The term was, originally, used for diffused astronomical objects beyond the Milky Way Galaxy. For instance, Andromeda Galaxy, which is the nearest major galaxy to us, was once referred to as Andromeda Nebula.
Nebulae are massive in size. Some of them have a diameter of hundreds of light years, while some are even visible to naked eye. The brightest nebula, Orion Nebula, occupies an area twice the diameter of the moon, and it can be viewed with our naked eye. The nearest nebula to earth is the Helix Nebula. It is 700 light years away from Earth. Most of them are less dense than any vacuum created on earth and a nebular cloud of the size of Earth would have a total mass of only a few kgs.
Formation of nebula
There are different ways and mechanisms in which nebulae are formed. Some are formed from gas that is already present in the interstellar space while some are produced with the gas and dust thrown out by dying stars. Regions where stars are formed also have nebulae.
Formation of a star out of a nebula
Nebulae are mostly made of hydrogen and helium. Gravity plays an important role in pulling all the dust and gases together. As they get together, they become denser and their gravitational pull also increases. Eventually the ball of gas and dust gets so big that it collapses from its own gravity. This leads to heating up of the materials at the centre of the cloud. The hot core starts the formation of a star.
What are the types of nebulae?
There are four major groups:
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H II regions which contain ionised hydrogen
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Planetary nebulae
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Supernova remnant
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Dark nebula
How do astronomers know what a nebula looks like?
Astronomers use powerful telescopes like NASA’s Spitzer Space Telescope and Hubble Space Telescope to take pictures.
