Lecture #11 -- Evolution Before and After the Main Sequence


I. Pre-Main Sequence
Before reaching the main sequence, stars are still quite bright. Their heat comes from gravitational collapse. Indeed, they tend to be a little above the main sequence on the HR diagram, so they are more luminous prior to hydrogen fusion. The more massive the star, the faster it will reach the main sequence, and it is possible for a cluster to have massive stars that have already left the main sequence before the low mass stars have even made it there yet. Before reaching the main sequence, stars are shrouded in dust so radiate mostly in the infrared, and they break out in the T Tauri phase. Sometimes this involves interesting dynamics like the formation of polar jets, such as seen in Herbig-Haro objects. Transport of mass and angular momentum away from the star occurs in this phase.
II. More than One Main Sequence: Population I and II, and H and He-burning
There are actually several different main sequences, depending on the composition of the star. In globular clusters we find the Population II main sequence, shifted slightly to the blue and comprising of stars with low metallicity formed in an earlier epoch of the galaxy when metals were rarer (remember metals are made over time in stars). Elsewhere we find the normal Population I main sequence. These are hydrogen-burning main sequences, but there is also a helium-burning main sequence for when helium is burned in the core after the star has evolved into a giant.
II. After the Main Sequence: What happens to the core
A star leaves the main sequence when it runs out of hydrogen in its core. Without this nuclear fuel, the core has no heat source, but the star continues to radiate and lose energy, so the core must collapse as pressure loses its battle with gravity. However, the collapse heats the interior, and soon a shell outside the core, which still has hydrogen because it was below 10 million K before, now goes above 10 million K and begins to fuse hydrogen into helium. This is called shell burning. The behavior of the star is then a little weird-- the region inside the shell continues to collapse and get hotter because it still has no energy source, but outside the shell there is lots of heat being pumped into the "envelope", and so the envelope does the exact opposite of the core-- it expands and cools. As the surface cools it gets redder, and the expansion of the envelope makes the star quite large-- hence it is called a red giant or even a red supergiant.
II. As the temperature rises...
The core continues to collapse and get hotter, and soon even helium can be fused into carbon, via the triple alpha process. This requires extremely high densities to happen fast enough, and indeed the only place in the history of the universe where the triple alpha process is important is in the cores of evolved stars. This is rather important because the triple alpha process is a key bottleneck in the path to making metals. (Read: no triple alpha, no metals.) The triple alpha process then provides the core with a new source of heat. For massive stars, this is the helium-burning main sequence. Less massive stars like the Sun have an interesting phase prior to the helium main sequence, which occurs because in these stars the core density gets so high in the core that by the time it gets hot enough to fuse helium the pressure takes on a very new character, called degeneracy pressure. The behavior of this new pressure is very odd, and does not have the safety-valve effect that stabilizes hydrogen fusion. Hence the helium fusion is unstable, and tends to have a runaway effect called the "helium flash". A small but energetically significant fraction of the helium in the core burns in an unbelievably short time! Why doesn't the whole star blow up? Because it takes even more energy than this to blow up a star, as the expanding envelope soaks up a lot of energy in the form of gravitational potential energy. Once the temperature in the core gets high enough the degeneracy is lifted, and this allows the core to begin to expand as well, and the fusion stabilizes and the star enters the helium-burning main sequence alongside the more massive stars.