Chapter 13 Practice Questions

Chapter 13 Practice Questions Review Questions 1. What is a molecular cloud ? Briefly describe the process by which a protostar forms from gas in a molecular cloud. 2. Why do protostars rotate rapidly? How can a close binary star system form? 3. Why does a spinning disk of gas surround a protostar? Describe key phenomena seen among protostars, such as strong stellar winds and jets. 4. What are the minimum and maximum masses for a star, and why do these limits occur? What is a brown dwarf ?

5. What is degeneracy pressure, and how does it differ from thermal pressure ? Explain why degeneracy pressure can support a stellar core against gravity even when the core becomes cold. 6. Briefly describe the Sun’s life stages after it exhausts its core hydrogen. Discuss both the changes occurring in the Sun’s core and the changes visible from outside the Sun. 7. Why does helium fusion require much higher temperatures than hydrogen fusion? Briefly explain why helium fusion in the Sun will begin with a helium flash.

13. Why can’t iron be fused to release energy? 14. Summarize some of the observational evidence supporting our ideas about how the elements form in massive stars. 15. What event initiates a supernova, and why is a neutron star or black hole left behind? What observational evidence supports our understanding of supernovae? 16. Why can the lives of close binary stars differ from those of single stars? Describe the Algol paradox and its resolution.

Test Your Understanding Does It Make Sense? Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer. 17. The iron in my blood came from a star that blew up more than 4 billion years ago. 18. I discovered stars being born within a patch of extremely low- density, hot interstellar gas. 19. Humanity will eventually have to find another planet to live on, because one day the Sun will blow up as a supernova. 20. I sure am glad hydrogen has a higher mass per nuclear particle than many other elements. If it had the lowest mass per nuclear particle, none of us would be here. 21. If the Sun had been born 4½ billion years ago as a high-mass star rather than as a low-mass star, Jupiter would have Earth-like conditions today, while Earth would be hot like Venus. 22. If you could look inside the Sun today, you’d find that its core contains a much higher proportion of helium and a lower proportion of hydrogen than it did when the Sun was born. 23. I just discovered a 3.5 M Sun main-sequence star orbiting a 2.5 M Sun red giant. I’ll bet that red giant was more massive than the 3.5 M Sun star when it was a main-sequence star. 24. Globular clusters generally contain lots of white dwarfs. 25. After hydrogen fusion stops in a low-mass star, its core cools off until the star becomes a red giant. 26. The uranium in nuclear reactors comes from supernova explosions.

32. What would stars be like if hydrogen had the smallest mass per nuclear particle? a. Stars would be brighter. b. All stars would be red giants. c. Nuclear fusion would not occur in stars of any mass. 33. What would stars be like if carbon had the smallest mass per nuclear particle? a. Supernovae would be more common. b. Supernovae would never occur. c. High-mass stars would be hotter. 34. What would you be most likely to find if you returned to the solar system in 10 billion years? a. a neutron star b. a white dwarf c. a black hole 35. Which of these stars has the shortest life expectancy? a. an isolated 1 M Sun star b. a 1 M Sun star in a close binary system with a 0.8 M Sun star c. a 1 M Sun star in a close binary system with a 2 M Sun star 36. What happens to the core of a high-mass star after it runs out of hydrogen? a. It shrinks and heats up. b. It shrinks and cools down. c. Helium fusion begins right away. Short-Answer 40. Brown Dwarfs. How are brown dwarfs like jovian planets? In what ways are brown dwarfs like stars?

Quantitative Problems Be sure to show all calculations clearly and state your final answers in complete sentences. 45. Density of a Red Giant. Near the end of its life, the Sun’s radius will extend nearly to Earth’s orbit. Estimate the volume of the Sun at that time using the formula for the volume of a sphere (V=4πr 3 /3) (V=4πr3/3). Using that result, estimate the average matter density of the Sun at that time. How does that density compare with the density of water (1 g/cm 3 )? How does it compare with the density of Earth’s atmosphere at sea level (about 10 −3 g/cm 3 )? 46. Supernova Betelgeuse. The distance from Earth of the red supergiant Betelgeuse is approximately 643 light-years. If it were to explode as a supernova, it would be one of the brightest stars in the sky. Right now, the brightest star other than the Sun is Sirius, with a luminosity of 26 L Sun and a distance of 8.6 light-years. How much brighter in our sky than Sirius would the Betelgeuse supernova be if it reached a maximum luminosity of 10 10 L Sun ?