A galaxy is a huge collection of stellar and interstellar matter isolated in space and bound together by its own gravity. Because we live within it, the Galactic disk of our own Milky Way Galaxy appears as a broad band of light across the sky, a band called the Milky Way. Near the center, the Galactic disk thickens into the Galactic bulge. The disk is surrounded by a roughly spherical Galactic halo of old stars and star clusters. Our Galaxy, like many others visible in the sky, is a spiral galaxy.
The halo can be studied using variable stars, whose luminosity changes with time. Pulsating variable stars vary in brightness in a repetitive and predictable way. Two types of pulsating variable stars of great importance to astronomers are RR Lyrae variables and Cepheid variables, whose characteristic light curves make them easily recognizable. All RR Lyrae stars have roughly the same luminosity. Astronomers can determine the luminosity of Cepheids by measuring the pulsation period and using the period—luminosity relationship, a simple correlation between period and absolute brightness. The brightest Cepheids can be seen at distances of millions of parsecs, extending the cosmic distance ladder well beyond our own Galaxy. RR Lyrae stars are fainter but much more numerous, making them very useful within the Milky Way.
In the early twentieth century, Harlow Shapley used RR Lyrae stars to determine the distances to many of the Galaxy's globular clusters. He found that the clusters have a roughly spherical distribution in space, but the center of the sphere lies far from the Sun. The globular clusters are now known to map out the true extent of the luminous portion of the Milky Way Galaxy. The center of their distribution is close to the Galactic center, which lies about 8 kpc from the Sun.
Disk and halo stars differ in their spatial distributions, ages, colors, and orbital motion. The luminous portion of our Galaxy has a diameter of about 30 kpc. The halo lacks gas and dust, so no stars are forming there. All halo stars are old. The gas-rich disk is the site of current star formation and contains many young stars. Stars and gas within the Galactic disk move on roughly circular orbits around the Galactic center. Stars in the halo and bulge move on largely random three-dimensional orbits that pass repeatedly through the disk plane but have no preferred orientation. Halo stars appeared early on, before the Galactic disk took shape, when there was still no preferred orientation for their orbits. As the gas and dust formed a rotating disk, stars that formed in the disk inherited its overall spin and so moved on circular orbits in the Galactic plane, as they do today.
In the vicinity of the Sun the Galactic disk is about 300 pc thick. Young stars, gas, and dust are more narrowly confined; older stars have a broader distribution. Intermediate between the young disk and the old halo, in both age and spatial distribution, are the stars of the thick disk, which is about 2-3 kpc thick.
Attempts to map out the Galactic disk by optical observations are defeated by interstellar absorption. Astronomers use radio observations to explore the Galactic disk because radio waves are largely unaffected by interstellar dust. Regions where most of the hydrogen is in atomic form may be studied using 21-cm radiation. Regions where the gas is mostly molecular are studied through radio molecular emission lines. Gas has been detected in the disk at up to 50 kpc from the Galactic center. Regions where the gas is mostly molecular are usually studied by observing radio emission lines from "tracer" molecules, such as carbon monoxide. The gas distribution fattens near the center into the Galactic bulge. Radio-emitting gas has been detected in the disk at up to 50 kpc from the Galactic center.
Radio observations clearly reveal the extent of our Galaxy's spiral arms. The spiral arms in spiral galaxies are regions of the densest interstellar gas and are the places where star formation is taking place. The spirals cannot be "tied" to the disk material, as the disk's differential rotation would have wound them up long ago. Instead, they may be spiral density waves that move through the disk, triggering star formation as they pass by. Alternatively, the spirals may arise from self-propagating star formation, when shock waves produced by the formation and evolution of one generation of stars triggers the formation of the next.
The Galactic rotation curve plots the orbital speed of matter in the disk versus distance from the Galactic center. By applying Newton's laws of motion, astronomers can determine the mass of the Galaxy. They find that the Galactic mass continues to increase beyond the radius defined by the globular clusters and the spiral structure we observe. The rotation curves of our own and other galaxies show that many, if not all, galaxies have an invisible dark halo containing far more mass than the visible portion of the galaxies. The dark matter making up these dark halos is of unknown composition. Leading candidates include low-mass stars and exotic subatomic particles. Recent attempts to detect stellar dark matter have used the fact that a faint foreground object can occasionally pass in front of a more distant star, deflecting the star's light and causing its apparent brightness to increase temporarily. This deflection is called gravitational lensing.
Astronomers working at infrared and radio wavelengths have uncovered evidence of energetic activity within a few parsecs of the Galactic center. The leading explanation is that a black hole 1ר million times more massive than the Sun resides at the heart of our Galaxy.
1. Cepheids can be used to determine the distances to the nearest galaxies. (Hint)
2. RR Lyrae stars are a type of cataclysmic variable. (Hint)
3. The Galactic halo contains about as much gas and dust as the Galactic disk. (Hint)
4. The Galactic disk contains only old stars. (Hint)
5. Population I objects are found only in the Galactic halo. (Hint)
6. Up until the 1930s, the main error made in determining the size of the Galaxy was due to an incorrectly calibrated method of determining stellar distances. (Hint)
7. Astronomers use 21-cm radiation to study Galactic molecular clouds. (Hint)
8. Radio techniques are capable of mapping the entire Galaxy. (Hint)
9. In the neighborhood of the Sun, the Galaxy's spiral density wave rotates more slowly than the overall Galactic rotation. (Hint)
10. The mass of the Galaxy is determined by counting stars. (Hint)
11. Dark matter is now known to be due to large numbers of black holes. (Hint)
12. A million—solar mass black hole could account for the unusual properties of the Galactic center. (Hint)
13. Cosmic rays are very energetic photons. (Hint)
14. Most of the mass of our Galaxy exists in the form of dark matter. (Hint)
15. The Galactic center has been extensively studied at visible and ultraviolet wavelengths. (Hint)
1. One difficulty in studying our own galaxy in its entirety is that we live _____. (Hint)
2. Herschel's attempt to map the Milky Way by counting stars led to an inaccurate estimate of the Galaxy's size because he was unaware of _____. (Hint)
3. The highly flattened, circular part of the Galaxy is called the Galactic _____. (Hint)
4. The roughly spherical region of faint old stars and globular clusters in which the rest of the Galaxy is embedded is the Galactic _____. (Hint)
5. Cepheids and RR Lyrae stars are observed to vary in _____ with periods of days to months. (Hint)
6. Cepheid pulsational periods range from _____ to _____. (Hint)
7. Cepheids and RR Lyrae variables lie in a region of the H—R diagram called the _____. (Hint)
8. According to the period—luminosity relation, the longer the pulsation period of a Cepheid, the _____ its luminosity. (Hint)
9. Harlow Shapley determined the distances to the globular clusters using observations of _____. (Hint)
10. The Sun lies roughly _____ pc from the Galactic center. (Hint)
11. The orbital speed of the Sun around the Galactic center is _____. (Hint)
12. The orbits of halo objects are _____ in direction. (Hint)
13. The original cloud of gas from which the Galaxy formed probably had a size and shape similar to the present Galactic _____. (Hint)
14. Rotational velocities in the outer part of the Galaxy are _____ than would be expected on the basis of observed stars and gas, indicating the presence of _____. (Hint)
15. Observations of the _____ of infrared spectral lines indicate that gas near the Galactic center is orbiting at extremely high speeds. (Hint)
1. What are spiral nebulae? How did they get that name? (Hint)
2. How are Cepheid variables used in determining distances? (Hint)
3. Roughly how far out into space can we use Cepheids to measure distance? (Hint)
4. What important discoveries were made early in this century using RR Lyrae variables? (Hint)
5. Why are the central regions of our Galaxy best studied using radio telescopes? (Hint)
6. Of what use is radio astronomy in the study of Galactic structure? (Hint)
7. Contrast the motions of disk and halo stars. (Hint)
8. Explain why Galactic spiral arms are believed to be regions of recent and ongoing star formation. (Hint)
9. Describe what happens to interstellar gas as it passes through a spiral density wave. (Hint)
10. What is self-propagating star formation? (Hint)
11. What do the red stars in the Galactic halo tell us about the history of the Milky Way? (Hint)
12. What does the rotation curve of our Galaxy tell us about its total mass? (Hint)
13. What evidence is there for that dark matter in the Galaxy? (Hint)
14. What are some possible explanations for dark matter? (Hint)
15. Why do astronomers believe that a supermassive black hole lies at the center of the Milky Way? (Hint)
1. Calculate the angular diameter of a prestellar nebula, of radius 100 A.U., lying 100 pc from Earth. Compare this with the roughly 6 ° diameter of the Andromeda galaxy (Figure 23.2a). (Hint)
2. What is the greatest distance at which an RR Lyrae star of absolute magnitude 0 could be seen by a telescope capable of detecting objects as faint as 20th magnitude? (Hint)
3. A typical Cepheid variable is 100 times brighter than a typical RR Lyrae star. How much farther away than RR Lyrae stars can Cepheids be used as distance-measuring tools? (Hint)
4. The Hubble Space Telescope can see a star like the Sun at a distance of 100,000 pc. The brightest Cepheids have luminosities 30,000 times greater than that of the Sun. How far away can HST see these Cepheids? (Hint)
5. Calculate the proper motion (in arc seconds/year) of a globular cluster with a transverse velocity (relative to the Sun) of 200 km/s and a distance of 3 kpc. Do you think that this motion is measurable? (Hint)
6. Calculate the total mass of the Galaxy lying within 20 kpc of the Galactic center if the rotation speed at that radius is 240 km/s. (Hint)
7. Using the data presented in Figure 23.16, calculate how long it takes the Sun to "lap" stars orbiting 15 kpc from the Galactic center. How long does it take matter at 5 kpc to lap us? (Hint)
8. A density wave made up of two spiral arms is moving through the Galactic disk. At the 8-kpc radius of the Sun's orbit around the Galactic center, the wave's speed is 120 km/s, and the Galactic rotation speed is 220 km/s. Calculate how many times the Sun has passed through a spiral arm since the Sun formed 4.6 billion years ago. (Hint)
9. Given the data in the previous question and the fact that O stars live at most 10 million years before exploding as supernovae, calculate the maximum distance at which an O star (orbiting at the Sun's distance from the Galactic center) can be found from the density wave in which it formed. (Hint)
10. Material at an angular distance of 0.1" from the Galactic center is observed to have an orbital speed of 1100 km/s. If the Sun's distance to the Galactic center is 8 kpc, and the material's orbit is circular and is seen edge-on, calculate the mass of the object around which the material is orbiting. (Hint)
1. If you are far from city lights, look for a hazy band of light arching across the sky. This is our edgewise view of the Milky Way Galaxy. The Galactic center is located in the direction of the constellation Sagittarius, highest in the sky during the summer, but visible from spring through fall. Look at the band making up the Milky Way and notice dark regions; these are relatively nearby dust clouds. Sketch what you see. Look for faint fuzzy spots in the Milky Way and note their positions in your sketch. Draw in the major constellations for reference. Compare your sketch with a map of the Milky Way in a star atlas. Did you discover most of the dust clouds? Can you identify the faint fuzzy spots?
