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A nebula is a cloud of gas or dust in space. Nebulae can be dark or, if illuminated by nearby stars or stars immersed in them, they can be bright. They are generally places where the formation of stars and planetary discs occurs, so that very young stars are usually found within it.

There is a great variety of nebulas accompanying the stars at all stages of their evolution. The vast majority correspond to gaseous clouds of hydrogen and helium that undergo a process of gravitational contraction towards a state of protostar. Thus, the so-called cocoon nebulae have a newly formed star inside. The nebula is not, in this case, but the remains of gas that has not collapsed. The gas in question, which can, by means of atomic collisions, form molecules and small solid particles of greater or lesser complexity, is heated by the radiation emitted by the new star enough to mask its presence, and what is observed is an image similar to that of a caterpillar cocoon.

Another type of nebulae, called Bok globules, are clouds of highly condensed gas, in the process of forming a protostar. They are revealed, when they are located on a light background, such as the Galaxy, as a darkening of the background, for example, the nebula called the Sack of Coal, next to the constellation Cruz del Sur, and the nebula called Head of a Horse, One of the most famous.

The so-called Herbig-Haro objects are small, variable nebulae that appear and disappear over a period of a few years, which appear to consist of lumps of gaseous matter ejected at the poles of a star in formation, mainly in the cocoon phase. Their luminosity is produced by collision with the surrounding gas cloud, as they produce a characteristic shock wave due to the high speed with which they are expelled.

Another type of nebulae, with a chemical composition rich in heavy chemical elements (helium, carbon and nitrogen mainly) are remains of star matter ejected by giant and supergiant stars at high speed (1000 km / s) in a type of stars called Wolf-Rayet similar to these they also occur in the last stellar stages, after the formation of novae and supernovae.

Planetary nebulae are called that because many of them resemble planets when viewed through a telescope, although in fact they are layers of material from which an evolved star of medium mass detached during its last stage of evolution of a red giant before becoming a white dwarf. The Ring nebula, in the constellation Lira, is a typical planetarium that has a rotation period of 132,900 years and a mass of about 14 times the mass of the Sun.

Several thousand planetary nebulae have been discovered in the Milky Way. More spectacular, but smaller in number, are the fragments of supernova explosions, and perhaps the most famous of these is the Crab Nebula. Nebulae of this type are intense radio sources, as a consequence of the explosions that formed them and the probable remains of pulsars into which the original stars became.

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