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Nebulae Nebulae are clouds of gas and dust, illuminated by starlight. In most regions of interstellar space, there is about 1 atom per cubic centimeter. However, in some regions, the density is over 1000 atoms per cubic centimeter. (This still about the density of the best vacuum made by humans!) These regions are called nebulae.

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Bright Nebulae

A nebula by itself cannot produce any light energy. But if there are some bright stars inside the nebula, the gases in the nebula may be ionized by energy from those stars and emit light. This is exactly how the gas in a neon tube emits light, when ionized by electricity. Thus, this kind of nebula is known as an emission nebula. The most common gas in interstellar space is hydrogen, which emits red light when ionized. So, emission nebulae usually appear reddish in color. A second kind of bright nebula reflects light from nearby stars in front of it, rather than emitting it from ionized gas. This kind of nebula is known as a reflection nebula, and is typically bluish in color. Emission and reflection nebulae are often found near each other - a classic example is the Trifid Nebula (M 20) in Sagittarius. The Trifid Nebula, M 20. (Jim Misti.) Nebulae are the birthplaces of the stars, and are often associated with the star clusters that are forming from it. The typical size of a nebula is about a hundred light years. Most nebulae are located in our Milky Way galaxy, and so (like open clusters) are concentrated along the plane of the Milky Way in our sky.