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A nebula is a localized cloud of dust and gases (mostly hydrogen) in intergalactic space. Nebulae are often responsible for star formation. Gravity causes a portion of a nebula's dust and gases to contract into a much smaller and denser mass. Pressure and temperature build up within the mass as contraction continues through millions of years. In time, the mass becomes hot enough to shine -- a star is born.
Types of Nebulae
Emission Nebulae are so-called because they generate and emit their own light. This type of nebula is always associated with an extremely hot, bright star. The intense ultraviolet light from the star energizes the gas atoms in the cloud, but since gases do not like being energized the atoms get rid of the extra energy by emitting it in the form of light. Most emission nebulae appear red because hydrogen is the most prominent gas in the universe and hydrogen atoms generate red light when energized.
Left: The Orion Nebula, which makes up part of the sword in the constellation Orion, is the most well-known example of an emission nebula.
A Reflection Nebula is a nebula near a cool star that does not emit enough ultraviolet light to energize the gas atoms in the cloud. Such a nebula is incapable of emitting its own light, but the dust particles can still reflect visible light, hence the name. This kind of nebula looks blue in color when viewed from Earth because the dust scatters the blue light while allowing the rest of the light spectrum to pass through undisturbed (the same phenomenon that gives our sky its blue color).
Right: A reflection nebula surrounds the Pleiades Cluster.
Planetary Nebulae are so-called because many of them superficially resemble planets when viewed through a telescope. They are formed when a red giant or supergiant star begins to shed its outer layers. When such a star begins to run out of fuel it will expand. After millions of years it will begin to shrink down to its normal size again, and as it shrinks its surface layers are shed, leaving a visible ring (sometimes multiple rings) behind. A planetary nebula is only visible from Earth for about 50,000 years because the stellar material gradually mixes into the surrounding space and the ring becomes too thin to see.
Left: The Cat's-Eye Nebula, in the constellation Draco, is a spectacular example of a planetary nebula.
A Supernova Nebula is, as the name implies, a nebula formed by the remnants of a supernova. Nebulae of this type are strong emitters of radio waves, as the result of the explosions that formed them and the pulsar remnants of the original star.
Right: The Crab Nebula, in the constellation Taurus, is the remnant of a supernova that was seen on Earth in 1054.
If a nebula is in an area that has no nearby stars, it can neither emit nor reflect enough light to be visible on its own. This kind of nebula can only be seen as a dark cloud because the dust particles blot out the light from stars behind them and is, therefore, known as a Dark Nebula.
Left: The Horsehead Nebula is a dark nebula that is only visible because it partially blocks light from the Orion Nebula.
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This page was last updated on 04/21/2017.