Nebula: Definition and Types of Nebulae

A nebula is a cloud or collection of interstellar medium in one location in space. Nebulae are produced in different ways. For instance, they can be gathered together by gravity, dispersed by stars, or lit up by a powerful radiation source nearby. Most of these beautiful nebulae contain only a few thousand atoms or molecules per cubic centimetre. This is many times sparser than even the best laboratory vacuum chambers on Earth can achieve.

Types of nebulae

There are different kinds of nebulae based on their appearance {such as dark nebulae, reflection nebulae and planetary nebulae} or the physical processes that create them {such as protostellar nebulae, protoplanetary nebulae, or supernova remnants}.

Dark nebulae

Dark nebulae look like black blobs in the sky. They are generally dark because they contain mainly cold, high-density, opaque gas, as well as enough dust to quench the light from stars behind them. One example of a dark nebula is the Coal Sack Nebula.

Reflection nebulae

Reflection nebula is lit by bright, nearby light sources because the dust particles in them act like countless microscopic mirrors, which reflect light from stars or other energetic objects toward Earth. To the human eye, reflection nebulae usually look bluish. This is because blue light is more effectively reflected in this way than red light.

Emission nebulae

An emission nebula is a glowing gas cloud with a strong source of radiation—usually a bright star—within or behind it. If the source gives off enough high-energy ultraviolet radiation, some of the gas is ionized, which means the electrons and nuclei of the gas molecules become separated and fly freely through the cloud.

When the free electrons recombine with the free nuclei to become atoms again, the gas gives off light of specific colors. What colors they emit depends on the temperature, density, and composition of the gas. For example the Orion Nebula glows mostly green and red.

Quasar

Quasar refers to quasi-stellar radio source. They were thought to be stars earlier but they are not stars at all, but rather active galactic nuclei. Nowadays, the word “quasar” is often used to mean any Quasi-Stellar Object (QSO), whether or not it emits radio waves.