Planetary nebula were so named by 18th century astronomers because through a telescope they appear as a disk not unlike the planets of our solar system. But even though they may have the appearance of planets, hence the name, they are not. They are instead, the remnants of stars with a mass less than eight solar masses that reached the end of their lives and have exploded. As such, “planetary nebula” have nothing at all to do with planets.
When a low mass star, those not more than eight times the mass of the Sun, consumes all the hydrogen fuel in its core, it begins to fuse helium into beryllium. Beryllium is relatively unstable and disintegrates back into helium atoms unless another helium atom fuses with the beryllium to form carbon. Sometimes an additional helium will fuse with the carbon atom to form oxygen. This fusion process is known as an alpha chain reaction.
There are two things that occur in a star once alpha chain reactions begin to occur. First of all the core temperature increases as most alpha chain reactions are exothermic. At the same time, the denser nuclei formed take up less space so the core contracts, generating even more heat. As the core begins to contract, residual hydrogen around the core is drawn in, heats up and begins to fuse into helium. The radiation produced heats the star’s interior, causing it to expand and become a red dwarf. As the star expands, it cools and the fusion of hydrogen around the core slows or even stops. Some of the mass of the star escapes into space, but the rest of it is drawn back into the star by its gravity. As the star collapses, it heats up again and the process starts all over.
The undulation of red dwarf stars continues until one of three things happen. If the stars mass decreases to about 7.5% of the Sun’s mass, all fusion ceases and the star becomes a burned out hulk. If, however, the star collapses with great enough inward velocity, the following explosion may blow the whole star apart resulting in an expanding nebula of gas rock and dust. The star has become what is referred to as a planetary nebula. The final fate for some hydrogen rich red dwarf stars, usually smaller than the Sun, is that they keep burning their hydrogen until it is all gone.
Red dwarfs are the most common types of stars in the Milky Way galaxy and probably the universe, but shining with a brilliance just 1/10,000th of the sun, most are invisible even with a relatively good telescope. Planetary nebula formed by exploding red dwarfs are even fainter, but there are actually quite a few within a few thousand light years of earth which can be viewed even with an amateur telescope. Of course, they will only appear as a faint disk, kind of like the planet Uranus.