In Astronomy, a nova is a star which very brutally becomes extremely brilliant, with a great increase in its glare, which can be about 10 magnitudes (illumination multiplied by 10000). This sharp luminosity lasts only a few days, and the star takes again its initial glare then gradually.
The word nova being borrowed from Latin, Latin plural is novæ , with a tolerance for novae , which is also the plural employed by the english-speaking. In French, novas is also a correct form.
The astronomers who discovered them regarded them as new stars, since they appeared where previously did not exist star, and thus called them novæ .
Some novæ are recurring, and underwent several explosions since they were observed, with intervals about several decades.
The knowledge of this phenomenon comes mainly from the spectographic study of the novæ. The stars which become novæ, called prénovæ , are in general of spectral Type has, and not very luminous.
At the time of the explosion, the spectrum approaches that of the Supergéante S, but with a displacement of all the lines towards the purple one, proportionally with their Wavelength. The interpretation of this shift is that the radiation is emitted by a gas expanding which escapes from the surface of star, with speeds of about 1000 km/s.
This enormous nuclear explosion, whose energy is about 1038 with 1039 J, would be the consequence of the Accrétion hydrogen on the surface of a white Naine.
When dwarf white has a star partner near which ages and increases volume while becoming a Géante red, the external atmosphere of this partner will be often attracted by the Gravité of dwarf white. The gases thus captured consist mainly of Hydrogène and Hélium, the two principal components of the Matière in the Univers. The gases are crushed on the surface of dwarf white by its enormous gravity, are compressed and heated at temperatures about ten million Kelvin S while additional matter continues to be added. At a certain time, the pressures and the temperatures of the layer of hydrogen become enough large to start a reaction of nuclear Fusion which quickly converts a great quantity of hydrogen into helium and others heavier elements.
The enormous quantity of energy released by this process expels remaining gases of the surface of dwarf white and produces an extremely luminous glare but of short duration.
Novæ recurring
A white Naine can produce novæ very often while additional hydrogen coming from its star partner continues to increase on its surface. An example is RS Ophiuchi, which one knows six eruptions (in 1898,1933,1958,1967,1985 and 2006). However, early or late, the star partner will have exhausted her matter or the dwarf white one will undergo a nova so powerful that it will be completely destroyed by this process.It is a little similar to a Supernova of the type I; however, in general the supernovæ imply different processes and energies much higher and should not be confused with the ordinary novæ.
See too
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