In the cold areas of the Cosmos, where gas and interstellar dust are very abundant, it usually happens that the material is concentrated and forms huge icy clouds.
The dark nebulae, also called absorption nebulaeThey are composed mostly of hydrogen, which is the fuel of the stars.
The gravitational attraction makes the material more and more grouped. As the amount and density of the material increase, in the center of the nebula the pressure and temperature also increase.
When the pressure and temperature are high enough, the nebula collapses. A star has just been born inside. The new star begins to heat the cold cloud. When heated, the dark nebula glows reddish colors and becomes a emission nebula.
Some dark nebulae
Dark nebulae only look like black spaces in front of an illuminated background, usually an emission nebula. Some of the best known dark nebulae are:
Horse Head Nebula: In the photo above, it is the most beautiful and famous dark nebula, for its curious shape of seahorse head. Is in the Orion constellation, 1,500 light years away from Earth.
Coal Sack Nebula: It is the best known dark nebula in the southern hemisphere. It is 600 light years away, in the constellation Cruz del Sur. It is easily recognized with the naked eye, as an unexpected dark area on the white strip of the Milky Way. It is seen in the following photo:
Cone Nebula: located 2,500 light years from Earth, in the constellation of Monoceros. Form a dark cone on top of the Christmas tree cluster. Interestingly, it was discovered on December 26.
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