The nebulae they are interstellar dust and gas structures. Depending on whether they are more or less dense, they are visible, or not, from Earth.
As part of galaxies, nebulae can be found anywhere in interstellar space. One of the most famous is the Horse Head nebula, in Orion.
Before the invention of the telescope, the term nebula it applied to all celestial objects of diffuse appearance. As a consequence of this, many objects that we now know to be clusters of stars or galaxies were called nebulae.
Nebulae have been detected in almost all galaxies, including ours, the Milky Way. Depending on the age of the associated stars, they can be classified into two large groups:
1.- Associated with evolved stars, such as planetary nebulae and supernova remnants.
2.- Associated with very young stars, some even still in the process of formation, such as Herbig-Haro objects and molecular clouds.
Classification of the nebulae according to their light
If you follow the process that originates the light they emit, the nebulae can be classified as:
The emission nebulae, whose radiation comes from dust and ionized gases as a result of the heating to which they are subjected by very hot nearby stars. Some of the most amazing objects in the sky, such as the Orion Nebula, are nebulas of this type.
The reflection nebulae they reflect and disperse the light of little hot stars from its surroundings. The Pleiades of Taurus They are an example of bright stars in a reflection nebula.
The dark nebulae they are little or not luminous clouds, which are represented as a dark spot, sometimes surrounded by a halo of light. The reason they don't emit light by themselves is that the stars are too far away to heat the cloud.
The entire dark strip seen in the sky when we look at the disk of our galaxy is a succession of dark nebulae.
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