1. The horizontal branch is where low-mass stars "go" (on the H-R diagram) after they have stopped climbing the red giant branch. On the H-R diagram (see Figure 22-1), the horizontal branch is located a bit above the main sequence, meaning that stars on the horizontal branch tend to be somewhat brighter in luminosity than main-sequence stars with the same temperatures, and somewhat larger in size. As Figure 22-3 shows, many horizontal branch stars are whitish or even a bit bluish in color, due to the fact that they have higher surface temperatures than red giants. Unlike main-sequence stars and red giant stars, horizontal branch stars have core helium fusion and shell hydrogen fusion going on simultaneously.

2. The asymptotic giant branch is where low-mass stars "go" (on the H-R diagram) after they leave the horizontal branch. On the H-R diagram (see Figures 22-1 and 22-3), it is located in the giant-star region (cool, large, and more luminous than main-sequence stars with the same temperature). It "approaches" the red giant branch from the left (hence its name). AGB stars have a core of carbon and oxygen that sustains no fusion reactions, and also two shells that are producing energy -- one helium-burning and one hydrogen-burning.

4. A planetary nebula is formed when the envelope of a low-mass star is gently ejected. This happens after the star becomes an AGB star. (And that's all you need to know about how PNs are formed -- don't worry about the details like "thermal pulses" and "helium shell flashes.")

6. A white dwarf's radius is inversely related to its mass -- that is, white dwarfs with more mass are smaller. This is because the more massive they are, the tighter gravity squeezes them. Main-sequence stars don't work this way; the more massive a main-sequence star is, the larger it is (check out Figures 19-15 and 19-23 to see that this is true).

As an aside, there is no simple relationship between mass and size for giants and supergiants. And please note that the fact that they are larger than main-sequence stars does NOT make them more massive! (Think about where giants and supergiants come from, and you should see why this is the case.)

7. The Chandrasekhar limit (1.4 solar masses) is the maximum mass that a white dwarf can be. Degenerate-electron pressure (which is what holds up a white dwarf against its own weight) is not strong enough to support more than this mass.

9. White dwarfs have low luminosities (generally 100-10,000 times less luminous than the Sun -- see Figure 19-14), so they are difficult to see at great distances.

10. White dwarfs are made of mostly carbon and oxygen, but they are not hot enough to fuse either of those elements and thereby make energy. The white dwarf's weight is supported not by thermonuclear fusion, but by degenerate-electron pressure -- the tendency of densely-packed electrons to resist being packed any more closely, due to the Pauli Exclusion Principle (two electrons can't be in the "same place" at the same time).

11. The mass of a star determines how strongly gravity compresses it. Since the star's pressure must balance gravity to maintain hydrostatic equilibrium, and since the temperature of the star's interior is related to its pressure, and since the temperature and pressure in the star's core determine the rate of nuclear fusion reactions there, the mass of a star determines how quickly it goes through its evolution. Also, since more massive stellar cores will be compressed more when the fusion reactions stop, higher temperatures will be reached, and heavier elements will be able to fuse to provide more energy for the star. Finally, when all available nuclear fuel is exhausted, the mass of the star's core determines whether it can be supported by degenerate-electron pressure (a white dwarf) or degenerate-neutron pressure (a neutron star) -- or whether it will collapse to a geometrical point of no size at all (a black hole singularity).

12. Heavier nuclei have more positive charge (more protons), and therefore must be moving faster to overcome electromagnetic repulsion and fuse with other nuclei. Since the temperature in the core is related to the speed of the nuclei there, it determines what types of nuclear reactions are possible.

13. Red supergiants are more luminous and larger than red giants (they both have roughly the same surface temperature, however). All stars become giant stars at some point in their evolution, but only high-mass stars can ever become supergiants (because they can activate more shells of nuclear fusion).

14. Nuclear density is the density with which protons and neutrons are packed together within an atomic nucleus. It is also the density with which neutrons are packed together in the neutronium core of a high-mass star that is about to go supernova. Degenerate-neutron pressure becomes possible at this density, and in fact it is this pressure which causes the high-mass star's collapsing neutronium core to "bounce", propelling a pressure-wave through the star's envelope and ripping it apart in a Type II supernova.

19. Our Sun cannot become a supernova. Only high-mass stars undergo Type II supernovae, but our Sun is a low-mass star. Its core will become a white dwarf, and since the Sun does not have a companion star that could add mass to it, it will never undergo a Type Ia supernova either.

Last edited 10 Apr 04 M. A. Weinstein.