One of the consequences of this faster power generation is that more photons are produced. Remember when we said that the dust tails of comets are swept away by light pressure from the Sun? In a similar way, the photons produced in the CNO cycle are numerous enough to cause a significant pressure, called radiation pressure, that helps support the star against gravity. Show During this stage, the star is burning hydrogen to helium in its core, and so it is sitting on the Main Sequence. Depending on its mass, it may live only 1-100 million years in this stage--much shorter than the Sun's lifetime. Once the high mass star starts to run out of hydrogen in the core, and starts burning hydrogen in the shell, it expands into a Red Giant stage just like we saw for low mass stars. But there is no helium flash. The helium core is so hot that nuclear fusion begins there slowly over time, without degeneracy pressure becoming a factor. The star slowly moves back toward the Main Sequence, and burns helium in its core, but the rate is so high that the star runs out of helium in just a 100,000 years or less. Once the high mass star reaches the Red Supergiant stage, and is burning helium in a shell around the inert carbon core, the core can reach a high enough temperature (600 million K) for carbon to fuse into heavier elements! This is different from the low-mass star case, where the temperature to fuse Carbon is never reached. But the carbon is exhausted in only a few hundred years, and the next stage of still heavier elements begins. Each stage lasts a shorter and shorter time, partly because the reactions are less and less efficient -- each reaction produces less energy than the previous one. The reactions can become quite complex inside the star, because as the inner core is producing energy from one type of reaction, an outer shell may be producing a lower temperature reaction (e.g. burning helium to carbon, or hydrogen to helium) until the central part of the star resembles an onion with several layers all going at once.
The simplest set of reactions are called helium capture reactions, where helium is captured by a series of more massive elements in the sequence carbon to oxygen to neon to magnesium. Other reactions, which can take place only at the highest temperatures and pressures, are C + O -> Si, and Si + Si -> Fe. Once silicon is fusing into iron, the game is up for the star. No reaction with iron can release more energy. Once this begins to happen, the star has only days until the end. The star Betelgeuse is a Red Supergiant star. It is a very red star in the constellation Orion. We know that it is very close to the end of its life. It certainly has no more than 1000 years to survive, and maybe far less. It could explode into a supernova tomorrow -- we have no way of knowing. In fact, perhaps it already has exploded and we just haven't found out yet. Betelgeuse is 560 lightyears away, and so it takes light 560 years to reach us. While all of this is happening, the outer appearance of the star changes relatively slowly. Remember than O and B stars are already very luminous, so as the outer layers expand they tend to move horizontally in the H-R diagram. Their size increases, but their temperature decreases such that their luminosity is virtually the same. They tend to follow a zig-zag horizontal path in the H-R diagram, as shown in the figure below. Iron: the End of the LineSupernova (Type II) Remember that degeneracy pressure is due to the electrons all being so close together that they have no other energy states to go into. This electron degeneracy exerts the pressure that keeps the star up, but when it fails the electrons are actually forced into the nucleus, all of the protons combine with the electrons to form neutrons, and the entire core becomes one big neutron nucleus! Such a neutron core has its own degeneracy, called neutron degeneracy, which may halt the collapse of the core, but by then the core has shrink (in a fraction of a second) from an object the size of the Earth to an object only 10 km across. To the outer layers of the star, it is as if the bottom fell out and it all goes crashing into the core. At the same time, the tremendous gravitational potential energy from the collapsing core is released in that fraction of a second -- over 100 times the amount of energy that the Sun would release over its entire 10 billion year lifetime. The energy drives the outer layers of the star away in a titanic explosion called a supernova. There is so much energy in a supernova explosion that elements higher in atomic number than iron (Fe) can be created. All of the atoms in the universe that are heavier than iron were created in supernova explosions! This is called the nucleosynthesis of heavy elements. eClose Binary Stars The description we gave of the lives of stars indicates that higher mass stars live a short time, burn through their hydrogen fuel very quickly, and then end their lives in a supernova explosion. Meanwhile, lower mass stars live much longer, with a lifetime that depends inversely on mass -- the smaller the mass the longer the lifetime. Such stars end their lives in a planetary nebula phase and then as a slowly cooling white dwarf. |