A nebula is a cloud of gas and dust in space. A nebula can be "lit up" by the energy of one or more nearby stars (including stars inside the nebula). In this state, the cloud of gas and dust becomes visible to us (although most are only visible with the aid of telescopes). The plural of nebula is nebulae (pronounced neb' u lee). Nebulae are found in galaxies. Some nebulae are small while others are huge. Some can contain thousands of stars while others surround just a single star. Some nebulae are "star nurseries," otherwise known as star birth nebulae, where new stars are formed. Other types of nebulae, called star death nebulae, are produced when a star loses its raw material at the end of its life.

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Star Birth Nebulae

A star birth nebula (or star formation region) is a cloud of gas and dust in which new stars are being formed from the raw material of the cloud. In places where the cloud has become compressed (where the gas and dust are very close together), the gas and dust start "clumping." At a certain stage, these clumps start heating up in the center, and when that happens, star formation begins. After many years of clumping and heating, a star will get so hot in the middle (over 10 million degrees Centigrade) that it will begin to make its own energy – it starts to shine. In many of the star forming regions depicted on the Cosmic Decoders Cards, new stars have already begun to shine, and their energy makes the clouds of gas and dust around them glow with beautiful colors.

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Star Death Nebulae

When a star nears the end of its existence, astronomers say it begins to "die." One characteristic of dying stars is that they lose the raw material that helped form them. A star death nebula is thus formed from the raw material expelled from a dying star. The way in which the raw material is expelled determines the type of resulting star death nebula. There are two types of star death nebulae: planetary nebulae and supernova remnants.

A planetary nebula has nothing to do with planets. Instead, it is created when a dying star loses material all around it. This material leaves the star in one or more "last gasps" before the star dies, and creates a sort of "shell" (made mostly of hot gas) around the dead star. Sometimes the gas comes off pretty much uniformly, giving the shell a ball or donut shape (as seen in the Retina Nebula from the Cosmic Decoders Card Set). At other times, old material that is already around the star closes off some "avenues of escape" for newly lost material (which creates yet its own shell). Then the shape of the nebula can be complicated, as you can see in the Saturn Nebula from the Cosmic Decoders Card Set. 

We can see the glowing shells of gas from planetary nebula because the atoms within are "excited" (energized by heat or radiation) by the energy of the hot dead star, or star "corpse," in the middle. The dead star is called a white dwarf because it is now much smaller and white-hot. Planetary nebulae and white dwarfs form when relatively smaller stars, like our Sun, die. However, when larger stars die, another type of star death nebula, called a supernova remnant, is formed.

A supernova remnant consists of the gas and dust left over from the explosion of a star at the end of its life. Instead of "quietly" letting off some material (as the smaller stars mostly do), the really big stars blow up when they die, scattering their fragments into space at great speed. Years after the explosion, we see these fragments, or remains, of gas and dust spread out over a great distance. We can see them because sometimes a star corpse still survives (known as a pulsar, a rapidly whirling object) and it provides energy to light up the gas and dust remains (as in the Crab Nebula). In other supernova remnants, the exploded material collides with other material in space, and it is the energy of these collisions that causes the gas to glow.

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Star Clusters