In about 5 billion years, when the sun shucks off its outer layers, it will create a beautiful shell of diffuse gas known as a planetary nebula. About 10,000 of these short-lived, glowing objects are estimated to exist in the Milky Way, although only about 1,500 have been detected; the unseen rest hide behind interstellar dust.
The term "planetary nebula" is a misnomer. It was coined by William Herschel, who also compiled an astronomical catalog. Herschel had recently discovered the planet Uranus, which has a blue-green tint, and he thought that the new objects resembled the gas giant.
The death of a star
At the end of its lifetime, the sun will swell up into a red giant, expanding out beyond the orbit of Venus. As it burns through its fuel, it will eventually collapse. The outer layers will be ejected in a shell of gas that will last a few tens of thousands of years before spreading into the vastness of space. The small core, a newly formed white dwarf, will illuminate those layers in a dazzling, predominantly blue-green display. [VIDEO: Earth to Be Consumed By Red Giant Star]
This process will be duplicated in stars that have up to eight times the mass of the sun. Massive stars, at the end of their evolutionary path, explode into supernovas. The expanding shell of gas forms another type of nebula: a supernova remnant. The Crab Nebula (M1) is a good example.
Other types of nebulae include emission nebulae, which are clouds of ionized gas emitting light of various colors; dark nebulae, which are clouds of gas so dense that background light is blocked; and protoplanetary nebulae, which occur when a star starts to shed its outer layers before becoming a planetary nebula.
No planets involved
The first planetary nebula to be discovered was the Dumbbell Nebula, M27, by Charles Messier in 1764. He eventually added four to his catalog of astronomical objects.
In 1790, Herschel found NGC 1514, a planetary nebula with a bright central star. He realized that these new objects were made up of gas or dust, rather than being clusters as thought at the time. Herschel identified 79 objects as planetary nebulae, but only 20 of them truly were, while 13 others that he had classified as other objects turned out to be these gaseous shells.
In living color
New technology has captured a number of phenomenal images of planetary nebulae in extreme depth. In doing so, it has revealed the complexities that could occur at the end of the life of the sun. Where scientists once thought that the gaseous layers came off evenly, images from the Hubble Space Telescope have revealed a wide array of possibilities that could be the fate of our closest star.
Dumbbell Nebula (M27): The first recorded planetary nebula, the Dumbbell Nebula lies 1,200 light-years away from Earth.
Ring Nebula (M57): The almost-perfect ring-like shape made naming M57 a no-brainer. The diffuse shell of gas and dust spread almost evenly after they were shucked off of their parent star.
NGC 1514: When William Herschel saw the bright star in the heart of this planetary nebula, he realized that he wasn't looking at clusters but through gas and dust. As a result, he coined the name "planetary nebula," because they shared the coloration of the recently discovered Uranus.
Saturn Nebula NGC 7009: Located in the constellation Aquarius, the Saturn Nebula, or NGC 7009, has a bright central star surrounded by a football shaped array of gas and dust.
Stingray Nebula (Hen-1357): The youngest known planetary nebula, Hen-1357 is as large as 130 solar systems.
SuWt2: A close binary star system creates a ring-like structure of dust and gas inside of this planetary nebula.
NGC 2818: This beautiful spreading planetary nebula is located 10,400 light-years away in the southern constellation of Pyxis, the Compass.
— Nola Taylor Redd, SPACE.com contributor