Novae and Supernovae

9 Apr

The traditional term “novae,” or “new stars,” is actually misleading. A nova is not a newly originating star, but one that, because of some internal cause not yet known in detail, exhibits an extremely intense outburst of light. This outburst occurs very suddenly, usually within a few hours of days, and the luminosity of the star increases 5000 to 100, 000 times (25, 000 times on the average). A few hours after maximum intensity the brightness begins to decrease; the greater the maximum brightness, the faster this occurs. For the luminosity to decrease by a factor of 10 takes a few days for the “fast” novae, and several years for the “slow” novae. The outburst is associated with a rapid expansion at velocities up to a couple of thousand kilometers per second. As a consequence, the radius of the star increases by 100 to 400 times and reaches the size of a giant star.

Little is known yet of the pre-nova state, the condition of such stars before the outburst. They are probably sub-dwarfs of spectral type A (see Hertzprung-Russell Diagram). Even during its outburst, the star shows a normal absorption spectrum, but a few hours or days after maximum brightness, the spectrum begins to change fundamentally. Strong emission lines appear, cause by highly ionized atoms, such as are found in the Sun’s corona or in the galactic emission nebulae. The temperature increases, and in the final state (ex-nova), frequently after only a few years’ time the nova has become an O star with a temperature of 30,000 to 60,000 degrees, although its luminosity is considerably less than that of a normal O star (see Fig. 34). The frequency of a novae in the Milky Way is about 25 per year, of which only about two or three are observable. Altogether, we know of some 100 novae within the Milky Way. Apart from these, a large number have been observed in extra-galactic systems, particularly in the Andromeda nebula. No systematic difference have been found, so that we are clearly dealing with the same type of phenomenon

 

During the continuous survey of extra-galactic systems, observers have found stars that showed light outbursts 10,000 times the intensity of an ordinary nova. Such stars are called supernovae. We know nothing about their state before outburst. The light increase takes place somewhat more slowly, but at maximum luminosity becomes so intense that the star appears as bright in itself as the whole galaxy of billions of stars in which it is situated. In 25 days a supernova radiates as much energy as the sun in a million years! The total energy emitted during such an outburst amounts to 5 times 10^48 erg. a few percent of a total energy content of a star (about 10^50 erg). The frequency of supernovae, however is much less than that of ordinary novae. According to a statistical survey by Zwicky, which is still incomplete, of course owing to lack of observational information, only 50 supernovae are known altogether, and of these only three are in our own galaxy; perhaps three supernovae appear in each Milky way system once every thousand years.

Astronomy A to Z

Edited by Lloyd Motz – Professor of Astronomy Columbia University (Grosset & Dunlap, Inc., Publishers New York © 1964 All Rights Reserved) pp. 160-161

@ 1957 By Fischer Bücherei KG Frankfurt-Main Hamburg

 

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