Before the era of astronomy, a star that suddenly appeared where nothing had been seen before, was called nova, or "new star." This is an inappropriate name, since these stars existed long before they could be seen with the naked eye. Astronomers believe that there may be a dozen novae in the Milky Way, the Earth's galaxy, every year, but two or three of them are too far away to be seen or obscured by interstellar matter.
In fact, novas are seen more easily in other nearby galaxies than in ours. They are called novas according to the year of their appearance and the constellation in which they arise. Typically, a nova increases its original brightness by several thousand times in a matter of days or hours. Then it enters a transition period, during which it pales, and becomes bright again; from there it pales little by little until it reaches its original level of brightness.
The novas are stars in a late period of evolution. They can be considered to be a type of variable stars. Apparently they behave this way because their outer layers have formed an excess of helium through nuclear reactions and expands too quickly to be contained. The star explosively emits a small fraction of its mass as a layer of gas and then normalizes. The remaining star is typically a white dwarf and is generally believed to be the smallest member of a binary system, subject to a continuous decline in matter from the largest star. Perhaps this phenomenon always happens with dwarf novae, which arise again and again at regular intervals of a few hundred days.
The novas in general show a relationship between their maximum brightness and the time it takes to pale in a certain amount of magnitudes. By measuring the closest novae of which we know distance and brightness, astronomers can use novae from other galaxies as indicators of the distance of those galaxies.
The explosion of a supernova is much more spectacular and destructive than that of a nova and much rarer. These phenomena are rare in our galaxy, and despite their increase in brightness by a factor of billions, only a few can be seen with the naked eye. Until 1987, only three were actually identified throughout history, the best known of which was the one that emerged in 1054 AD. C. and whose remains are known as the Crab Nebula.
Supernovae, like novae, are seen more frequently in other galaxies. Thus, the most recent supernova, which appeared in the southern hemisphere on February 24, 1987, emerged in a satellite galaxy, the Large Magellanic Cloud. This supernova, which exhibits some unusual features, is today the subject of intense astronomical study.
The mechanisms that produce supernovae are less known than those of the novae, especially in the case of stars that have more or less the same mass as the Sun, the middle stars. However, stars that have much more mass sometimes explode in the last stages of their rapid evolution as a result of a gravitational collapse, when the pressure created by nuclear processes within the star can no longer support the weight of the outer layers . This is called Type II supernova.
A Type I supernova originates similarly to a nova. It is a member of a binary system that receives the flow of pure fuel by capturing material from its partner.
There are few remains of the explosion of a supernova, except for the expanding layer of gases. A famous example is the Crab Nebula; In its center there is a pulsar, or neutron star that spins at high speed. Supernovae are significant contributors to the interstellar material that forms new stars.
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