Astronomy Picture of the Day 3-14-03

Explanation: Large, massive stars end their furious lives in spectacular supernova explosions -- but small, low mass stars may encounter a similar fate. In fact, instead of simply cooling off and quietly fading away, some white dwarf stars in binary star systems are thought to draw enough mass from their companions to become unstable, triggering a nuclear detonation. The resulting standard candle stellar explosion is classified as a Type Ia supernova and perhaps the best example yet of the aftermath is this expanding cloud of shocked stellar debris, DEM L71, in the nearby Large Magellanic Cloud. The sharp false-color x-ray image from the orbiting Chandra Observatory shows the predicted bright edges of the outer blast wave shock region and the x-ray glow of an inner region of reverse shock heated gas. Based on the Chandra data, estimates for the composition and total mass of expanding gas strongly suggest that this is all that remains of a white dwarf star. Light from this small star's self-destructive explosion would have first reached Earth several thousand years ago.

The classic supernova that we all know (Type II) results from a massive star running out of fusion fuel. This usually leaves a remnant: neutron star or black hole, depending on the star's mass.

The above photo is a different mechanism. A member of a binary star goes through its life, but is not massive enough to supernova. Instead it loses mass (which the other star may accumulate) after becoming a red giant. All that is left is the white dwarf. The other star is now bigger than before if it got some of the now-white dwarf's matter. If it sweeps up enough stellar material it may become big enough to spill over into the white dwarf's gravitational well. As the material swirls off the larger star, it heats up then is pulled onto the white dwarf's surface. If enough accumulates, it becomes unstable and results in a thermonuclear explosion. Some white dwarfs do this on a repeating basis; we call them recurrent novae. But if the material accumulates to a great degree, it may trigger a much more massive explosion that rips the white dwarf to shreds. That's the Type 1a supernova.

Mechanism for Type 1A Supernova Explosions

In a Type IA Supernova, accretion is thought to be onto a white dwarf, but at a rate different from that for a nova. Instead of igniting a thin surface layer, as in the white dwarf, in this case the star becomes unstable and essentially the entire star is consumed in a gigantic thermonuclear explosion.

This may be likened to the explosion of a hydrogen bomb approximately the size of the Earth but containing the mass of the Sun.

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