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Stars form when an interstellar cloud collapses under its own gravity and breaks up into pieces comparable in mass to our Sun. Heat, rotation, and magnetism all compete with gravity to influence the cloud’s evolution. The evolution of the contracting cloud—the changes in its temperature and luminosity—can be conveniently represented as an evolutionary track on the Hertzsprung–Russell diagram. A cold interstellar cloud containing a few thousand solar masses of gas can fragment into tens or hundreds of smaller clumps of matter, from which stars eventually form.

As a collapsing prestellar fragment heats up and becomes denser it eventually becomes a protostar—a warm, very luminous object that emits radiation mainly in the infrared portion of the electromagnetic spectrum. At this stage of its evolution, the protostar is also known as a T Tauri star, after the first object of this type discovered. Eventually, a protostar’s central temperature becomes high enough for hydrogen fusion to begin, and the protostar becomes a star. For a star like the Sun, the whole formation process takes about 50 million years. Protostellar winds encounter less resistance in the directions perpendicular to a protostar’s disk. Thus they expel two jets of matter in the directions of the protostar’s poles. As the protostellar wind gradually destroys the disk, the jets widen until, with the disk gone, the wind flows away from the star equally in all directions.

More massive stars pass through similar stages, but much more rapidly. Stars less massive than the Sun take much longer to form. The zero-age main sequence is the region on the H–R diagram where stars lie when the formation process is over. Mass is the key property for determining a star’s characteristics and life span. The most massive stars have the shortest formation times and main-sequence lifetimes. At the other extreme, some low-mass fragments never reach the point of nuclear ignition. The universe may be populated with a vast number of brown dwarfs—objects that are not massive enough to fuse hydrogen to helium in their interiors.

Many of the objects predicted by the theory of star formation have been observed in real astronomical objects. The dark interstellar regions near emission nebulae often provide evidence of cloud fragmentation and protostars. Radio telescopes are used for studying the early phases of cloud contraction and fragmentation; infrared observations allow us to see later stages of the process. Many well-known emission nebulae, lit by several O-type stars, are partially engulfed by molecular clouds, parts of which are probably fragmenting and contracting, with smaller sites forming protostars. Shock waves can compress other interstellar clouds and trigger star formation. Star birth and the production of shock waves are thought to produce a chain reaction of star formation in molecular cloud complexes.

A single collapsing and fragmenting cloud can give rise to hundreds or thousands of stars—a star cluster. The formation of the most massive stars may play an important role in suppressing further formation of lower-mass cluster members. Open clusters, with a few hundred to a few thousand stars, are found mostly in the plane of the Milky Way. They typically contain many bright blue stars, indicating that they formed relatively recently. Globular clusters are found mainly away from the Milky Way plane and may contain millions of stars. They include no main-sequence stars much more massive than the Sun, indicating that they formed long ago. Globular clusters are believed to date from the formation of our Galaxy. Loosely bound groups of newborn stars are called stellar associations. Infrared observations have revealed young star clusters or associations in several emission nebulae. Eventually, clusters break up into individual stars, although the process may take billions of years to complete.


1. Given the typical temperatures found in interstellar space, a cloud containing as few as 1,000,000 atoms has sufficient gravity for it to begin to collapse. HINT

2. Both rotation and magnetic fields act to accelerate the gravitational collapse of an interstellar cloud. HINT

3. The time a solar-type star spends in formation is relatively short compared to the time it spends as a main-sequence star. HINT

4. Most stars form as members of groups or clusters of stars. HINT

5. A stage 4 protostar may have a luminosity 1000 times that of the Sun. HINT

6. As it evolves along the Hayashi track from stage 4 to stage 6, a protostar’s luminosity stays roughly constant. HINT

7. The rate of evolution of a stage 5 object is rapid compared with the rates at previous stages. HINT

8. Brown dwarfs take a long time to form, but will eventually arrive as stars on the lower main sequence. HINT

9. Stages 1 and 2 of star formation can be observed using optical telescopes. HINT

10. Shock waves produced by emission nebulae can initiate star formation in nearby molecular clouds. HINT

11. The formation of the first high-mass stars in a collapsing cloud tends to inhibit further collapse within that cloud. HINT

12. G-, K-, and M-type stars form more frequently than O- and B-type stars. HINT

13. The gas in an emission nebula eventually dissipates into space, leaving behind a star cluster. HINT

14. Star clusters eventually dissipate, leaving behind individual stars like the Sun. HINT

15. Typical open clusters contain millions of stars. HINT


1. Atoms in an interstellar cloud have random motions, with an average velocity determined by the cloud’s _____. HINT

2. A(n) _____ plots a star or protostar’s changing location on the H–R diagram as the object evolves. HINT

3. In stage 1 of prestellar evolution, a typical interstellar cloud has the following properties: temperature _____ K, size _____ pc, mass _____ solar masses. HINT

4. In stage 2 of prestellar evolution, a contracting interstellar cloud _____ into smaller pieces. HINT

5. During stage 3 of prestellar evolution, as each piece of the original interstellar cloud continues to contract, its central density and temperature _____. HINT

6. At stage 4 of prestellar evolution, a piece of the interstellar cloud becomes a _____. HINT

7. A stage 4 object is plotted in the _____ (upper/lower) _____ (right/left) part of the H–R diagram. HINT

8. At stage 6 the central temperature of the object reaches _____ K. HINT

9. At this temperature, a stage 6 object begins to _____. HINT

10. When hydrogen is fusing stably in the core, the star has reached the _____. HINT

11. The T Tauri phase of a star occurs during stage _____. HINT

12. It takes a star like the Sun a total of about _____ million years to form. HINT

13. More massive stars evolve more _____. HINT

14. Astronomers look for emissions at _____ wavelengths to identify interstellar clouds in stages 1 and 2. HINT

15. At stages 4, 5, and 6, objects emit a great deal of radiation in the _____ part of the electromagnetic spectrum. HINT


1. Briefly describe the basic chain of events leading to the formation of a star like the Sun? HINT

2. What is the role of heat in the process of stellar birth? HINT

3. What is the role of rotation in the process of stellar birth? HINT

4. What is the role of magnetism in the process of stellar birth? HINT

5. What is an evolutionary track? HINT

6. Why do stars tend to form in groups? HINT

7. Why does the evolution of a protostar slow down as it approaches the main sequence? HINT

8. In what ways do the formative stages of high-mass stars differ from those of stars like the Sun? HINT

9. What are brown dwarfs? HINT

10. What are T Tauri stars? HINT

11. Stars live much longer than we do, so how do astronomers test the accuracy of theories of star formation? HINT

12. At what evolutionary stages must astronomers use radio and infrared radiation to study prestellar objects? Why can’t they use visible light? HINT

13. Why has it been difficult until recently to demonstrate that stars and protostars actually exist within star-forming regions? HINT

14. What is a shock wave? Of what significance are shock waves in star formation? HINT

15. Explain the usefulness of the H–R diagram in studying the evolution of stars. Why can’t evolutionary stages 1–3 be plotted on the diagram? HINT

16. Compare the times necessary for the various stages in the formation of a star like the Sun. Why are some so short and others so long? HINT

17. What do star clusters and associations have to do with star formation? HINT

18. Compare and contrast the observed properties of open star clusters and globular star clusters. HINT

19. How can we tell if a star cluster is young or old? HINT

20. In the formation of a star cluster with a wide range of stellar masses, is it possible for some stars to die out before others have finished forming? Do you think this will have any effect on the cluster’s formation? 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. In order for an interstellar gas cloud to contract, the average speed of its constituent particles must be less than half the cloud’s escape speed. Will a (spherical) molecular hydrogen cloud of mass 1000 solar masses, radius 10 pc, and temperature of 10 K begin to collapse? Why or why not? (See More Precisely 8-1.) HINT

2. Under the same assumptions as in Problem 1, estimate the minimum mass needed to cause a 1000 K, 1 pc cloud to collapse. HINT

3. Use the radius–luminosity–temperature relation to explain how a protostar’s luminosity changes as it moves from stage 4 (temperature 3000 K, radius 2 108 km) to stage 6 (temperature 4500 K, radius 106 km). What is the change in absolute magnitude (Sec. 17.5) HINT

4. A protostar on the Hayashi track evolves from a temperature of 3500 K and a luminosity 5000 times that of the Sun to a temperature of 5000 K and a luminosity of 3 solar units. What is its radius (a) at the start, and (b) at the end of the evolution? HINT

5. What is the (approximate) absolute magnitude of a stage-5 protostar? (See Figure 19.7.) HINT

6. Use the H–R diagrams in this chapter to estimate by what factor a 1000-solar luminosity, 3000 K protostar is larger than a main-sequence star of the same luminosity. HINT

7. By how many magnitudes does a 3-solar-mass star decrease in brightness as it evolves from stage 4 to stage 6? (See Figure 19.8.) HINT

8. As a simple model of the final stage of star formation, imagine that between stages 6 and 7 a star’s surface temperature increases with time at a constant rate while the luminosity remains constant at the stage 7 level. The stage 7 radius is equal to the solar value. Using the temperatures given in Table 19.1, calculate the star’s radius at a time exactly halfway between these two stages. HINT

9. What is the luminosity, in solar units, of a brown dwarf whose radius is 0.1 solar radii and whose surface temperature is 600 K (0.1 times that of the Sun)? HINT

10. What is the maximum distance at which the brown dwarf in the previous problem could be observed by a telescope of limiting apparent magnitude (a) 18, (b) 30? HINT

11. A shock wave from a supernova explosion moves at a speed of about 5000 km/s. How long will such a disturbance take to cross a molecular cloud 20 pc in diameter? HINT

12. The luminosity of a hypothetical star-forming region is dominated by five bright O-type stars, each of absolute magnitude 28. What is the net absolute magnitude of the region? HINT

13. If the star-forming region in Problem 12 is in a galaxy 10 Mpc (1 megaparsec = 1,000,000 pc) from Earth, calculate its apparent magnitude. HINT

14. An open cluster has a diameter of 5 pc and a mass 1000 times the mass of the Sun. (a) Estimate the typical speed of its component stars. (b) Based on this speed, estimate the number of times a typical star orbits the center of the cluster in the 500 million years it takes for the cluster to dissolve in the galactic tidal field. HINT

15. Approximating the gravitational field of our Galaxy as a mass of 1011 solar masses at a distance of 8000 pc (see Chapter 23), estimate the “tidal radius” of a 20,000-solar-mass open-star cluster—that is, the distance from the cluster’s center, outside of which the galactic tidal force overwhelms the cluster’s own gravity. HINT


1. Prestellar Evolution. As a group, create a large pie chart showing the amount of time spent in each of the stages 1 through 6 listed in Table 19.1 for a solar-type star over a total period of 30 million years. Each group member should label one or two of the stages and clearly describe the characteristics of each.

RESEARCHING ON THE WEB To complete the following exercises, go to the online Destinations Module for Chapter 19 on the Companion Website for Astronomy Today 4/e.

1. Access the "Hertzsprung–Russell Diagram" page and describe the current evolutionary stage of 80 percent of the stars in our galaxy.

2. Access the "All Star Line Up" page and describe how Alpha Centauri relates to our Sun in terms of size, brightness, and spectral class.

3. Access the "What is a Star?" page and describe how stars 10 times more massive and 1/10th the Sun’s mass will be different than the Sun in terms of brightness and lifetime.


1. The Trifid Nebula, otherwise known as M20, is a place where new stars are forming. It has been called a “dark night revelation, even in modest apertures.” An 8- to 10-inch telescope is needed to see the triple-lobed structure of the nebula. Ordinary binoculars reveal the Trifid as a hazy patch located in the constellation Sagittarius. This nebula is set against the richest part of the Milky Way, the edgewise projection of our own Galaxy around the sky. It is one of many wonders in this region of the heavens. What are the dark lanes in M20? Why are other parts of the nebula bright? There have been reports of large-scale changes occurring in this nebula in the last century and a half. The reports are based on old drawings, which show M20 looking slightly different from how it appears today. Do you think it possible for a cloud in space to undergo a change in appearance on a time scale of years, decades, or centuries?

2. Summer is a good time to search with binoculars for open-star clusters. Open clusters are generally found in the plane of the Galaxy. If you can see the hazy band of the Milky Way arcing across your night sky—in other words, if you are far from city lights and looking at an appropriate time of night and year—you can simply sweep with your binoculars along the Milky Way. Numerous “clumps” of stars will pop into view. Many will turn out to be open-star clusters.

3. Globular star clusters are harder to find. They are intrinsically larger, but they are also much farther away and therefore appear smaller in the sky. The most famous globular cluster visible from the Northern Hemisphere is M13 in the constellation Hercules, visible on spring and summer evenings. This cluster contains half a million or so of the Galaxy’s most ancient stars. It may be glimpsed in binoculars as a little ball of light, located about one-third of the way from the star Eta to the star Zeta in the Keystone asterism of the constellation Hercules. Telescopes reveal this cluster as a magnificent, symmetrical grouping of stars.

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. Appendix 3 contains a list of the 20 brightest stars. Make your own table of these stars with columns for information on the range of dates each is visible in your area at a time convenient for observation, and another column with information on how to locate each one. Make note of those stars that are not available to you because they are in the Southern Hemisphere. Use SkyChart III and the chart you have made to locate the bright stars available at this time. Along with being able to recognize the constellations, it is rewarding to know the names of the bright stars. A small effort is usually sufficient to develop a satisfying familiarity.

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.