5%

d. 5%

Which of the following statements best summarizes current evidence concerning dark matter in individual galaxies and in clusters of galaxies? a. Within individual galaxies, dark matter is always concentrated near the galactic center, and within clusters it is always concentrated near the cluster center. b. Dark matter is present between galaxies in clusters, but not within individual galaxies. c. Dark matter is present in individual galaxies, but there is no evidence that it can exist between the galaxies in a cluster. d. Dark matter is the dominant form of mass in both clusters and in individual galaxies.

d. Dark matter is the dominant form of mass in both clusters and in individual galaxies.

What do we mean when we say that the rotation curve for the Milky Way galaxy is "flat"? a. The disk of a spiral galaxy is quite flat rather than spherical like the halo. b. All the galaxy's mass is concentrated in its flat, gaseous disk. c. The amount of light emitted by stars at different distances is about the same throughout the galaxy. d. Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.

d. Gas clouds orbiting far from the galactic center have approximately the same orbital speed as gas clouds located further inward.

What is the distinguishing characteristic of what we call ordinary (or baryonic) matter? a. It can attract other matter through the force of gravity. b. It emits a great deal of light. c. It is made of subatomic particles that scientists call WIMPs. d. It consists of atoms or ions with nuclei made from protons and neutrons.

d. It consists of atoms or ions with nuclei made from protons and neutrons.

Which of the following best sums up current scientific thinking about the nature of dark matter? a. Dark matter consists primarily of a mysterious form of energy that is causing the expansion of the universe to accelerate. b.There is no longer any doubt that dark matter is made mostly of WIMPs. c. Dark matter probably does not really exist, and rather indicates a fundamental problem in our understanding of gravity. d. Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.

d. Most dark matter probably consists of weakly interacting particles of a type that we have not yet identified.

What do we mean when we say that particles such as neutrinos or WIMPs are weakly interacting? a. They interact with other matter only through the weak force and not through gravity or any other force. b. They are only weakly bound by gravity, which means they can fly off and escape from galaxies quite easily. c. The light that they emit is so weak that it is undetectable to our telescopes. d. They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.

d. They respond to the weak force but not to the electromagnetic force, which means they cannot emit light.

Which of the following best summarizes what we mean by dark matter? a. matter for which we have theoretical reason to think it exists, but no observational evidence for its existence b. matter that may inhabit dark areas of the cosmos where we see nothing at all c. matter consisting of black holes d. matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light

d. matter that we have identified from its gravitational effects but that we cannot see in any wavelength of light

In terms of the "eras" that scientists use to describe different times in the history of the universe, we live in the __________.

era of galaxies

How do we determine the Milky Way's mass outside the Sun's orbit?

from the orbits of stars and gas clouds orbiting the galactic center at greater distances than the Sun

Galactic Fountains

galactic fountain scenario: supernova explosions in the galactic disk heat the interstellar medium and can drive hot gas out of the disk, creating so-called galactic fountains that contribute to the formation of a halo of hot gas around the Milky Way. As the gas rises above and below the disk, reaching heights of a few kiloparsecs (kpc), it emits radiation and thus becomes cooler, condensing into clouds which then fall back into the disk, in a fashion that resembles a fountain

All the following types of objects are found almost exclusively in the disk (rather than the halo) of the Milky Way except _________.

globular clusters

Consider a spinning disk of pizza dough, as shown here. What would the rotation curve for the spinning dough look like?

graph increases from reft to light

Notice the blue ovals (such as those indicated by the arrows) in this image of a galaxy cluster. The oval structures are not really located where they appear to be, but instead are multiple images of a single galaxy that lies directly behind the cluster. What do we call the process that creates these multiple images?

gravitational lensing

The primary source of a quasar's energy is

gravitational potential energy.

Each day the moon is roughly

half a zodiacal constellation further east than it was the day before.

A more massive main sequence star uses up its core material faster than a lower mass main sequence star because the more massive star

has a hotter core

Over time, the star-gas-star cycle leads the gas in the Milky Way to _________.

have a greater abundance of heavy elements

Which of these are ideal properties for distance?

high luminosity(intrinsic) distance-independent property(period, rise/decline)

If observations had shown that the cosmic microwave background was perfectly smooth (rather than having very slight variations in temperature), then we would have no way to account for _________.

how galaxies came to exist

If observations had shown that the cosmic microwave background was perfectly smooth (rather than having very slight variations in temperature), then we would have no way to account for _____________________.

how galaxies came to exist.

Collisions between galaxies typically unfold over a period of _________.

hundreds of millions of years Remember that typical galaxies are 100,000 light-years in diameter, so even at the speed of light a collision would take hundreds of thousands of years. Galaxies actually collide at relative speeds less than 1/1000 of the speed of light, so the collisions unfold over hundreds of millions of years

Where could we measure strong magnetic fields in the Sun?

in a sunspot

Consider the light curve of a variable star seen above. Where in the Hetrzprung-Russell Diagram would this star be found?

in the Cepheid instability strip

Where are most of the Milky Way's globular clusters found?

in the halo

Telescopes designed to study the earliest stages in galactic lives should be optimized for observations in ______. visible light X-rays radio waves infrared light

infared light We must observe very distant galaxies to see how galaxies looked when they were very young, and these galaxies have such large redshifts that any light they emitted as visible or ultraviolet has been shifted into the infrared.

Types of Nebulae

ionization (emission) and reflection.

If we say that a galaxy has a lookback time of 1 billion years, we mean that it is now 1 billion light-years away it is 400 million years old its light traveled through space for 1 billion years to reach us it was 1 billion light-years away when the light left the galaxy

its light traveled through space for 1 billion years to reach us Lookback time is more meaningful than distance, because the galaxy's current distance is different from what it was at the time it emitted the light we now see (because the universe is expanding).

This figure shows observations over more than a decade that have allowed us to determine the orbits of several stars around Sgr A* (the central object). What can we learn about Sgr A* by analyzing these orbits?

its mass

What distance method can give us the most accurate distance to the planet Venus?

main sequence fitting

Approximately what time would the third quarter moon rise?

midnight

Which of these pieces of evidence supports the idea that inflation really happened?

observations of the cosmic microwave background that indicate a flat geometry for the universe

Experiments allow physicists today to reproduce (on very small scales) energy and temperature conditions thought to have prevailed in the early universe as far back in time as about __________.

one ten-billionth (10-10) of a second after the Big Bang

Red and orange stars are found evenly spread throughout the galactic disk, but blue stars are typically found _________.

only in or near star-forming clouds

Measurements of how orbital speeds depend on distance from the center of our galaxy tell us that stars in the outskirts of the galaxy

orbit the galactic center just as fast as stars closer to the center.

On Earth we specify a location by its latitude and longitude. In the sky we use:

right ascension and declination

In the figure above are shown some measurements of the angular size of the Moon, phased with a derived period of 27.504 days. On the basis of these observations, what is the fractional variation of the Earth-Moon distance?

roughly 10 to 13 percent

Approximately how far from the Sun are the nearest (other) stars?

several light-years

How much mass will Sirius lose due to its stellar wind and the production of a planetary nebula?

somewhere between 0.6 and 1.4 solar masses

The four fundamental forces that operate in the universe today are __________.

strong force, weak force, electromagnetic force, gravity

Based on current evidence, which of the following is considered a likely candidate for the majority of the dark matter in galaxies?

subatomic particles that we have not yet detected

Suppose that inflation did not occur. In that case, the fact that the cosmic microwave background has the same temperature in opposite directions of the sky would be considered ___________.

surprising, because those locations would never have been close enough to have any light or matter exchanged between them

Suppose that inflation did not occur. In that case, the fact that the cosmic microwave background has the same temperature in opposite directions of the sky would be considered _________________.

surprising, because those locations would never have been close enough to have any light or matter exchanged between them.

Based on current evidence, a supercluster is most likely to have formed in regions of space where ___________________.

the density of dark matter was slightly higher than average when the universe was very young.

Which of the following is getting bigger as a result of the expansion of the universe?

the distance between the Milky Way and a galaxy 100 megaparsecs away

Laboratory experiments conducted with a particle accelerators confirm predictions made by the theory that unifies ________________

the electromagnetic and weak forces into the electroweak force

Laboratory experiments conducted with particle accelerators confirm predictions made by the theory that unifies _________.

the electromagnetic and weak forces into the electroweak force

Cosmological redshift is the result of ______. very old, red stars in distant galaxies the high speeds at which galaxies move within clusters supermassive black holes the expansion of the universe

the expansion of the universe right answer feedback: It differs from what we usually think of as a Doppler shift because the galaxies are not really moving through the universe; rather, they only appear to be moving away from us because space itself is growing between the galaxies.

Applying the Newton's version of Kepler's third law (or the orbital velocity formula) to the a star orbiting 40,000 light-years from the center of the Milky Way galaxy allows us to determine ______.

the mass of the Milky Way galaxy that lies within 40,000 light-years of the galactic center

The best measurements of the mass of the black hole at the galactic center come from

the orbits of stars in the galactic center.

The critical density of the universe is the __________.

the total density of matter and energy needed to give the universe a "flat" geometry (in spacetime).

Notice the distorted galaxy images, such as the large arc-shaped structure, in this image of a galaxy cluster. What can astronomers learn by carefully measuring the distortions in this image?

the total mass of the cluster

Which of the following does inflation help to explain?

the uniformity of the cosmic microwave background

Suppose that the universe were infinite in both extent and age. In that case, we would expect the night sky to be ___________.

uniformly bright

Suppose that the universe were infinite in both extent and age. In that case, we would expect the night sky to be _________________.

uniformly bright

A "GUT" (grand unified theory) refers to theories that __________.

unify the strong force with the electromagnetic and weak forces

A "GUT" (grand unified theory) refers to theories that _________.

unify the strong force with the electromagnetic and weak nuclear forces

Dark matter is inferred to exist because

we can observe its gravitational influence on visible matter.

Suppose that Earth's atmosphere had no greenhouse gases. Then Earth's average surface temperature would be

well below the freezing point of water

The final end state of a single star with more than 8 solar masses is

white dwarf

Fossil evidence suggests that life on Earth arose

within a few hundred million years after Earth formed.

Say an asteroid orbits the Sun and has a mean distance of 4 AU. What is the orbital period in years?

Ch 18 Reading/Concept, Astro Chapters 15-19, Astro Final, 16, Pre-Lecture Chapter 17 + Visual Quiz, ASTR FINAL, Astronomy Ch. 17, ASTR=Chapter 18 Visual Quiz, Module 13: Exploring Our Galaxy: The Milky Way, Chapter 18 : Astronomy Reading Quiz

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