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 any 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 important 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 simple evolutionary sequence that links spiral, elliptical, and irregular galaxies. Most astronomers believe 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, and 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. Mergers between spirals most likely result in elliptical galaxies.

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 50 and 80 km/s/Mpc; we adopt a valve of 65 km/s/Mpc, in the middle 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 gaseous disk. HINT

3. There is little or no interstellar dust in elliptical galaxies, but there may be substantial amounts of interstellar gas. HINT

4. Most ellipticals contain only young stars. HINT

5. Most galaxies are spirals. HINT

6. Irregular galaxies, although small, have lots of star formation taking place in them. HINT

7. Elliptical galaxies evolve into spirals. 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 contains about a million individual galaxies. HINT

14. Most galaxies are 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 _____ cool gas and dust than do irregulars. HINT

5. The Milky Way Galaxy, the Andromeda Galaxy, and roughly 40 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. HINT

7. The center of the Local Supercluster lies in _____. HINT

8. When galaxies collide, the star formation rate often _____. HINT

9. Mass determinations from rotation curves, line broadening, and galaxies in binaries and clusters all make use of _____ laws. HINT

10. More than 90 percent of the universe is made up of _____. 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. _____ galaxies may be formed by mergers between spiral systems. HINT

14. Hubble’s law is a correlation between the redshifts and the _____ of galaxies. HINT

15. On the largest scales, galaxies in the universe appear to be arranged on huge_____ surrounding nearly empty _____. HINT


1. What distinguishes one type of spiral galaxy from another? HINT

2. Describe some similarities and differences between elliptical galaxies and the halo of our own Galaxy. HINT

3. Describe the four rungs in the distance-measurement ladder used to determine the distance to a galaxy lying 5 Mpc away. HINT

4. Describe the contents of the Local Group. How much space does it occupy compared to the volume of the Milky Way? HINT

5. What are standard candles, and why are they important to astronomy? HINT

6. How is the Tully-Fisher relation used to measure distances to galaxies? HINT

7. What is the Virgo Cluster? HINT

8. Describe two techniques for measuring the mass of a galaxy. HINT

9. Why do astronomers believe that galaxy clusters contain more mass than we can see? HINT

10. Why are galaxies at great distances from us generally smaller and bluer than nearby galaxies? HINT

11. What evidence do we have that galaxies collide with one another? HINT

12. Describe the role of collisions in the formation and evolution of galaxies. HINT

13. Give an example of how mergers can transform one type of galaxy into another. HINT

14. Do you think that collisions between galaxies constitute “evolution” in the same sense as the evolution of stars? HINT

15. What are starburst galaxies, and what do they have to do with galaxy evolution? HINT

16. What is Hubble’s law? HINT

17. How is Hubble’s law used by astronomers to measure distances to galaxies? HINT

18. What is the most likely range of values for Hubble’s constant? Why is the exact value uncertain? HINT

19. What are voids? HINT

20. Describe the distribution of galactic matter on very large (more than 100 Mpc) scales. HINT

PROBLEMS Algorithmic versions of these questions are available in the Practice Problems Module of the Companion Website.

The number of squares preceding each problem indicates the approximate level of difficulty.

1. A supernova of luminosity one 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 800 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. HINT

4. Based on the data in Figure 24.18, estimate the mass of the galaxy NGC 4984 inside 20 kpc. HINT

5. Based on the data in Figure 24.18, estimate the amount of line broadening (maximum minus minimum wavelength) of the 656.3 nm H line observed in NGC 4984. HINT

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. HINT

7. Use Kepler’s third law (Section 23.6) to estimate the mass required to keep a galaxy moving at 750 km/s in a circular orbit of radius 2 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? HINT

8. Assuming that the average speed of the protons in the (ionized) X-ray-emitting gas is the same as the mean orbital speed of the stars, estimate the temperature of the gas in the X-ray halo of an elliptical galaxy of mass 1012 solar masses and radius 25 kpc. (More Precisely 8-1) HINT

9. Calculate the average speed of hydrogen nuclei (protons) in a gas of temperature 20 million K. Compare this with the speed of a galaxy moving in a circular orbit of radius 1 Mpc around a galaxy cluster of mass 1014 solar masses. HINT

10. 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? HINT

11. 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

12. 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

13. Assuming Hubble’s law with H0 = 65 km/syMpc, what would be the angular diameter of an E0 galaxy of radius 40 kpc, if its 656.3 nm H line is actually observed at 700 nm? HINT

14. A small satellite galaxy is moving in a circular orbit around a much more massive parent, and just happens to be moving exactly parallel to the line of sight as seen from Earth. The recession velocities of the satellite and the parent galaxy are measured to be 6450 km/s and 6500 km/s, respectively, and the two galaxies are separated by an angle of 0.1° on the sky. Assuming H0 = 65 km/s/Mpc, calculate the mass of the parent galaxy. HINT

15. Galaxies in a distant galaxy cluster are observed to have recession velocities ranging from 12,500 km/s to 13,500 km/s. The cluster’s angular diameter is 55'. Use these data to estimate the cluster’s mass. What assumptions do you have to make in order to obtain this estimate? Take H0 = 65 km/s/Mpc. HINT


1. Sampling the Sky. Figure 24.23 is called the Hubble Deep Field. The Picture shows far too many galaxies for one person to easily count. Each group member should count the galaxies in a random area 3 cm 3 cm and then determine a group average. Since the entire image is approximately 500 cm2, multiply your group’s average number of galaxies in a 3 cm 3 cm area by 55.5 to estimate the number of galaxies in the image. How does your value compare to that of another group?

2. Classification Schemes. Edwin Hubble classified galaxies into three broad categories based on their appearance. Classify all the books used this term by the group member who is taking the most classes, using categories such as color, size, thickness, and cost. Use any scheme that adequately describes the collection. Clearly define each category.

RESEARCHING ON THE WEB To complete the following exercises, go to the online Destinations Module for Chapter 24 on the Companion Website for Astronomy Today 4/e.

1. Access the “Galaxy Types” page and make a labeled sketch of the three types of galaxies defined on this page.

2. Access the “Cosmology of the Local Group” page and list the three brightest and the three closest galaxies of the Local Group.

3. Access the “Our Hierarchical Universe” pages and list the top six important questions in cosmology.


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 signs 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 the 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, 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, M58, M59, M60, M84, M86, M87 (a giant elliptical thought to have a massive black hole at its center), M89, and M90. Examine each galaxy for unusual features; some have very bright nuclei.

SKYCHART III PROJECTS The SkyChart III Student Version planetarium program on which these exercises are based is included as a separately executable program on the CD in the back of this text.

1. Use SkyChart III to observe the distribution of visible galaxies. Deep Sky and Grid Lines should be selected under DRAW. Select all grid lines under DRAW/Symbols & Grids and set for 180º field of view. Deselect all other parameters under DRAW. Set a time near midnight and select a time-step of one month. Repeatedly press the F3 key until the faintest objects are visible. Set ANIMATION/1 Month and step the time forward one month at a time by pressing the F6 key. Note the distribution of galaxies, and observe the relationship of the galactic equator to the observed galaxies. Explain the distribution you find.

2. The Large and Small Magellanic Clouds are two irregular galaxies prominent to the unaided eye in the Southern Hemisphere. Set up the software by selecting VIEW/180º Field. Select DRAW/Sky Background/Black, DRAW/Constellations and DRAW/Deep Sky Objects; deselect Stars and Horizon Mask. Change the display with the down arrow key until the Southern Hemisphere is centered in the display. The Large Magellanic Cloud is at about the same hour angle as Orion, and the Small Magellanic Cloud is at about the same hour angle as Cassiopeia. They are present in the display as ovals when DRAW/Deep Sky Objects is selected. Select DRAW/Grid Lines and observe the relationship between the positions of these irregular galaxies and the Galactic equator.

Alternatively, locate the Magellanic Clouds by using VIEW/Center Object/Magellanic Cloud. If this method is used for locating the Magellanic Clouds, then orient the screen with North up and move the display so that you can observe the relative size and position of the clouds to constellations you are familiar with.

3. The Local Group consists of several nearby galaxies orbiting one another in an area about 1 Mpc in diameter. The largest members of this cluster are the Milky Way and Andromeda Galaxies. Print a chart labeling the locations of the nearest members of the Local Group: The Large and small Magellanic Clouds, and the Sculptor (NGC253) and Fornax (NGC1316) galaxies.

4. Locate the Andromeda Galaxy (M31) and print a chart labeling members of the Local Group nearest Andromeda (i.e., M32, M33, NGC 147, NGC 185).

5. Collisions of galaxies are rare and can last many millions of years. Some collisions can result in a change in shape in one or both galaxies, as in the M51 and NGC 5195 galaxies. Others collide more violently, resulting in new star formation, as is the case with galaxies NGC 4038 and NGC 4039, as well as NGC 3690 and IC 694. Find these galaxies in SkyChart III.

In addition to the Practice Problems and Destinations modules, the Companion Website at provides for each chapter an additional true-false, multiple choice, and labeling quiz, as well as additional annotated images, animations, and links to related Websites.