Eventually stars die. We've discussed what happens to stars until just before death, and now we'll explore how stars end their existence. Again, there are differences that depend on the mass of the star, so we'll begin with low mass stars and move to higher masses.
The manner in which a star dies depends on its mass after its used up all its nuclear fuel (triple alpha process, CNO, SI, etc.). Recall that as a star progresses to fusing heavier elements, much of its outer portion is ejected, forming a (planetary) nebula. Low-mass stars are those with a mass less than 1.4 Msun at the end. Models indicate that such stars begin their life on the main sequence with masses of 5 or 10 Msun.
As a star, like our Sun, approaches its end, it switches from fusing hydrogen to fusing helium (the triple alpha process). The product is carbon and oxygen. When the helium is exhausted, the core collapses again, but stars with less than 1.4 Msun cannot generate high enough temperatures to begin fusing carbon and oxygen. The core collapses, but now there's no fusion process that can kick in to provide energy to balance gravity and prevent further collapse.
Fusion will not keep the core from collapsing, but a weird effect of quantum physics will. Electrons are a member of a class of particles called Fermions, in honor of the Italian-American physicist Enrico Fermi. Fermions have the odd behavior that two of them cannot occupy the same space - think of this as the ultimate in anti-social behavior. This rule is why an oxygen atom is behaves like oxygen, rather than like 8 hydrogen atoms stuck together. This rule enables transistors to have the properties they do, and therefore is vital to all modern electronic devices. This rule is necessary for superconductors to exist!
Particles that aren't Fermions are Bosons, named after the Indian physicist Bose. Bosons like to be in the same place, the ultimate in social behavior. Photons are bosons. The tendancy of photons to want to be in the "same place" is exploited in lasers. Since photons never stand still, for them the term "same place" means moving in the same direction, and a laser is a beam of photons that are all moving in the same direction, rather than spreading out in all directions like light from a lightbulb or a star.
So, what does this have to do with the collapse of a low-mass star. The core eventually stops collapsing because the electrons in the atoms get squeezed so close together that to squeeze them more would require that two electrons are in the same place. This is forbidden by the rules of quantum physics, so it stops the collapse of the core. We say that the core is supported by the pressure of a degenerate electron gas.
A star supported by the pressure of a degenerate electron gas is a white dwarf. The Indian-American physicist S. Chandrasekhar calculated how a star would shrink into a white dwarf. The surprising result, shown in Figure 22.1, is that the larger the mass, the smaller the white dwarf! This is exactly the opposite behavior we are used to.
For comparison, the radius of the Earth is about 0.02 Rsun. A white dwarf in the LMC is shown in Figure 22.3.
The radius of the white dwarf becomes zero at about 1.4 Msun. This mass is known as the Chandrasekhar limit. Stars with masses larger than the Chandrasekhar limit have enough gravity to overcome the pressure of the degenerate electron gas, and their fate will be discussed shortly.
White dwarfs shine due to the heat generated during gravitational collapse. Recall that the core is heated during collapse, but the temperature doesn't rise enough to enable the fusion of carbon or oxygen to begin. As white dwarfs shine, the heat is radiated away, cooling the star. Eventually, over billions of years, it cools off and stops shining. We than call it a black dwarf, a cold stellar corpse with the mass of a star compressed into something the size of a planet.
Observations of open clusters suggest that stars with initial masses of 8 to 10 Msun will eject enough material that their final mass is below the Chandrasekhar limit. What happens to more massive stars?
In heavier stars, the core is heated to a sufficient temperature during collapse to allow the fusion of heavier elements. Carbon can fuse to Neon, then we can get Sodium, Sulfur, Silicon, and eventually Iron. Fusing Silicon into Iron is the last energy producing reaction possible. The iron nucleus is the most tightly bound of any nucleus. Producing heavier nuclei requires a net input of energy, rather than a net output of energy.
A star that has reached this stage has the onion-like structure shown in Figure 22.4. The center is iron. Surrounding this is Silicon and Sulfur. Surrounding this is Oxygen, Neon, and Sodium, further surrounded by Carbon, Oxygen, and Neon. Further out is Helium and Nitrogen, and finally the outermost layer is composed of the remaining Hydrogen and Helium. Not shown is the surrounding nebula of material ejected earlier.
Fusion of Iron doesn't generate energy. There is nothing left to keep the core from collapsing into a white dwarf composed of iron. Fusion can still occur in the shells surrounding the white dwarf core, creating more iron that falls into the white dwarf at the center causing it to shrink more (remember the mass-radius relation for white dwarfs). Eventually, when the mass of iron in the white dwarf exceeds 1.4 Msun, it can no longer remain a white dwarf. The electrons are squeezed so tightly that they have no where to go except to combine with the protons in the iron nucleus.
When an electron and proton combine, they form a neutron (plus a neutrino). In a very rapid reaction, less than a second, all the electrons and protons combine into neutrons. Neutrons are fermions, just like electrons, so they can be squeezed, but only up to a point. Then the degenerate neutron gas pushes back and stops further contraction. It stops, that is, if the mass of the ball of neutrons is less than 3 Msun. If the mass is less than 3 Msun, the contraction stops and we're left with what is called a neutron star. If the mass is greater than 3 Msun, the contraction continues, and now there is nothing left to stop the collapse -- the core turns into a black hole.
The collapse to a neutron star is very rapid. In less than a second, the white dwarf core that was about the size of the Earth collapses to a ball less than 20km in diameter. Material falling inward reaches speeds of ¼ the speed of light. When it reaches the surface of the neutron star, it suddenly stops and rebounds. The rebounding effect propagates outward, accelerating material in the outer layers away from the neutron star. This is a supernova explosion.
A supernova explosion is characterized by a burst of neutrinos, and a tremendous release of energy as the outer material of the original star is blown away from the neutron star that remains at the core. The heating results in a tremendous increase in the brightness of the star for a short period of time.
Supernova explosions are fairly common, being as dramatic as they are. They've been recorded throughout history as the appearance of new stars. (Nova in Latin means new, and early observers believed they were seeing the birth of a new star when a supernova suddenly appeared where no star previously was seen.) There's about one supernova every 25 to 100 years in our galaxy, and they occur frequently throughout the universe.
Only about 10% of the original mass remains in the neutron star or black hole. About 90% of the original mass of a star is blown away in a supernova explosion. This material contains many of the fusion products, heavier elements like carbon, oxygen, silicon, and iron. In addition, the supernova sends off lots of neutrons, and these can be absorbed in the heavy nuclei to make still heavier nuclei. This is how we get important elements like copper, silver, gold, and lead. We are nost simply made from the stuff of stars, but also from the stuff of supernovae!
Figure 22.5 shows a before and after picture of SN1987a in the LMC. This was the most recent nearby supernova. SN1987a has been more studied than any previous supernova. We observed its visible, and infrared light, gamma rays, and neutrinos.
The table below summarizes what happens to stars (and non-stars) of different masses.
|Initial Mass||Final Mass||State at End of Its Life|
|0.01 to 0.08||0.01 to 0.08||Brown Drawf|
|0.08 to 0.25||White dwarf made mostly of helium|
|0.25 to 8 or 10||White dwarf made mostly of carbon and oxygen|
|8 or 10 to 12||<1.4||White dwarf made mostly of oxygen - neon - magnesium|
|12 to 40||<3||Supernova explosion that leaves a neutron star|
|>40||Supernova explosion that leaves a black hole|