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 located outside our Galaxy; these 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. The light we see is nothing but the photons emitted by the free electrons in the ionized gas as they recombine with protons to form neutral atoms. HII regions (hydrogen-two, ionized Hrydrogen) and planetary nebulae are important examples.

Some emission nebulas radiate not via recombination, but because their free electrons emit photons while gyrating around magnetic field lines through a mechanism called synchrotron. Remnants of supernovae are of this ype, examples include the famous Crab nebula.

Reflection nebulas are cold collections of gas and dust near bright stars. They could be even remnants of the interstellar cloud from which the star formed. Their dust scatters the light coming from nearby stars and we see the scattered light.