Diffuse Nebulae⏩
Bright nebulae are usually vast concentrations of gas and dust in which stars have been or are being formed. There are three main types of bright diffuse nebulae:
Emission Nebulae
Interstellar clouds of hydrogen which glow because of the intense radiation of hot stars inside the nebula. Emission nebulae are typically red.
Reflection Nebulae
Clouds of gas and dust which emit no light of their own, but shine because they reflect light from nearby stars. Reflection nebulae are typically blue.
Supernova remnants
A supermassive star at the end of its life will explode ejecting the outer layers of the star into interstellar space. The most famous example is the Crab nebula which is the supernova remnant of a star that exploded in 1054.
Three famous nebulae. On the left is the Cocoon nebula (IC 5146) - a dense emission nebula in Cygnus. In the middle is M78 - a reflection nebula near the Orion nebula. On the right is the Crab Nebula - the remnant of an exploded star.
Planetary Nebulae⏩
These nebulae are formed during the last stages of a star's life when a red giant star casts of its outer layers in a process which leads to the star becoming a white dwarf. The shell of gas expands outwards and is visible for a few thousand years. The remnant of the star that is left is an intensely hot white dwarf with a surface temperature as high as 100 000K.
Three planetary nebulae imaged by the Hubble Space Telescope. On the left is IC 418 or the 'Spirograph Nebula'; in the middle is NGC 2440; and on the right is NGC 3132 or the 'Eight-Burst Nebula'.
Dark Nebulae⏩
Sometimes known as absorption nebulae these are dark concentrations of gas and dust which often have stars forming inside them. Dark nebulae can be seen if they obscure part of a bright nebula (eg. the Horsehead nebula) or if they block out background stars (eg. the Coalsack).
The Horsehead Nebula⏩
many of the dark nebula regions within 2000 light years plotted onto the galactic plane. the positions of 742 dark nebulae listed by Dutra and Bica. A few bright nebulae are also marked. These dark nebulae are the important regions of star formation in our local part of the Orion Arm. For example, the famous Pleiades star cluster consists of hot young stars that formed among the dark clouds of Taurus.
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