A nebula is defined as a cloud of dust and gas present in the interstellar space. It is observed as dark or bright haze or mist surrounding the stars in the space. The plural of nebula is nebulae.
Generally the astronomers divide nebulae into two broad categories:
- Bright nebulae which either reflect or emit light
- Dark nebulae which are highly dense and block out the light of other stars.
Specifically nebulae can be divided into following types:
The absorption nebulae are composed of dense clouds of dust and gas. These nebulae are very dense and are capable of blocking out the light of not only the stars near them but also the light being emitted from other bright nebulae around them.
Mostly the absorptive nebulae occur in a form of clusters which are related to giant molecular clouds which are large clouds of dust and gas associated with star formation.
The absorption nebulae also exist in highly dense and isolated form called Bok globules.
These contain not more than a few hundred solar masses and under the right conditions they are capable of undergoing a gravitational collapse and form new stars.
Examples of absorption nebulae are: Cone nebula, Elephant trunk nebula and Horse head nebula.
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A star is a component of universe which is bigger than planets and has its own light and does not obtain energy form another source. Sun is also a star around which all the planets revolve.
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The emission nebulae contrary to the absorption nebulae are light emitting and luminous nebulae.
The light emitting factor for these nebulae is believed to be ultraviolet florescence caused by a hot star or a group of hot stars present inside these nebulae.
These stars emit a large quantity of ultraviolet radiation which collides with the gas and dust particles of the nebula, this energizes the particles and the nebula starts glowing, as a result.
Common examples are: The Carina nebula, The flaming star nebula, The lagoon nebula and Tarantula nebula.
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The planetary nebulae are found around the surroundings of a dying star. These nebulae are formed by the outer layers of these stars which are on the verge of diminishing. The planetary nebulae are often observed in various colors and shapes. It is important to mention that most images seen of these nebulae are colored by the astrologists themselves to highlight certain chemicals and are not the actual colors of the nebulae.
The planetary nebulae contain high portions of rare elements such as oxygen and sulfur. Common planetary nebulae are: The Ant nebula, The Dumbbell nebula, The Cat’s eye nebula, and The Eskimo nebula.
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The reflection nebulae are present around the stars and reflect the light that is emitted by them.
These stars are often confused with the emission nebulae but these two are totally different as; firstly the reflection nebulae don’t emit light instead it reflects light and secondly the emission nebulae are surrounded by highly energized stars which energize the nebulae particles as well where as the reflection nebulae surround stars which are not hot enough to them glow.
However both emission and reflection nebulae can be termed as bright nebulae.
Examples include: Corona Australis Complex and The Rho Ophiuchi Complex.
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The diffuse nebulae are highly complex nebulae which are formed by the diffusion of reflection, emission and dark dense nebulae.
They consist of emission lines from the emission nebulae which join into reflection nebulae which contain weak streaks of absorption nebulae and then join in to dark and dense nebulae which contain dense clouds of dust and gas.
The diffuse nebulae are of various shapes and may have dark or bright formations.
The supernovae remnants are special types of nebulae which are released as the debris by supernovae.
The supernovae remnants are composed of the heaviest elements in the universe and that too in very high concentrations.
This makes these nebulae very interesting and highly special.
Since there are two types of supernovae, the super novae remnants are also of two types.
The most famous supernovae remnant is from the type II supernova and is called the jelly fish supernova remnant.
Wolf-Rayet Generated Nebulae
The wolf Rayet generated nebulae are also a very special type of nebulae.
These nebulae are dense with clouds of dust and gas and are emitted by the Wolf-Rayet stars and shed into the space.
Common example is Crescent nebula .
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