Module 10 ASTR 100
What is a galaxy? About how many galaxies fill the observable universe?
A galaxy is a gravitationally bound grouping of stars, dust, and gas that is recognizable as an object distinct from its surroundings. Thousands of billions of galaxies fill the vast observable universe.
What is luminous matter?
A galaxy's spectrum is due primarily to starlight, so astronomers can find out what types of stars are in the galaxy. Once this is known, the theory of stellar evolution is used to calculate the combined mass of all the stars. The physics of radiation from interstellar gas at X-ray, infrared, and radio wavelengths enables us to estimate the mass of the gas and dust. Together, the stars, gas, and dust in a galaxy are called luminous matter (or sometimes normal matter).
Describe how Type Ia supernovae can be used to estimate distances to very distant galaxies.
A globular cluster is a sphere of stars that contains between tens of thousands and a million stars. About one-fourth of the globular clusters in the Milky Way reside in or near the disk. The rest occupy the halo of the Milky Way. Shapley made a three-dimensional map of globular clusters. This map showed that globular clusters occupy a roughly spherical region of space with a diameter of about 300,000 light-years. These globular clusters trace out the halo of the Milky Way Galaxy, which reflects the modern view of the globular-cluster distribution. Globular clusters orbit the gravitational center of the galaxy, so the center of the distribution of globular clusters is the same as the gravitational center of the galaxy.
What are globular clusters? Where are they mostly found in our galaxy? How can we use globular clusters to estimate our Sun's location in the galaxy?
A globular cluster is a sphere of stars that contains between tens of thousands and a million stars. About one-fourth of the globular clusters in the Milky Way reside in or near the disk. The rest occupy the halo of the Milky Way. Shapley made a three-dimensional map of globular clusters. This map showed that globular clusters occupy a roughly spherical region of space with a diameter of about 300,000 light-years. These globular clusters trace out the halo of the Milky Way Galaxy, which reflects the modern view of the globular-cluster distribution. Globular clusters orbit the gravitational center of the galaxy, so the center of the distribution of globular clusters is the same as the gravitational center of the galaxy.
If all the matter in a galaxy was luminous matter, what should happen to the rotation speed of the galaxy as you move out from the center? What do we actually see? What does this tell us about the matter in a galaxy?
A graph showing how the orbital velocity of stars and gas in a galaxy changes with distance from the galaxy's center is called a rotation curve, and the disk is often said to "rotate," even though the disk is not a solid object like a wheel. Instead, the stars all travel in the same direction around the center of the galaxy, like cars on a racetrack. The universe is full of a substance called dark matter, which accounts for what we actually see.
What happens to the magnetic field and rate of rotation/spin of a star when it becomes a neutron star? Explain how this can create a pulsar.
A neutron star has a magnetosphere just like Earth and several other planets do, except that the neutron star's magnetosphere is vastly stronger and is whipped around many times a second by the spinning star. Just like in planets, in stars the magnetic axis is often not aligned with the rotation axis. Electrons and positrons move along the magnetic-field lines and are "funneled" by the field toward the magnetic poles of the system. The particles produce beams of radiation along the magnetic poles of the neutron star. As the neutron star rotates, these beams sweep through space much like the rotating beams of a lighthouse. The neutron star appears to flash on and off with a regular period equal to the period of rotation of the star (or half the rotation period, if we see both beams). These rotating neutron stars are known as pulsars.
What is a neutron star? What range of masses can a neutron star have? What hold a neutron star up from collapsing?
A neutron star is a giant ball of neutrons. It is incredibly dense, and can have a mass up to 3 solar masses. The collapse is halted when neutrons are packed as tightly together as the rules of quantum mechanics allow. Held up by neutron degeneracy.
What are the two most common ideas of what makes up dark matter?
A number of suggestions are under investigation, including Jupiter-sized objects, numerous black holes, large numbers of white dwarf stars, and exotic unknown elementary particles. These candidates can be lumped into two categories, called MACHOs and WIMPs. Dark matter candidates such as small main-sequence M stars, planets, white dwarfs, neutron stars, or black holes are collectively referred to as MACHOs, which stands for "massive compact halo objects. This leaves the exotic unknown elementary particles commonly known as WIMPs, which stands for "weakly interacting massive particles." These particles are predicted to be something like neutrinos; they would barely interact with ordinary matter yet would have some mass. WIMPs are currently the favored explanation because there are not enough MACHOs to account for the effects we observe.
What is Hawking radiation?
After a very, very long time, the black hole would become small enough that it would become unstable and explode. Although the light that emerges, known as Hawking radiation, is of considerable interest to physicists and astronomers, in a practical sense the low intensity of Hawking radiation means it is not a useful way to "see" a black hole.
What is the only difference between a normal galaxy and an active galaxy? Describe how interactions between galaxies can lead to an active galaxy.
Apparently, the only difference between a normal galaxy and an active galaxy is whether the supermassive black hole at its center is being fed at the time we see that galaxy.
What does general relativity say should happen to the elliptical orbit of a planet around the Sun? Do we have evidence of this?
Curved spacetime does have observable consequences. General relativity predicts that an elliptical orbit, in which the planet swings in closer and then farther from the Sun, should process; that is, the long axis should slowly change its direction. As light travels through distorted spacetime, its path bends. This bending of the light path as the light passes through bent spacetime is called gravitational lensing because lenses also bend light paths
How does the speed of light vary (if at all) for observers moving at different speeds?
During the closing years of the 19th century and the early years of the 20th century, the results of laboratory experiments with light puzzled physicists. They expected that the speed of light should differ from one observer to the next as a result of the observer's motion. Instead, they found that all observers measure exactly the same value for the speed of a beam of light, regardless of their motion!
What are quasars? Are quasars mostly close or far away from us? What does that tell us about when in the universe quasars were most common?
Extraordinarily luminous objects at enormous distances. These "quasi-stellar radio sources" were dubbed quasars. Today we know that quasars result from extreme activity in the nuclei of galaxies, which often results from interactions with other galaxies. Together, quasars and their less luminous but still active cousins are called active galactic nuclei (AGNs). Quasars are quite far from us. Because we see quasars only at great distances, we know that they are quite rare in the universe at this time but were once much more common. This discovery of the changing occurrence of quasars was one of the first pieces of evidence to demonstrate that the universe has evolved over time.
T/F: Our galaxy is mostly luminous matter and only has a small amount of dark matter.
False: Our galaxy is mostly made up of dark matter.
How do cosmic-ray muons give evidence for time dilation?
Fast particles called cosmic-ray muons are produced 15 km up in Earth's atmosphere when high-energy cosmic rays strike atmospheric atoms or molecules. Muons at rest decay very rapidly into other particles. This happens so quickly that, even if they could move at the speed of light, virtually all muons would have decayed long before traveling the 15 km to reach Earth's surface. However, time dilation slows the muons' clocks, so the particles travel further during their longer lifetime and reach the ground.
What is a galaxy group? What galaxy group is our galaxy a part of? What other large galaxy is also part of the same group?
Galaxies do not exist in isolation. The vast majority of galaxies are parts of gravitationally bound collections of galaxies. The smallest and most common of these are called galaxy groups. A galaxy group contains as many as several dozen galaxies, most of which are dwarf galaxies. The Milky Way is a member of the Local Group, first identified by Edwin Hubble in 1936. There are about 50 members of the Local Group, including two large barred spiral galaxies. The Milky Way and the Andromeda Galaxy, accounting for nearly 98% of the mass of the Local Group.
What are the three main types of galaxies? Who first grouped galaxies into these types?
Galaxies that are elliptical in three dimensions, something like an egg, are called elliptical galaxies. These galaxies have a circular or elliptical outline on the sky and have no outer disk. They have numbered subtypes ranging from nearly spherical to quite flattened. Elliptical galaxies have few young stars. Spiral galaxies, designated with an initial S, are characterized by a flattened, rotating disk. The spiral arms that give these galaxies their name lie in this disk. In addition to disks and arms, spiral galaxies have central bulges, which extend above and below the disk. Galaxies that fall into none of these classes are called irregular galaxies. As their name implies, irregular galaxies are often without symmetry in shape or structure. Often, irregular galaxies are the result of the collision of two or more galaxies. Edwin Hubble created this method of organizing the galaxies.
What is the difference between a dwarf and a giant galaxy? What types of galaxies (elliptical, spiral, irregular) can be both dwarf and giant.
Galaxies with relatively low luminosity are dwarf galaxies. Galaxies that are 1 billion times as luminous as the sun are called giant galaxies. Elliptical galaxies are the only galaxies that can be both.
What is Hubble's Law? Describe how Hubble discovered this law. What does it tell us about the universe?
Hubble found that the recession velocity of a galaxy is proportional to the distance of that galaxy. A galaxy at a distance of 30 Mpc from Earth moves away from us twice as fast as a galaxy at a distance of 15 Mpc from Earth. This relationship between distance and recession velocity has become known as Hubble's law. Hubble interpreted Slipher's redshifts as Doppler shifts, and he concluded that almost all of the galaxies in the universe are moving away from the Milky Way. For one thing, Hubble's law helps us test the prediction that the universe is both isotropic and homogeneous. Observations in different directions confirm that the same Hubble law applies in every direction, so the universe is isotropic. Hubble's law also shows that the universe is homogeneous.
What happens if a neutron star is heavier than 3 Mo?
If the mass of a neutron star exceeds about 3 MSun, then gravity will be stronger than the neutron degeneracy pressure. These larger cores shrink quickly under their own gravity, and gravity increases at an ever-accelerating pace. Eventually, gravity is so strong that the escape velocity exceeds the speed of light. From this point on, nothing can escape from the collapsing object. The object is now called a black hole: an object with gravity so strong that even light cannot escape it. Black holes are so strange, so far from the common understanding of reality, that the laws of Newtonian physics cannot be used to describe them.
What are open clusters? Where are they mostly found in our galaxy? How are they different in size and age than globular clusters?
In contrast, open clusters are much less tightly bound collections of a few dozen to a few thousand stars that are found in the disk of a spiral galaxy. As with globular clusters, the stars in an open cluster all formed in the same region at about the same time, and so their ages can be found from the main-sequence turnoff. Open clusters have a wide range of ages. Some contain the very youngest stars known, while others contain stars somewhat older than the Sun.
How does time change when you are near a very massive object? How does the wavelength/color of light emitted change if the light is coming from near a very massive object?
Mass distorts the geometry of time as well. Deep within the gravitational field of a massive object, clocks run more slowly from the perspective of a distant observer. This effect is called general relativistic time dilation. Because time is running slowly on the surface of the neutron star, the light that reaches the distant observer will have a lower frequency than when it was emitted. A lower frequency means a longer wavelength, so the light from the source will be seen at a longer, redder wavelength than the wavelength at which it was emitted. The reddening of light as it climbs out of a gravitational well is called the gravitational redshift because the wavelengths of light from objects deep within a gravitational well are shifted to longer, redder wavelengths. The effect of gravitational redshift is similar to the Doppler redshift.
Describe where most stars are found in the disk of a spiral galaxy. Where are most new stars found in the disk?
Most stars that are found in the disk of a spiral galaxy are the younger stars in the galaxy.
Describe how 21-cm radiation from neutral hydrogen has helped us map our galaxy. Why can't we use visible light to do this?
Neutral hydrogen in the interstellar medium emits radiation at a wavelength of 21 cm, in the radio region of the spectrum. It's mapped in 21-cm radiation from neutral hydrogen. Because of its long wavelength, 21-cm radiation freely penetrates dust in the interstellar medium, enabling us to see neutral hydrogen throughout our galaxy. The maps showed spiral structure in the other galaxies and suggested spiral structure in the Milky Way. At about the same time, observations of ionized hydrogen gas in visible light showed two spiral arms with concentrations of young, hot O and B stars. These observations together confirmed that the Milky Way is a spiral galaxy
What age stars (old or young) are found in elliptical galaxies? Describe the difference between an E0/E1 galaxy and an E6/E7 galaxy.
Old stars are found in elliptical galaxies. The difference between an E0 galaxy and an E7 galaxy is a scale of spherical to flat. E0 is a sphere, E7 is flat.
What is Cygnus X-1? How did astronomers determine that it is likely a black hole?
One line of evidence for black holes that result from supernovae comes from X-ray binary stars. The radio emission from the Cygnus X-1 X-ray binary system flickers rapidly, changing in as little as 0.01 second. The X-ray emission from Cygnus X-1 arises when material from the B0 supergiant falls onto an accretion disk surrounding the black hole.
How did Eddington use a solar eclipse to confirm one of the predictions of general relativity?
Several months before the total solar eclipse of 1919, Sir Arthur Stanley Eddington measured the positions of stars in the direction of the sky where the eclipse would occur. He then repeated the measurements during the eclipse. Light from distant stars curved as it passed the Sun, causing the measured positions of the stars to shift outward. From his measurements, Eddington concluded that the stars appeared farther apart in his second measurement than in his first, as predicted by general relativity. During the eclipse, the triangle formed by Earth and any two stars contained more than 1808.
Describe the parts of a spiral galaxy. What is the difference between a barred and unbarred spiral galaxy? Which one is the Milky Way?
Spiral galaxies, designated with an initial S, are characterized by a flattened, rotating disk. The spiral arms that give these galaxies their name lie in this disk. In addition to disks and arms, spiral galaxies have central bulges, which extend above and below the disk. Roughly half of all galaxies with spiral arms have a bulge that is bar-shaped when viewed from above the plane of the galaxy; these galaxies are known as barred spirals. Both spirals and barred spirals are subdivided into types a, b, and c, according to the prominence of the central bulge and how tightly the spiral arms are wound. The Milky Way falls is a loosely wound barreled galaxy.
Why is star formation more active in the inner part of the disk compared with the outer part?
Star formation is generally more active in the inner parts of the Milky Way than in the outer parts because interstellar gas is denser in the inner parts.
Describe how the amount of massive elements in a star can indicate how long ago the star formed.
Stars add heavy elements to the interstellar medium as part of a cycle in which material is taken from the interstellar medium to form stars, processed through the life and death of a star, and then lost to the interstellar medium again. When the universe was very young, only the least massive (light) elements existed. All elements more massive than boron must have formed in the cores of stars. For this reason, the abundance of heavy elements in the interstellar medium provides a record of all the star formation that has taken place up to the present time. Gas that is rich in heavy elements has gone through a great deal of stellar processing, whereas gas that is poor in heavy elements has not.
According to the special theory of relativity, what happens to time for a moving observer? Why does this not appear obvious in our everyday life?
The distance between two events depends on the motion of the observer, but the time between the two events does not. The only way that all observers will measure the speed of light to be the same is if the passage of time is different from one observer to the next. For moving observers, the time is stretched out, so that each second is longer, a phenomenon known as time dilation. In our everyday lives, we experience a Newtonian world because we never encounter speeds approaching that of light.
What are gravitational waves?
The equations of general relativity predict that if you accelerate the fabric of spacetime then waves in spacetime, called gravitational waves, will move outward at the speed of light. These gravitational waves are like electromagnetic waves in some respects. Accelerating an electrically charged particle gives rise to an electromagnetic wave. Accelerating a massive object gives rise to gravitational waves, which alternately stretch and compress spacetime as they pass through it.
What is the connection between the mass of a supermassive black hole and the mass of an elliptical galaxy or bulge of a spiral galaxy?
The mass of the supermassive black hole seems to be related to the mass of the elliptical-galaxy or spiral-galaxy bulge in which it is found. Most large galaxies probably contain supermassive black holes.
What is dark matter? Where do we find most dark matter in a spiral galaxy? Describe the dark matter found in elliptical galaxies?
The material in space which does not interact with light, and therefore reveals itself only by the influence of its gravity, is called dark matter. Astronomers currently estimate that as much as 95 percent of the total mass in some spiral galaxies consists of a greatly extended dark matter halo far larger than the visible spiral portion of the galaxy located at its center. Instead, we measure it from the variations among the motions of individual stars, and we find that elliptical galaxies are mostly made of dark matter. Another avenue of exploration confirms this result. An elliptical galaxy's ability to hold on to its hot gas depends on its mass: If the galaxy is not massive enough, the hot atoms and molecules will escape into intergalactic space. To find the mass of an elliptical galaxy, first we find the total amount of luminous hot gas from X-ray images, such as the blue and purple halo. Next we calculate the mass that is needed to hold onto that gas. We then compare the luminous mass with the gravitational mass. The amount of dark matter is the difference between the observed luminous mass and the amount of mass that is needed to hold on to the hot gas.
Describe how mass distorts spacetime according to general relativity.
The more mass there is, the slower time will flow. Rather than thinking of gravity as a "force" that "acts on" objects, it is more accurate to say that gravitation results from the shape of spacetime that objects move through. The warping of spacetime leads directly to the gravity that holds you to Earth. This theory, called the general theory of relativity, or sometimes just "general relativity," describes how mass distorts spacetime and is another of Einstein's great contributions to science.
Explain how we estimate the mass of the supermassive black hole at the center of our galaxy.
The motions of stars closest to the Sgr A* source suggest a central mass very much greater than that of the few hundred stars orbiting there. Stars less than 0.1 light-year from the galaxy's center follow Kepler's laws. The closest stars studied are only about 0.01 light-year from the center—so close that their orbital periods are only about a dozen years. The positions of these stars change noticeably over time, and we can see them speed up as they whip around what can only be a supermassive black hole at the focus of their elliptical orbits. Using Kepler's third law, we estimate that the black hole at the center of our own galaxy is relatively lightweight, having a mass of "only" about 4 million MSun.
What is a supermassive black hole? Describe how a supermassive black hole can produce an AGN or quasar?
The observation that AGNs emit so much light from such a small region of space implies that these galaxies contain supermassive black holes—black holes with masses from thousands to tens of billions of solar masses. Violent accretion disks around these black holes power AGNs. Gas in the accretion disk or in nearby clouds orbiting the central black hole at high speeds produces emission lines that are smeared out by the Doppler effect into the broad lines seen in many AGN spectra. This accretion disk surrounding a supermassive black hole is the "central engine" that produces AGNs
What is the event horizon of a black hole? How does the radius of the black hole change as the black hole's mass increases?
The radius where the escape velocity equals the speed of light is called the Schwarzschild radius, named for physicist Karl Schwarzschild (1873-1916), and it is proportional to the mass of the black hole. The sphere around the black hole at this distance is called its event horizon. The radius and mass of a black hole are proportional.
If you imagine a cross section of our disk, where do we find the youngest stars? Where do we find the oldest stars?
The youngest stars in our galaxy are most strongly concentrated in the galactic plane, defining a disk more than 100,000 light-years across, but only about 1,000 light-years thick, which is very thin by comparison. The older population of disk stars, distinguishable by lower abundances of heavy elements, has a much "thicker" distribution: about 12,000 light-years thick. The youngest stars are concentrated closest to the plane of the galaxy because this is where the molecular clouds are. Older stars make up the thicker parts of the disk.
What would happen to someone as they fell into a black hole?
They would get stretched like a noodle and ripped apart very slowly in a process called spaghettification.
T/F: Hubble's Law means that we are in the center of an expanding universe. Explain.
True: Hubble's law actually says that the expansion always looks the same, regardless of location.
What is an x-ray binary?
X-ray binary, a binary system in which mass from an evolving star spills over onto a collapsed companion such as a white dwarf, neutron star, or black hole.
Which of the following is NOT true about black holes? a) A person falling into a black hole would be compressed (pushed into a ball shape) as they fell towards the black hole. b) An outside observer would think that a person falling into the black hole was moving in slow motion. c) A black hole can be formed after a Type II supernova. d) The heaviest stars form black holes at the end of their lives.
a) A person falling into a black hole would be compressed (pushed into a ball shape) as they fell towards the black hole.
Tell whether each description best matches an elliptical galaxy or a spiral galaxy. Some descriptions will have more than one answer. a) Stars orbit in random directions around the center b) Stars in disk all orbit in the same direction around the center c) Stars in bulge orbit in random directions around the center d) Large amounts of dust and cold gas e) Dark band running through the middle f) Reddish colors for stars g) Bluish colors for stars h) Not much recent star formation i) Currently forming a good amount of stars
a) Elliptical b) Spiral c) Spiral d) Spiral e) Spiral f) Elliptical g) Spiral h) Elliptical i) Spiral, Irregular
Which of these is true about globular clusters? a) Globular clusters contain more stars than open clusters. b) Globular clusters are younger than open clusters. c) Globular clusters are more spread out than open clusters. d) All of the above.
a) Globular clusters contain more stars than open clusters.
Which of these is NOT true about colors in galaxies? a) Older groups of stars often look blue because of how many low mass stars there are. b) Younger stars are normally bluer. c) The disk of a spiral galaxy has a bluer color to it. d) The bulge of a spiral galaxy has a redder color to it.
a) Older groups of stars often look blue because of how many low mass stars there are.
Where in our galaxy is the Sun located? a) The disk b) The halo c) The bulge d) It depends on the time of year.
a) The disk
Which of these objects would most likely have the highest escape velocity? a) The Earth b) A planet 3 times heavier than the Earth c) A planet half the weight of the Earth. d) They would all have the same escape velocity.
b) A planet 3 times heavier than the Earth
Where is the mass located in a black hole? a) In a dense iron core in the inner 100 miles of the black hole. b) All located at a singularity in the center of a black hole. c) Equally spread out over all the area inside the event horizon. d) Only at the event horizon.
b) All located at a singularity in the center of a black hole.
Edwin Hubble discovered that: a) Almost all galaxies are moving towards us. b) Almost all galaxies are redshifted. c) Galaxies that are farther away are moving toward us slower. d) All of the above.
b) Almost all galaxies are redshifted.
Which of these is NOT true about galaxies? a) Elliptical galaxies contain very little gas and dust. b) Elliptical galaxies normally have a blue color to them. c) Elliptical galaxies normally have only older stars in them. d) Most (around 90%) of a normal galaxy's mass is dark matter.
b) Elliptical galaxies normally have a blue color to them.
Which of the following is NOT true about our galaxy? a) Our galaxy is a spiral galaxy. b) Globular clusters are mostly located in the disk of our galaxy. c) Our Sun rotates around the center of our galaxy, which contains a 4 million Mo black hole. d) Dwarf galaxies (such as the Magellanic Clouds) formed from the same large cloud of gas and dust as our galaxy.
b) Globular clusters are mostly located in the disk of our galaxy.
Why do spiral galaxies not always look the same in the sky? a) Spiral galaxies have the biggest range in sizes, from dwarf spirals to giant spirals. b) We see spiral galaxies from different orientations or angles. c) Some spiral galaxies do not have a disk. d) Some spiral galaxies do not contain any gas or dust.
b) We see spiral galaxies from different orientations or angles.
A pulsar is: a) A very heavy black hole. b) A white dwarf slowing cooling off. c) A spinning neutron star. d) Another name for a yellow giant.
c) A spinning neutron star.
Imagine that you see a star with a bright blue color. The most likely reason the star is blue is: a) It is moving toward us. b) It is moving away from us. c) It is hot. d) It is cold.
c) It is hot.
Which of the following is true about neutron stars? a) Neutron stars are approximately the same diameter as the Earth. b) A neutron star heavier than 3 solar masses will turn into a white dwarf. c) Neutron stars can spin very quickly because they are quite small. d) Neutron stars do not collapse because of all the fusion that is occurring inside them.
c) Neutron stars can spin very quickly because they are quite small.
Which of these is true about the formation of our galaxy? a) Our galaxy actually formed before our universe formed. b) The disk of our galaxy formed first and then the halo and bulge formed. c) The formation of our galaxy is similar to the formation of stars but on a larger scale. d) All of the above.
c) The formation of our galaxy is similar to the formation of stars but on a larger scale.
Irregular galaxies: a) Have many young stars. b) Do not have a disk. c) Are the least common of the 3 galaxy types. d) All of the above.
d) All of the above.
The event horizon of a black hole: a) Is the point of no return - nothing can escape from inside the event horizon. b) Gets larger as the weight of the black hole increases. c) Is only a few miles in size for a black hole that weighs one solar mass. d) All of the above.
d) All of the above.
Which of the following is true about cosmology? a) Cosmology is the study of the creation and evolution of the universe. b) Hubble's observations challenged the idea that the universe is static. c) General relativity predicted that the universe should be expanding. d) All of the above.
d) All of the above.
Imagine that we see three stars. One star is red, one is yellow, and one is blue. All three are moving toward us at the same speed. Which star will have the largest blueshift? a) The red star. b) The blue star. c) The yellow star. d) They will all have the same blueshift. e) Trick question - they are actually redshifted.
d) They will all have the same blueshift.
Which of these is true about general relativity? a) Objects with mass change the shape of space/time and this creates gravity. b) Light coming from near a heavy object is shifted to redder wavelengths. c) General relativity says that light should be affected by gravity. d) General relativity has been tested by noticing that the light from distant stars is sometimes bent by the Sun. e) All of the above.
e) All of the above.