A planetary nebula is one stage of a star’s slow death. It has nothing to do with planets. The astronomer William Herschel named this kind of nebula for what he saw as a resemblance to the planet Uranus, as it appeared through primitive eighteenth century telescopes.
Perhaps 10,000 planetary nebulae shine at any one time in our Milky Way galaxy. Compared to the more than billion-year lifespan of many stars, the planetary nebula phase is short. It lasts less than 50,000 years.
How a planetary nebula forms
Fusion makes a star shine. A thermonuclear reaction in its core converts hydrogen to helium, and the reaction creates energy that radiates into space. However, as the hydrogen in the core is used up, a star’s construction changes. Many stars evolve into red giants, then into planetary nebulae, and then at last into white dwarfs.
As a star runs out of hydrogen in its core, it expands. That is, its outer layers expand while its core shrinks. Hydrogen fusion continues, outside the inner core, and that causes expansion. The core itself shrinks and heats, but the star begins to turn red as its outer layers cool. It becomes a red giant.
Huge red giants will continue to grow denser, fusing the materials they are made of into heavier and heavier elements. Helium becomes carbon, and carbon becomes nitrogen and oxygen. Eventually iron forms. Then fusion stops, and the iron begins to soak up energy. Eventually, the star explodes. The largest red giants become supernovas, vast spectacular star explosions.
Less massive red giants take a different path. The hydrogen atoms of the outer areas of the star continue to fuse into helium. Farther in though, helium is being transmuted to carbon. This fusion continues to produce energy, which powers a stellar wind that lifts away the outer parts of the star.
Hanging veils of cooler material show around the glowing core, held off by stellar wind and illuminated by starlight. That is, ultraviolet radiation emitted from the unshielded core ionizes the detached surrounding layers, and sets them aglow. The veils take many forms, as the energy emitted by the dying star interacts with gravity.
Examples of planetary nebula
One magnificent planetary nebula is the Cats Eye Nebula in Draco. The Cats Eye is the planetary nebula that William Herschel observed and likened to the planet he had discovered, Uranus.
Another is the Helix Nebula in Aquarius, which is one of the closest to earth. The Dumbbell Nebula, the first to be discovered, can be seen with binoculars.
The Ring Nebula in Lyra is just one of many nebulae that appear to be rings. They are not; they are spheres of matter spread across space. They look like rings because it is easier for observers see their sides than their centers, just as it is with terrestrial soap bubbles.
The end of a red giant
As a star approaches the end of its lifespan, heavy elements near the surface of the star are driven away by a strong stellar wind. The glowing interior is revealed, as we have seen, often through a veil of heavy elements. A dazzling sight, it enriches the universe with elements constructed by transmutation in the slowly dying star.
A star that is not massive enough to become a supernova slowly builds an inert core of carbon and oxygen. These remnant elements lack sufficient mass to fuel a nuclear fire. What is left is an incredibly dense white dwarf, which glows with residual heat. It slowly cools to its end state, and will become, when the universe is old enough, a black dwarf.
What is a planetary nebula? It is one of the last stages of a star’s long life. Lasting 50,000 years or less, the planetary nebula phase transforms a star from a red giant to a fading white dwarf.