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About Stars: Novae

A white dwarf mostly is an inactive star. At least when left alone. But many are part of a double or multiple system.
When two stars are close together, it is not unusual when they interchange matter. But if one of the partners is a white dwarf, the gas can't fall directly from the other star to its very small surface. It aggregates in a disk around it, before trickling down like in a funnel. Thereby the gas, mostly hydrogen, heats up and is pressed together. Now it can fusion at once in an explosion - a nova.

Nova means 'new', a historically founded wrong expression. A nova radiates for a short time 100 000 times brighter as the star did before. So a 'new' star can be seen in the sky where before was visually nothing. Novae are relatively frequent, several times each century one can be seen with the naked eye.

Despite the explosion the mass of the white dwarf increases. If it had a mass of slightly less than 1.44 solar masses (Chandrasekhar limit), it can happen that it excesses this value now. Now the star can't resist its own gravitation any more and collapses. But it is no iron core like at a type Ib or II supernova of a supergiant, but mostly helium and carbon. This still can fusion and under such circumstances does it immediately. The result is a type Ia supernova, a billion times brighter than the Sun. The white dwarf is disrupted completely, throwing large amounts of heavy elements into space - the basis for new planets like the Earth.

Example: Nova Persei

Back: About Stars | Continue: Brown Dwarfs
    Supernova remnant
Type Ia supernova remnant.
Photo: Nasa, Hubble Teleskop

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