The Hubble classification scheme divides galaxies into several classes, depending on their appearance. Spiral galaxies have flattened disks, central bulges, and spiral arms. They are further subdivided on the basis of the size of the bulge and the tightness of the spiral structure. The halos of these galaxies consist of old stars, whereas the gas-rich disks are the sites of ongoing star formation. Barred-spiral galaxies contain an extended "bar" of material projecting beyond the central bulge.
Elliptical galaxies have no disk and contain no gas or dust. In most cases, they consist entirely of old stars. They range in size from dwarf ellipticals, which are much less massive than the Milky Way Galaxy, to giant ellipticals, which may contain trillions of stars. S0 and SB0 galaxies have properties intermediate between those of ellipticals and spirals. They have extended halos and stellar disks and bulges (and bars, in the SB0 case) but little or no gas and dust.
Irregular galaxies are galaxies that do not fit into either of the other categories. Some may be the result of galactic collisions or close encounters. Many irregulars are rich in gas and dust and are the sites of vigorous star formation. The Magellanic Clouds, two small systems that orbit the Milky Way Galaxy, are examples of this type of galaxy.
Astronomers often use standard candles as distance-measuring tools. These are objects that are easily recognizable (by their light curves, spectra, or some other directly observable characteristic) and whose luminosities are known to lie in some reasonably well-defined range. Comparing luminosity and apparent brightness, astronomers determine the distance using the inverse-square law. Type I supernovae are particularly useful for measuring distances to faraway galaxies. They are bright and hence easily seen, and have a relatively narrow spread in luminosity. An alternative is the Tully-Fisher relation, an empirical correlation between rotational velocity and luminosity in spiral galaxies. By measuring the rotation speed of a spiral and using this relationship, astronomers can determine the galaxy's luminosity and hence its distance.
The Milky Way, Andromeda, and several other smaller galaxies form the Local Group, a small galaxy cluster. Galaxy clusters consist of a collection of galaxies orbiting one another, bound together by their own gravity. The nearest large galaxy cluster to the Local Group is known as the Virgo Cluster. Galaxy clusters themselves tend to clump together into superclusters. The Virgo Cluster, the Local Group, and several other nearby clusters form the Local Supercluster.
The masses of nearby spiral galaxies can be determined by studying their rotation curves. For more distant spirals, masses can be inferred from observations of the broadening of their spectral lines. On larger scales, astronomers use studies of binary galaxies and galaxy clusters to obtain statistical mass estimates of the galaxies involved. As in the Milky Way Galaxy, measurements of the masses of other galaxies and of galaxy clusters reveal the presence of large amounts of dark matter that is presently undetectable at any electromagnetic wavelength. The fraction of dark matter apparently grows as the scale under consideration increases. Large amounts of hot X-ray-emitting gas have been detected among the galaxies in many clusters, but not enough to account for the dark matter inferred from dynamical studies.
Researchers know of no evolutionary sequence that links spiral, elliptical, and irregular galaxies, and the process of galaxy formation is still only poorly understood. There is growing evidence that large galaxies formed by the merger of smaller ones in a process that may be continuing today. Collisions and mergers of galaxies play very important roles in galactic evolution. Interactions between galaxies appear to be very common. A starburst galaxy may result when a galaxy experiences a close encounter with a neighbor. The strong tidal distortions caused by the encounter compress galactic gas, resulting in a widespread burst of star formation.
Distant galaxies are observed to be receding from the Milky Way at rates that increase proportional to their distances from us. This relationship between recessional speed and distance is called Hubble's law. The constant of proportionality in the law is Hubble's constant. Its value is believed to lie between 45 and 90 km/s/Mpc, with most astronomers now favoring a value closer to the lower end of this range. Astronomers use Hubble's law to determine distances to the most remote objects in the universe. The redshift associated with the Hubble expansion is called the cosmological redshift.
On very large scales, galaxies and galaxy clusters are not spread randomly throughout space. Instead, they are arranged on the surfaces of enormous "bubbles" of matter surrounding vast low-density regions called voids. The origin of this structure is thought to be closely related to conditions in the very earliest epochs of the universe.
1. Barred-spiral galaxies have the same properties as normal spirals, except for the "bar" feature. (Hint)
2. Elliptical galaxies do not contain a flattened disk. (Hint)
3. There is no interstellar dust in elliptical galaxies, but there are substantial amounts of interstellar gas. (Hint)
4. Most ellipticals contain only old stars. (Hint)
5. Most galaxies are spirals. (Hint)
6. Irregular galaxies, although small, have lots of star formation taking place in them. (Hint)
7. Spiral galaxies evolve into ellipticals. (Hint)
8. Type I supernovae can be used to determine distances to galaxies. (Hint)
9. Every galaxy is a member of some galaxy cluster. (Hint)
10. Galaxy collisions can occur, but they are extremely rare. (Hint)
11. Galaxy collisions have little or no effect on the stars and interstellar gas in the galaxies involved. (Hint)
12. Distant galaxies appear to be much larger than those nearby. (Hint)
13. A typical galaxy cluster has a mass of about 1011 solar masses. (Hint)
14. Most galaxies appear to be receding from the Milky Way Galaxy. (Hint)
15. Hubble's law can be used to determine distances to the farthest objects in the universe. (Hint)
1. Galaxies are categorized by their _____ classification. (Hint)
2. Spiral galaxies with tightly wrapped spiral arms tend to have _____ central bulges. (Hint)
3. Spiral galaxies of type _____ have the least amount of gas; type _____ have the most. (Hint)
4. Elliptical galaxies generally contain far _____ gas and dust than do irregulars. (Hint)
5. The Milky Way Galaxy, the Andromeda Galaxy, and 18 other galaxies form a small cluster known as the _____. (Hint)
6. In the Tully—Fisher relation, a galaxy's luminosity is found to be related to the _____ of its 21-cm line.
7. The diameter of the Local Supercluster is about _____ . (Hint)
8. When galaxies collide, the star formation rate often _____ . (Hint)
9. Galaxy mass determinations from rotation curves, line broadening, and binary galaxies all make use of _____ laws. (Hint)
10. Dark matter may make up as much as _____ percent of the entire universe. (Hint)
11. Intergalactic gas in galaxy clusters emits large amounts of energy in the form of _____. (Hint)
12. By which process do galaxies form: fragmentation (large objects breaking up into small) or mergers (small objects accumulating into large)? _____ (Hint)
13. Hubble's law is a correlation between the redshifts and the _____ of galaxies. (Hint)
14. Hubble's constant is believed to lie in the range _____ to _____ km/s/Mpc.
15. The largest known structures in the universe, such as voids and the Great Wall, have sizes on the order of _____. (Hint)
1. In what sense are elliptical galaxies "all halo"? (Hint)
2. Describe the four rungs in the distance-measurement ladder used to determine the distance to a galaxy lying 5 Mpc away. (Hint)
3. Describe the contents of the Local Group. How much space does it occupy compared to the volume of the Milky Way?(Hint)
4. How is the Tully—Fisher relation used to measure distances to galaxies?
5. What is the Virgo Cluster? (Hint)
6. Describe two techniques for measuring the mass of a galaxy. (Hint)
7. Why do astronomers believe that galaxy clusters contain more mass than we can see? (Hint)
8. Why are galaxies at great distances from us generally smaller and bluer than nearby galaxies? (Hint)
9. What evidence do we have that galaxies collide with one another? (Hint)
10. Describe the role of collisions in the formation and evolution of galaxies. (Hint)
11. Do you think that collisions between galaxies constitute "evolution" in the same sense as the evolution of stars? (Hint)
12. What is Hubble's law? (Hint)
13. How is Hubble's law used by astronomers to measure distances to galaxies? (Hint)
14. What is the most likely range of values for Hubble's constant? Why is the exact value uncertain? (Hint)
15. What are voids? What is the distribution of galactic matter on very large (more than 100 Mpc) scales? (Hint)
1. A supernova of luminosity 1 billion times the luminosity of the Sun is used as a standard candle to measure the distance to a faraway galaxy. From Earth the supernova appears as bright as the Sun would appear from a distance of 10 kpc. What is the distance to the galaxy? (Hint)
2. A Cepheid variable star in the Virgo cluster has an absolute magnitude of —5 and is observed to have an apparent magnitude of 26.3. Use these figures to calculate the distance to the Virgo cluster. (Hint)
3. The Andromeda Galaxy is approaching our Galaxy with a radial velocity of 266 km/s. Given the galaxies' present separation of 930 kpc, and neglecting both the transverse component of the velocity and the effect of gravity in accelerating the motion, estimate when the two galaxies will collide.
4. Calculate the average speed of hydrogen nuclei (protons) in a gas of temperature 50 million K (see More Precisely 8-1). Compare this with the average orbital speed of 1000 km/s of galaxies in a typical galaxy cluster.
5. Based on the data in Figure 24.19, estimate the amount of line broadening (maximum minus minimum wavelength) of the 656.3 nm H 
line observed in the galaxy NGC 4984.
6. Two galaxies are orbiting each other at a distance of 500 kpc. Their orbital period is estimated to be 30 billion years. Use Kepler's law (as stated in Section 23.6) to find the total mass of the pair.
7. Use Kepler's third law (Section 23.6) to estimate the mass required to keep a galaxy moving at 1000 km/s in a circular orbit or radius 3 Mpc around the center of a galaxy cluster. Given the approximations involved in determining this figure, do you think this is a good estimate of the cluster's true mass?
8. In a galaxy collision, two similar-sized galaxies pass through each other with a combined relative velocity of 1500 km/s. If each galaxy is 100 kpc across, how long will the event last?
9. According to Hubble's law, with H0 = 65 km/s/Mpc, what is the recessional velocity of a galaxy at a distance of 200 Mpc? How far away is a galaxy whose recessional velocity is 4000 km/s? How do these answers change if H0 = 50 km/s/Mpc? If H0 = 80 km/s/Mpc? (Hint)
10. According to Hubble's law, with H0 = 65 km/s/Mpc, how long will it take for the distance from the Milky Way Galaxy to the Virgo Cluster to double? (Hint)
1. Look for a copy of the Atlas of Peculiar Galaxies by Halton Arp. It is available in book form or on laser disk. Search for examples of interacting galaxies of various types: (1) tidal interactions, (2) starburst galaxies, (3) collisions between two spirals, and (4) collisions between a spiral and an elliptical. For (1) look for galactic material pulled away from a galaxy by a neighboring galaxy. Is the latter galaxy also tidally distorted? In (2) the surest sign of starburst activity are bright knots of star formation. In what type(s) of galaxies do you find starburst activity? For (3) and (4) how do collisions differ depending on the types of galaxies involved. What typically happens to a spiral galaxy after a near-miss or collision? Do ellipticals suffer the same fate?
2. Look for the Virgo Cluster of galaxies. An 8-inch telescope is a perfect size for this project, although a smaller telescope will also work. The constellation Virgo is visible from the United States during much of fall, winter, and spring. To locate the center of the cluster, first find the constellation Leo. The eastern part of Leo is composed of a distinct triangle of stars, Denebola ( 
), Chort ( 
), and Zosma ( 
). Go from Chort to Denebola in a straight line east and continue on the same distance as between the two stars and you will be approximately at the center of the Virgo Cluster. Look for the following Messier objects that make up some of the brightest galaxies in the cluster: M49, 58, 59, 60, 84, 86, 87 (giant elliptical thought to have a massive black hole at its center), 89, and 90. Examine each galaxy for unusual features; some have very bright nuclei.
