There are two types of bright nebulae that are associated, not with star birth, but with star death. The first of these are planetary nebulae, so called because they're round objects that resemble planets. A planetary nebula is the detached outer atmosphere of a red giant star, which is one of the final stages in a medium-sized star's lifecycle. This is how planetary nebulae come to be:
- An aging star, low on hydrogen as fuel, begins to burn helium.
- It continues to burn hydrogen in its outer layers and, as it does, it swells up to a giant size.
- The surface cools and reddens.
- The giant star becomes unstable and ejects its outer layers.
- This ejected material forms a planetary nebula, which surrounds a hot, bluish-white core.
- Heat from the core makes the nebula glow.
Courtesy NASA and STScI
A good example of a planetary nebula is the Eskimo Nebula, which is located about 5,000 light-years from Earth in the constellation Gemini. Discovered by William Herschel in 1787, the nebula gets its name because, when viewed through ground-based telescopes, it resembles a face surrounded by a fur parka. The parka is actually a ring of material streaming away from a central, dying star.
If a star is massive enough, it doesn't die as a red giant, but as a supernova. A supernova occurs when a star explodes and throws off most of its material into space. When a supernova involves a binary, or two-star system, it's known as a Type 1 supernova. When a supernova involves a lone star, it's known as a Type 2 supernova.
In Type 1 supernovas, one star in the binary system is a white dwarf, a dying star that has consumed almost all of its hydrogen. The white dwarf pulls material from the outer layers of its companion star. This material burns in the dwarf's outer regions, causing its core to heat up to extreme temperatures. As the white dwarf is consumed in a runaway reaction, it explodes, expelling its remains in a vast cloud -- a nebula. On average, a Type 1 supernova occurs in a galaxy once every 140 years [source: Ronan].
Type 2 supernovas occur more frequently, perhaps once every 91 years in a galaxy [source: Ronan]. In a Type 2 supernova, a single star experiences a sudden collapse. The core of such a star becomes massively dense -- a tightly packed ball of neutrons. As the rest of the star's material falls inward under its own weight, it hits the core with such force that it "bounces" back outward again in a magnificent explosion. This explosion forms a visible nebula that can be observed easily from Earth.
The best-studied Type 2 supernova is the Crab Nebula, discovered in A.D. 1054 by Chinese and Arab astronomers, who believed they were looking at a new star. The "star" became brighter during several weeks and, by July, could be observed for 23 days even in the daytime. It remained visible to the naked eye for about two years. The supernova SN1987A, in the Large Magellanic Cloud, is another Type 2 supernova that exploded in 1987. Its nebula expanded to the diameter of Earth's orbit around the Sun -- 300 million kilometers -- in just 10 hours [source: Ronan]. You might think that such discoveries are rare, but as we'll see in the next section, astronomers continue to find new nebulae and find out new things about nebulae that have been studied for years.
Courtesy NASA and STScI
You might think that such discoveries are rare, but as we'll see in the next section, astronomers continue to find new nebulae and find out new things about nebulae that have been studied for years.