Lecture 7. Death of High Mass Stars
Massive stars live fast, and die young! They are more massive, so the inward pressure on the core due to gravity is larger, so to balance this the thermal pressure of the hot gas in the core needs to also be bigger, so they are hotter. Fusion reactions are very sensitive to temperature and go MUCH faster when the temperature is larger. And the higher temperatures mean that the star can burn Hydrogen into Helium via the faster Carbon-Nitrogen-Oxygen (CNO or carbon) cycle, as well as by the proton-proton chain. So the star gets through its Hydrogen fuel very quickly compared to the solar mass stars, and leaves the main sequence. High mass stars follow the same sort of evolution as low mass stars (except that they are so luminious that they are called supergiants rather than red giants) up to the point where the low mass star had a carbon/oxygen core. For low mass stars this is the end since their gravity is not strong enough to heat the core up enough to start other nuclear fusion reactions. But in high mass stars gravity is stronger, so the core temperature can rise so that the star can fuse Helium onto carbon to get oxygen and Helium onto oxygen to get Neon, and Helium onto Neon to get Magnesium.... all the way up the periodic table in steps of 2 (since the Helium nucleus has two protons), making all the heavy elements by nucleosynthesis But there must be a problem somewhere, since we know we can get energy from FISSION (splitting apart) of high atomic number elements (atomic number is just the number of protons in the nucleus) like uranium. Uranium doesn't give us energy is we add a Helium nucleus on - it gives us energy if we take one away!!! There must then be a crossover point somewhere, where we get very little energy for either fusion or fission. This crossover is at iron. When the star starts fusing lower atomic number elements into iron then the iron core just builds up. There are no more sources of fusion energy. The iron core contracts and heats up, but there is nowhere to go to get energy. In the end it is held up by electron degeneracy pressure, but the iron ash from nuclear reactions in shells around the core keeps on falling onto the core making it more and more massive, so gravity squashes it more and more...
How much mass can the electron degeneracy pressure hold ? When gravity can only be balanced if the average electron is moving at the speed of light then it all goes horribly wrong (Einsteins theory of special relativity - but ignore the maths). For a core mass of about 1.4 times the mass of the Sun the electrons can't hold up any more. The core collapses catastrophically , falls in, bounces, and the explosion tears apart the outer layers of the star in a dramatic supernova explosion . (More pictures of supernovae .
Much of the star gets ripped apart, forming an X-ray hot supernova remnant , which then cools as it expands. The temperatures and pressures in this explosion are enough to fuse higher atomic number elements, even though this takes energy, rather than providing energy. All the elements formed in the star get scattered across space in the explosion: this is the way in which the elements heavier than Helium formed (including those in us)!!
An overview of stellar evolution for different mass stars as seen in the Hertzsprung-Russell diagram for different star clusters
But what happens to the core ? The collapsing electrons get close enough to the protons to find life easier if they interact, forming a neutron (and a neutrino). Neutrons are like electrons in that they don't like being stuck in a box either, but they are more massive particles, so they have more energy, so the wavelength associated with them is smaller. So they can fit in a much smaller box. This is a neutron star , held up by neutron degeneracy pressure. But again, there is a limit to the mass of the stellar core which can be held up by this - when the mass is so big that gravity can only be balanced by neutron degeneracy pressure from neutrons approaching the speed of light. It all goes horribly wrong (again), but the core has nowhere left to go. A white dwarf has the mass of the Sun, squashed into an object the size of the Earth (the Earths diameter is 100x smaller than the Sun's). A neutron star is 2000x smaller than a white dwarf, so we have the mass of the Sun, squashed into a region the size of Newcastle (20 km diameter). And now gravity is very very strong, and we have to use Einsteins general theory of relativity to describe it. We have only to squash a neutron star by another factor of 3 before the gravity is so strong that light cannot escape. This is the definition of a black hole, space so warped that even light is forever trapped inside.
More on black holes and a virtual trip into a black hole