F g h

Answer: (a) Protostar, gravitational contraction of cloud of gas and dust; (b) stable main sequence star, shining by nuclear fusion (converting hydrogen to helium); (c) evolution to red giant when helium core forms; (d) red giant, shining by helium fusion; (e) variable star, formation of carbon core; (f) planetary nebula, enriched hydrogen envelope ejected into space; (g) white dwarf, mass packed into star about the size of Earth; (h) dead black dwarf in space.


A supernova is a gigantic stellar explosion. It may outshine its whole galaxy for a short time.

Most stars of eight or more times the Sun's mass die in a spectacular explosion called a Type II supernova. Their carbon cores contract gravita-tionally in the same way that a smaller star's does. But in a massive star, the core temperature continues to rise to 600 million K. Then the carbon fuses into magnesium. The collapse stops when the carbon in the core is used up. A new cycle begins—core contraction with rising temperature and onset of new nuclear reactions, fusion of heavier elements such as oxygen and silicon, new shells of lighter elements, and a halt in the collapse.

Iron ends these cycles, because it requires rather than releases energy in nuclear reactions. The doomed core collapses for the last time. When it cannot be compressed any further, it rebounds, sending out a shock wave. The outer layers explode violently. Light from the supernova can reach 100 billion times the Sun's luminosity.

Most of the energy released in the explosion is invisible. A great amount is carried away at the speed of light by high-energy radiation and neutrinos ejected from the collapsing core. This energy holds clues to the causes of stellar explosions and the kinds and amounts of chemical elements manufactured and sprayed into space by supernovas.

Supernova 1987A, the first bright supernova in the sky since the telescope was invented, appeared in the Large Magellanic Cloud in 1987. It was visible from the southern hemisphere for months and is the best-observed supernova to date (Figure 5.11). Neutrinos were detected exactly as theory predicted. The core temperature at explosion must have been 200 billion K! Now astronomers are using Supernova 1987A data to refine and test theories of star death.

What kind of stars die as Type II supernovas?_

Answer: Very massive stars (about 8 or more times the Sun's mass).


You might say that you are made of star dust.

Hydrogen and helium were probably the only elements in the universe when it began. Elements such as carbon, oxygen, and nitrogen, essential for life, are made inside the fiery cores of aging stars. The heaviest elements of all,

Figure 5.11. Supernova 1987A (top) in 1969 before the explosion was seen and (bottom, right of center) a week afterward, in February 1987.

such as gold and lead, are produced in the extremely high temperatures and intense neutron flux of a supernova explosion.

The supernova explosion sprays all these new elements out into space. They mix with the hydrogen, helium, and dust already there. All the material scattered into space by exploding massive stars becomes available again to be used in the formation of new stars and planets. Our Sun and Earth were formed about 5 billion years ago from a cloud of hydrogen and helium enriched in this way.

In a.d. 1054, Chinese and Native American observers recorded seeing a brilliant new star blaze in the sky even during daylight hours. The Crab Nebula in Taurus, a gas cloud expanding at 1600 km (1000 miles) per second, is observed today at the site of that supernova. It is about 3 pc (10 light-years) across, with the remnant core of the exploded star still at the center (Figure 5.12).

Which do you think are more abundant in the universe, elements lighter than iron or those heavier than iron? Why?_

Answer: Lighter elements. These elements have much more time to form. Elements lighter than iron are produced from primordial hydrogen over a long period of time inside the cores of massive stars, while those that are heavier are produced only during the brief interval when the star explodes (supernova) at the end of its life.


When a very massive star explodes, it may leave behind a star of more mass than the Sun squeezed tightly together into a ball only about 16 km (10 miles) across. This extremely dense star is made mostly of neutrons, uncharged atomic particles. It was named a neutron star when it was first hypothesized.

Pulsars, pulsating radio stars, were first observed in 1967 by Jocelyn Bell, a graduate student at Cambridge University, England. Pulsars send sharp, strong bursts of radio waves to Earth with clocklike regularity, at intervals between milliseconds and 4 seconds. Magnetars have abnormally strong magnetic fields, which lead to X-ray and gamma ray emission.

Figure 5.12. The Crab Nebula (M1) and pulsar (white dot near center) in Taurus are bright sources of radiation in all wavelengths. (a) Optical image. (b) X-ray image. (c) Infrared image.

Axis Of rotation

Observer ^ radio telescope

Figure 5.13. A pulsar or neutron star. Astronomers observe regular pulses of radiation emerging from the rotating star's magnetic poles as they sweep past Earth.

Axis Of rotation

Observer ^ radio telescope

Figure 5.13. A pulsar or neutron star. Astronomers observe regular pulses of radiation emerging from the rotating star's magnetic poles as they sweep past Earth.

Theory predicted that a neutron star should exist at the center of the Crab Nebula. A pulsar was found there in 1968 (Figure 5.12). The Crab Pulsar has since been observed over all electromagnetic wavelengths from radio to gamma.

A pulsar is a rapidly rotating, highly magnetic neutron star (Figure 5.13). Its characteristic short, regular pulses come from radiation beams, emitted by very energetic accelerated charged particles, sweeping past Earth as the neutron star periodically spins. The rotation and pulse rates gradually slow down as energy is radiated away.

X-ray bursters blast X-rays randomly. X-ray bursts come from an accreting neutron star in a binary system when its big, hot helium buildup explodes.

How would you expect the force of gravity on the surface of a pulsar to compare to the force of gravity on Earth?

Answer: Much greater on a pulsar. The force of gravity is stronger the closer matter is packed, and a pulsar is extremely dense.


A really massive star may continue to collapse after the pulsar stage to become a bizarre object called a black hole (Figure 5.14).

Figure 5.14. Artist's conception of detection of a black hole.

If black holes do exist, they are not holes at all. On the contrary, a black hole is a large mass contracted to extremely small size and enormous density. The force of gravity in such an object would be so great that, according to Einstein's theory of relativity, it would suck in all nearby matter and light.

A black hole can never be seen, because no light, matter, or signal of any kind can ever escape from its gravitational pull—hence its name. The surface of a black hole, or the boundary through which no light can get out, is called the event horizon.

The Schwarzschild radius (RS) is the critical radius at which a spherically symmetric massive body becomes a black hole. The equation is:

where G is the gravitation constant, M is the mass of the body, and c is the speed of light (Appendix 2). The Schwarzschild radius for the Sun is about 3 km (2 miles) while for Earth it is about 1 cm (0.4 inch).

Theory predicts that a star of over three solar masses at its final collapse must cross its event horizon and disappear from view. No known force could stop further collapse, so the star may continue to shrink to a spot at the center called a singularity.

Cygnus X-1 is an intense X-ray source over 2500 pc (8000 light-years) distant in Cygnus. Discovered in 1966, it is an eclipsing binary star (period 5.6 days) whose unseen component was the first black hole reported. The visible primary star is a blue supergiant that shows variations in spectral features from one night to the next. Presumably, when the black hole sucks in material gravitationally from its visible companion, the observed X-rays are emitted.

You will surely hear more about these intriguing black holes in the future as scientists investigate them further.

What do you think would happen if an unlucky spaceship passed very close to a black hole in space?_

Answer: The strong gravitational pull of the black hole would pull the spaceship in, producing a destructive force that would increase as the ship fell in and that would eventually tear it apart.

This self-test is designed to show you whether or not you have mastered the material in Chapter 5. Answer each question to the best of your ability. Correct answers and review instructions are given at the end of the test.

1. Define stellar evolution._

2. How do astronomers check a theory of stellar evolution?

3. List the three main steps in the birth of a star.

4. What is the main source of the energy that a main sequence star shines into space? _

5. For stars of the same initial chemical composition, what property determines the length of time it takes for the stars to evolve?_

6. Why will the Sun stop shining as a main sequence star about 5 billion years from now?_

List seven stages in the life cycle of a star like our Sun in order from birth to death.

8. List seven stages in the evolution of very massive stars in order from birth to death.

9. Why are elements that are lighter than iron, such as hydrogen, helium, carbon, and oxygen, so much more abundant in the universe than are the elements heavier than iron?_

10. Match the eight items from the theory of stellar evolution to a real sky object.


Birthplace of stars.


Betelgeuse in Orion.


Black hole candidate.


Crab Nebula in Taurus.


Blue giant.


Crab pulsar in Taurus.


Main sequence star.


Cygnus X-1.


Neutron star.


Mira in Cetus.


Pulsating variable star.


Orion Nebula.


Red giant.


Rigel in Orion.


Supernova remnant.




Compare your answers to the questions on the self-test with the answers given below. If all of your answers are correct, you are ready to go on to the next chapter. If you missed any questions, review the sections indicated in parentheses following the answer. If you missed several questions, you should probably reread the entire chapter carefully.

1. The changes that take place in stars as they age—the life cycle of stars. (Section 5.1)

2. They predict what changes in luminosity and temperature should take place in stars as they age. Then they compare these theoretical tracks of evolution on H-R diagrams with H-R diagrams for groups of real stars. (Section

3. (1) Gravitational contraction of a cloud of gas and dust; (2) rise in interior temperature and pressure; (3) nuclear fusion. (Section 5.3)

4. Nuclear fusion reactions in the core (hydrogen is converted into helium). (Sections 5.3, 5.5)

6. The Sun will leave the main sequence when all the available hydrogen fuel in its core is used up so that it no longer has an internal energy source. (Sections 5.6, 5.7)

7. (1) Protostar; (2) main sequence star; (3) red giant; (4) variable star; (5) planetary nebula ejected; (6) white dwarf; (7) dead black dwarf. (Sections 5.3, 5.5 through 5.13)

8. (1) Protostar; (2) main sequence; (3) red giant; (4) variable star; (5) Type II supernova; (6) pulsar/neutron star; (7) possible black hole. (Sections 5.3, 5.5 through 5.7, 5.9, 5.14, 5.16, 5.17)

9. Hydrogen and some helium were probably the original elements in the universe. The other elements that are lighter than iron are formed inside aging stars over a period of time. Elements heavier than iron are formed only during the brief time of a supernova. (Section 5.15)

10. (a) 6; (b) 4; (c) 7; (d) 8; (e) 3; (f) 5; (g) 1; (h) 2. (Sections 5.2, 5.5 through 5.7, 5.9, 5.14 through 5.17)

11. A superdense, gravitationally collapsed mass from which no light, matter, or signal of any kind can escape. (Section 5.17)

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