Nebula
The nebulae are regions of the interstellar medium made up of gases (mainly hydrogen and helium) as well as chemical elements in the form of cosmic dust. They have a notable cosmological importance because many of them are the places where stars are born due to condensation and aggregation of matter; on other occasions it is the remains of already extinct or extinct stars.
Nebulae associated with young stars are located in the discs of spiral galaxies and in any area of irregular galaxies, but they are not usually found in elliptical galaxies since these hardly have any star formation phenomena and are dominated by very small stars. old. The extreme case of a galaxy in which many nebulae exhibit intense episodes of star formation is called a starburst galaxy.
Before the invention of the telescope, the term "nebula" was applied to all diffuse-appearing celestial objects. For this reason, galaxies (groups of billions of stars, gas and dust held together by gravity) are sometimes wrongly called nebulae; it is a legacy of 19th century astronomy that has left its mark on contemporary astronomical language.
Nebulae can be classified into three broad categories according to the nature of their emission (or lack thereof).
Dark Nebulae
A dark nebula (also known as an absorption or inspiration nebula), is an accumulation of interstellar gas or dust not related to or far from any star, in such a way that it is not disturbed by its energy, so their presence can only be noticed by contrast with a more distant stellar background or emission nebula.
In this case, the nebula does not emit or reflect any light because it is far from the stars, but it does absorb light from objects behind it. Therefore, its existence is deduced by the presence of a dark region that stands out against the background of the starry sky. A typical example is the so-called Coal Sack in the constellation of the Southern Cross, and the Horsehead Nebula in the constellation of Orion is also very famous. Numerous dark nebulae can also be seen above the bright fringe of the Milky Way stretching across the sky.
Reflection nebulae
These nebulae reflect light from nearby stars that are not hot enough to emit the ultraviolet radiation needed to excite the gas in the nebula. Generally, these nebulae are formed by the remains of the gas that gave rise to the star, and their spectrum is similar to that of the stars whose light they reflect. The most representative case is the nebula around the star Mérope in the open cluster of the Pleiades (M45).
Emission nebulae
In this most common case, the gas that makes up the nebula glows as a result of the transformation it undergoes by the intense ultraviolet radiation from hot neighboring stars. In Astrophysics these objects are called H II Regions and are essential when analyzing the chemical composition and physical properties of nebulae (and the galaxies in which they are found) thanks to the analysis of their spectrum, made up of a multitude of lines. of emission of the chemical elements that they house. The brightest and most important emission line is H-alpha (from the hydrogen Balmer Series), located in the red zone of the spectrum (at 6562.82 Å), this being the reason why this color dominates in the images. traditional emission nebulae. But emission lines of helium, oxygen, nitrogen, sulfur, neon or iron are also detected. Depending on the nature of the emission nebula, they are subdivided into two totally different groups.
1) Emission nebulae associated with star formation regions, that is, in the presence of very young, massive and hot stars, even in the process of formation (Herbig-Haro objects) and molecular clouds. The most famous case is the Orion Nebula (M42), the closest to Earth, but other notable examples are the Eagle Nebula (M16, in the Serpent Constellation), the Trifid Nebula (M20, in Sagittarius) or the Lagoon Nebula (M8, also in Sagittarius).
2) Emission nebulae associated with dying or extinct stars are called planetary nebulae and supernova remnants. The former have nothing to do with planets: they are the envelopes of low or intermediate mass stars ejected into space at the end of their evolutionary cycles. In them, the gas is excited by a very small and hot object, a white dwarf, which is the exposed core of the dead star. Known examples of this type of nebula are the Ring Nebula (M57, in Lyre) and the Helix Nebula (NGC 7293, in Aquarius).
The supernova remnant (or "remnant") is the material released in the titanic explosion that puts an end to massive stars. The gas in this type of nebula can be affected both by the energy delivered by the supernova itself, and by the emission of a possible neutron star (a pulsar) within it. Perhaps the most famous example of a supernova remnant is the Crab Nebula (M1, in the constellation Taurus).