ASTR=Chapter 18 Visual Quiz, Module 13: Exploring Our Galaxy: The Milky Way, Chapter 18 : Astronomy Reading Quiz
Galaxy's Bulge Mass
The larger a galaxy's bulge mass, the larger the supermassive black hole is at its center.
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.
Overall composition of the MWG
70% H, 28% He, 2% heavy elements
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.
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
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.
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.
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.
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)
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).
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.
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.
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.
(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
Disk
Active regions of star formation.
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.
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
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.
Most abundant molecule in gas clouds
CO
Interstellar medium
Clouds of gas and dust
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.
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.
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.
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.
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.
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.
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.
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.
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.
Population I stars
Mix of old and new; heavy element concentrations like that found in Sun; in disk.
The MWG
Most likely a barred spiral 120,000 l-y across, containing 200-400 billion stars
Galactic Center
Most likely contains supermassive black hole.
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.
Halo
No star formation.
Dusty Gas Clouds
Obscure our view in visible light to the center of our galaxy
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.
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
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.
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.
Galaxy's Core
Stars seen swooping toward a small central region supports conclusion that a supermassive black hole exists at our galaxy's core.
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
Sun orbiting the MWG
Sun orbits the MWG once every 230 million years
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.
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.
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.
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 .
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.
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.
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.
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).
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.
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.
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.
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
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.
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
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
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
Types of Nebulae
ionization (emission) and reflection.
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.
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.
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)
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.
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.
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.
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.
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
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.
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.
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.
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.
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).
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.
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).
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.
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.
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.
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.
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.
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.
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.
Milky Way from Above
If we could view the Milky Way from above the disk, we would see its spiral arms.
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.
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.
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.
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).
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.
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.
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.
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).
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.
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.
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.
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.
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.
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.
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.
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)
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.
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.
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.
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.
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.
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 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.
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)
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
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.
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.
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.
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.
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.
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
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
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
