A cloud-like region of gas and dust that shines either by its own light (emission nebula) or by reflected light (reflections nebula), if it is a bright nebula, or simply absorbs light falling onto it if it is a dark nebula.

A nebula seen in silhouette as it absorbs light from behind; also, called a dark nebula.

1. A nebula which, as a result of ionized gas within it, shines by its own light.
2. A gas cloud that receives energy from a hot star, allowing it to give off radiation in emission lines; such as, those of hydrogen.

The characteristic reddish radiation of many emission nebulae is mostly from the hydrogen-alpha line.

1. The cloud of expanding gas surrounding a star that has blown off its outer layers, possibly in a nova stage.
2. A shell of gas thrown off by a star at the end of its life.

Planetary nebulae have nothing to do with planets. They were named by William Herschel, who thought their rounded shape resembled the disk of a planet.

After a star; such as, the sun has expanded to become a red giant, its outer layers are ejected into space to form a planetary nebula, leaving the core as a white dwarf at the center.

A nebula that shines as the result of the scattering of the light of a star or group of stars nearby.

Such scattering is usually caused by dust within the nebula.

A supernova remnant formed as a result of a supernova explosion about 30,000 years ago.

Still expanding, the nebula will eventually become indistinguishable from interstellar gas.