17. Drs. Harden, Petrie, and Rozyczka discover a binary system consisting of a white dwarf of about 1.4 solar masses and a red giant of about 2 solar masses. They report that the red giant seems to be losing mass, which is going onto the white dwarf. What will happen to this system when the white dwarf exceeds the Chandrasekhar limit?
a. the white dwarf will collapse into a black hole
b. a Type I supernova
c. a Type II supernova
d. mass will start flowing back onto the red giant

A Type I supernova occurs when the mass of a white dwarf exceeds the Chandrasekhar limit. In fact, the Chandrasekhar limit is about 1.4 solar masses, so this system is going to have a Type I supernova relatively soon.

18. Which of the following fusion reactions cannot produce energy to keep a star from collapsing in on itself?
a. hydrogen into helium
b. helium into carbon
c. silicon into iron
d. iron into tellurium

Fusion of iron takes up energy. All the others produce energy. Hydrogen fusion powers Main Sequence stars, helium fusion powers horizontal branch stars, and silicon fusion powers very massive stars just before they explode in a Type II supernova.

19. How big is a neutron star?
a. about the size of San Francisco (about 12 kilometers across)
b. about the size of the Earth
c. about the size of the Sun
d. about the size of the orbit of Mars

Neutron stars are incredibly dense. They have a mass of about 3 solar masses, in an object the size of San Francisco. This is the density of atomic nuclei.

20. Why are black holes black?
a. because they are white dwarfs that have lost all their remnant heat
b. because they are so hot they only emit light in the ultraviolet
c. because their escape velocity is greater than the speed of light, so any light they emit can't escape
d. because they are moving away from us so quickly, their light is redshifted out of the visible range

A black hole is defined as an object whose escape velocity is greater than the speed of light. Something inside the black hole could be emitting all kinds of light, but the light cannot escape, so no observer outside the black hole could ever see it.