The Lives of Stars		Here is a table showing the difference in the lives of low, intermediate, and high mass stars: Low Mass Intermediate High Mass Mass (Suns) 0.1-2 2-8 8-~100 M.S. Fusion p-p chain CNO cycle CNO cycle Later Fusion triple alpha triple alpha, helium capture, etc. triple alpha, helium capture, etc. Develops degenerate core? Yes Yes No Helium flash Yes No No Fusion stops at: Carbon Carbon Iron Corpse White Dwarf White Dwarf Neutron star or Black hole Exceptions: some very low mass stars don't fuse up to carbon and leave helium white dwarfs (less than 0.5 solar masses.) Flowchart on stellar evolution from U of Washington.
Low Mass Stars -- Evolution		A low-mass star (like our Sun for example)--any star less than about twice as massive as our Sun follows through the following life steps: Main Sequence -- "burning" (fusing actually) hydrogen to form helium. Helium forming means there are fewer particles (1 helium is formed from 4 hydrogens), and this means that the core will have less pressure, so it shrinks and heats up, speeding up the fusion. The Sun's luminosity has already increased about 40% since it was born. The Earth doesn't appear to be that much hotter though so perhaps the Earth used to have a stronger Greenhouse effect? Going up the Red Giant Branch: helium builds up in the core, and fusion of hydrogen to helium (still through the p-p chain) starts in a shell surrounding the core. Over the next 100s of millions of years, a star like the Sun would have its core contract to 1/3 of its size, and heat up by 10 times--meanwhile the outer layers expand and cool Red Giant--all the fusion is in the shell surrounding the core. The Sun will then have expanded to around the orbit of Venus, around 100 times bigger than it is now. Helium Flash--the core has become degenerate. The life of a star is marked by a battle between two forces--gravitation pulling the star together and presure pushing it apart. For our Sun, they are in balancy because of a "thermostat" mechanism. If the Sun heats up in the core, it will expand, causing it to cool and the fusion to slow down. If the Sun's core were to shrink it would heat up, fusion would speed up, and that would cause it to expand again. The Sun's pressure is thermal pressure, pressure from the random motion of a hot gas pushing against its boundaries. As the core becomes more and more dense as a star evolves, however, a different kind of pressure takes over, called degenerate pressure. Degenerate pressure is not caused by heat but by the Pauli Exclusion Principle, a basic law of physics. The Pauli Exclusion Principle is what keeps all the electrons in an atom from falling down to the lowest level, thus making chemistry possible. It says that no two fermions can be in exactly the same state. First, what's a fermion? And second, what's a state? The particles of physics come in two categories, bosons and fermions. Bosons, light the photons that make up light, can be stacked exactly on top of each other. This is what happens in a laser. The individual photons "lose their individuality." Fermions, however, include electrons, protons, and neutrons--the particles of normal matter. They can not be stacked into exactly the same state. "State"--what does that mean? Well, to understand this properly you'd need to worry about quantum mechanics. Think of the "state" of a particle as being all the physical information about a particle that is smeared out into a cloud of probability: the state says where a particle could be and how fast it could be moving.