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Nebulae are gas and interstellar dust. Depending more or less dense, they are visible, or not, from the Earth.
Nebulas can be found anywhere in interstellar space. Before the invention of the telescope, the nebulous term applied to all celestial objects diffuse appearance. As a result, many objects that we now know are clusters of stars or galaxies were called nebulae.
Nebulae in almost all galaxies, have been detected including our own, the milky way. Depending on the age of the associated stars, they can be classified into two major groups:
1.- Associated with evolved stars, as the planetary nebulae and supernovae remnants.
2.- Associated with very young stars, some even still in process of formation, such as molecular clouds and Herbig-Haro objects .
Classification of the nebulae according to your light
If you use the process that causes light-emitting, nebulae are classified as:
The emission nebulae, whose radiation comes from dust and gases ionized as a consequence of the warming that is being tested by nearby hot stars. Some of the most striking objects in the sky, like the Orion Nebula, are such nebulae.
The Reflection nebulae reflect and scatter the light of nearby little hot stars. The Taurus Pléyades are an example of bright stars in a reflection nebula.
The dark nebulae are clouds, little or nothing bright, which is represented as a dark spot, sometimes surrounded by a halo of light. The reason why not emit light themselves is that the stars are too remote to heat cloud. One of the most famous is the Horsehead, Orion Nebula. Throughout the dark strip that is observed in the sky when we look at the disk of our Galaxy is a succession of dark nebulae.