In space it is possible to locate impressive cosmic formations in every way: from their fantastic appearance to their exquisite chemical composition. There are many, millions, but, among them, stands out a group, called nebulae, that look like colorful clouds floating in the interstellar medium. They are composed mainly of hydrogen and helium, in addition to various chemical elements reduced to cosmic dust. They are closely related to stars, since they are born from nebulae or, well, they turn into nebulae towards the end of their days.
The man's curiosity led him to devise and build the telescope, with which he could finally observe the stars up close. However, it was not only these that they had the pleasure of observing, since they also noticed the presence of other galaxies, black holes, asteroids and, of course, nebulae. It should be noted that, previously, "nebula" was a word used to name any body that had a somewhat diffuse or blurred appearance; this changed in the 19th century, in which, step by step, the appropriate terms were coined for each formation.
In our days, nebulae have been classified into three groups, taking into account their emission and absorption of light. The first of these, the dark or absorption nebulae, are distinguished by being far from the stars and absorbing much of the energy that they radiate. Then there are reflection nebulae, which are characterized by reflecting the light of nearby stars, but whose intensity is not strong enough to excite the gases of the same. Finally, there is the emission nebula, the best-known class, whose gases glow intensely as a product of the emission of UV rays by nearby hot stars.
