Nebula (Latin for ‘cloud’) is the an umbrella term for many different types of diffuse aggregations of gas and dust found in the interstellar medium of a galaxy. They can be either nurseries of new stars and planetary systems or graveyards of old stars and their cadavers. Until the 19th century, any extended fuzzy object whose component parts could not be resolved using a telescope was called a nebula. Some of them later turned out to be outside our Galaxy; they were initially named ‘extragalactic nebula’ but now we call them simply galaxies.

Nebulas can be either bright or dark. Bright nebulas are seen against the background of a darker sky, but the dark nebulas are seen in silhouette against a brighter background. There are two types of bright nebulae: emission and reflection. The gas in an emission nebula radiates its own light, but the reflection nebulas are seen mainly because they reflect the light coming from nearby stars.

Emission nebulas shine because their gas has become ionized due to ultraviolet radiation from nearby stars. HII (hydrogen-two, ionized hrydrogen) regions and planetary nebulae are important examples.