Chapter 18

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. Which of these objects has the largest radius? (a) a $1.2M_{\text{Sun }}$ white dwarf (b) a $1.5M_{\text{Sun }}$ neutron star (c) a $3.0M_{\text{Sun }}$ black hole.

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. What would happen if the Sun suddenly became a black hole without changing its mass? (a) The black hole would quickly suck in Earth. (b) Earth would gradually spiral into the black hole. (c) Earth would remain in the same orbit.

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. Which of these neutron stars must have had its angular momentum changed by a binary companion? (a) a pulsar that pulses 30 times per second (b) a pulsar that pulses 600 times per second (c) a neutron star that does not pulse at all.

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. What would happen to a neutron star with an accretion disk orbiting in a direction opposite to the neutron star's \(\operatorname{spin}\{(a)\) Its spin would speed up. (b) Its spin would slow down. (c) Its spin would stay the same.

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. Which of these binary systems is most likely to contain a black hole? (a) an X-ray binary containing an O star and another object of equal mass (b) a binary with an X-ray burster (c) an X-ray binary containing a G star and another object of equal mass.

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. Which of these black holes exerts the weakest tidal force on an object near its event horizon? (a) a $10M_{\text{Sun }}$ black hole (b) a $100M_{\text{sun black hole }}(\mathrm{c})$ a ${10}^6{\mathrm{M}}_{\text{sun }}$ black hole.

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. Where do gamma-ray bursts tend to come from? (a) neutron stars in our galaxy (b) binary systems that also emit X-ray bursts (c) extremely distant galaxies.

Unanswered Questions. We have seen in this chapter that theoretical models make numerous predictions about the nature of black holes but leave many questions unanswered. Briefly describe one important but unanswered question related to black holes. If you think it will be possible to answer that question in the future, describe how we would find an answer, being as specific as possible about the evidence necessary to answer the question. If you think the question will never be answered, explain why you think it is impossible to answer.

Life Stories of Stars. Write a one to two-page life story for the scenarios in Problems 39 through $42.$ Each story should be detailed and scientifically correct but also creative. That is, it should be entertaining and at the same time prove that you understand stellar evolution. Be sure to state whether "you" are a member of a binary system. You are a neutron star whose mass is $1.5{\mathrm{M}}_{\mathrm{Sun}}$.

Be sure to show all calculations clearly and state your final answers in complete sentences. A Black Hole I? You've just discovered a new X-ray binary, which we will call Hyp-X1 ("Hyp" for hypothetical). The system Hyp-X1 contains a bright, B2 main- sequence star orbiting an unseen companion. The separation of the stars is estimated to be 20 million kilometers, and the orbital period of the visible star is 4 days. a. Use Newton's version of Kepler's third law to calculate the sum of the masses of the two stars in the system. (Hint: See Mathematical Insight $15.4.$ ) Give your answer in both kilograms and solar masses $(M_{\text{Sun }}=2.0\times {10}^{30}\mathrm{kg})$ b. Determine the mass of the unseen companion. Is it a neutron star or a black hole? Explain. (Hint: A B2 mainsequence star has a mass of about $10M_{\text{Sun. }}.)$