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As an infrared source, the star continues to lose mass until eventually, so much of the stellar envelope is gone, the hot stellar interior is exposed. The temperature of the core is so hot, it produces enough ultraviolet radiation to ionize the shell of gas and dust around it.
In a classic example of astronomers' poor choice of names, these ionized shells are called planetary nebulae. Why? In the smaller telescopes used in the 19th century, these nebulae resembled planets some. They didn't move around the sky like planets do, but they did look like discs of light instead of point sources.
For some pretty nice pictures of some planetary nebulae taken by the Hubble Space Telescope, go to the planetary nebula page.
Some people mistakenly call the phase when a star evolves into a planetary nebulae a "nova." I can recall an episode of Star Trek where Scotty warns Captain Kirk to get off of a planet because its sun is "gonna go nova any minute now."
Actually, while it must be a beautiful event to witness, it takes thousands of years for a dying star to produce a thick circumstellar dust shell and then ionize it. The details of this process remain poorly understood.
The hot stellar interior of a planetary nebula collapses into what is called a white dwarf. Imagine half the mass of the sun crammed into a volume the size of the Earth! At this point, every electron in every atom has been compressed down as close to the atomic nucleus as it can get (according to the Pauli Exclusion Principle). The force preventing any further collapse is called electron degeneracy pressure. For most stars, this is the final resting place, a dense, degenerate object slowly cooling into what one day could be described as a black dwarf.
Sometimes in binary systems, you'll get a white dwarf in orbit around a red giant. If the giant expands enough, it'll start to dump material onto the surface of the white dwarf. When this gets hot enough, it'll ignite in a runaway thermo-nuclear reaction, and that's what a nova really is.
In the 1930s, a young astrophysicist named Chandrasekhar showed that if a white dwarf was more massive than 1.44 times the mass of the Sun, something amazing happened. The force of gravity was enough to overcome the electron degeneracy pressure, and the star would collapse into an even denser state. Stars more massive than approximately 5-7 times the mass of the Sun don't become giants - they become supergiants. Their mass is so tremendous, the core can get hot enough to fuse elements even larger than helium into more massive elements. As the star evolves, it will get hot enough in its core to fuse heavier elements, one at a time. Its interior will be layered like an onion, with progressively hotter layers deeper and deeper, fusing heavier and heavier elements. Each layer moves outward from the core and represents a temporary source of energy to halt the collapse process. The dying star exhausts each more massive fuel source in the core more furiously and faster than the one before.
The problem with this strategy is that it is doomed to failure. As long as you're fusing elements lighter than iron, you get energy out of the reaction. But try a reaction where the end product is more massive than iron and you'll discover that the reaction absorbs energy instead of producing energy. The same principle explains why both hydrogen fusion and uranium fission produce energy. As long as you're moving to atomic numbers closer to iron, you're generating energy.
So all of a sudden, the star, trying to hold itself up, runs out of fuel. The consequence is sudden and dramatic: a supernova. The star collapses in on itself, and material rebounds off of the central core on a tremendous explosion. When a nearby star goes supernova, it will shine so brightly that we can see it during daylight. If a supernova happens really close, we'd be in VERY BIG TROUBLE!
In the core, the force of gravity is sufficient to overcome the electron degeneracy pressure, and the electrons are driven into the atomic nuclei. Each electron combines with a proton, producing a massive sphere of neutrons. A typical neutron star weighs more than 1.44 times the mass of the sun, and yet it's so dense, it's only a few kilometers across! Most towns are larger than that.
You might guess that with sufficient mass, even the neutron degeneracy pressure holding up a neutron star can be overcome. When this happens, the star collapses in on itself completely. It becomes so dense, only objects travelling faster than the speed of light could escape its gravity. And since nothing can travel faster than the speed of light (according to Einstein), nothing can escape, not even light itself.
It's a common misconception that all stars eventually become black holes. (The people at Trivial Pursuit seem to think so - I once lost a game because of that question). Actually, most stars will eventually become white dwarfs. Most of what's left become neutron stars. Only the most massive of stars become black holes.
Last modified 11 January, 2021. © Gregory C. Sloan.