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. So now the word ‘nebula’ is used only for interstellar clouds.

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.

Sometimes protoplanetary disks are also called protoplanetary nebulas, the gas and dust surrounding a protostar that will soon give rise to planets and asteroids. However, the usage getting out of fashion and these objects are now mainly known as proplyds.

Emission nebulas shine because their gas has become ionized due to ultraviolet radiation from nearby stars. The light we see is made of the photons emitted by electrons in the ionized gas as they recombine with protons. HII regions (hydrogen-two, ionized Hrydrogen) and planetary nebulae are important examples. Helix nebula is a planetary nebula in our galaxy and NGC 604 is a well-known HII region in the Triangulum galaxy.

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

Reflection nebulas are cold conglomerations of gas and dust near bright stars. They could even be remnants of the interstellar cloud from which the nearby star formed. Their dust scatters the light coming from these stars and we see the scattered light. Because short waves are scattered more than long waves, reflection nebulas are usually bluish. The With Head nebula in the Orion constellation possibly reflects the light of Rigel, one of the brightest stars in the sky.

They are also known as absorption nebulas because they absorb the light coming from neighboring stars. The absorbed visible light is then either scattered or re-emitted at infrared wavelengths. A famous example is the Horsehead nebula which is seen below in the infrared vision of Hubble.