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Final countdown to dramatic supernova
Posted: 05 January 2010, updated 11 January 2010

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A white dwarf star that will one day explode in a giant thermonuclear explosion has been discovered to be relatively close to our Solar System. Fortunately it’s not due to explode for another million years.

An artist’s impression of a binary system with a white dwarf harvesting gas from a companion star, which forms a disc around the white dwarf. Image: ESO/L Calçada.

The white dwarf is found in the binary system T Pyxidis, in the Southern Hemisphere constellation of Pyxis, along with a fairly normal Sun-like companion star. The white dwarf – the burnt out remains of an ancient star – is leeching off its partner, stealing gas from it that consequently builds up on the surface of the white dwarf. Roughly every twenty years, this build up of gas becomes a little too dense and part of the surface explodes in what we call a nova. T Pyxidis was seen going nova in 1890, 1902, 1920, 1944 and 1967, but there has been nothing since, and nobody knows why we are overdue. However, new analysis of spectral observations of the white dwarf made with the now-defunct International Ultraviolet Explorer (IUE) have revealed that it is approaching the time when it will upgrade from a nova to a full blown supernova.

This happens when the white dwarf accumulates so much gas that it crosses the Chandrasekhar limit, which is 1.4 times the mass of our Sun, and matter becomes so dense within the star that a thermonuclear explosion engulfs the star, utterly destroying it. The IUE data implies that it is close to this limit, and is continuing to collect more gas from its companion at a rate of 200 trillion kilograms per second. Furthermore, the observations answer a long-standing puzzle; do the recurrent nova rid the white dwarf of all its extra mass, meaning it has to start over again after each eruption, or does it have some left over after each nova? The observations conclusively show that the white dwarf doesn’t shed all its extra mass with each eruption, so over time it is gaining mass, thus explaining how it can build up to the Chandrasekhar limit.

The new analysis of the data also places this doomsday star within 1,000 parsecs (3,260 light years) of Earth. White dwarf supernovae are among the most powerful supernovae in the Universe, but fortunately we are far enough away for this to just be a spectacular display in the night sky, assuming the human race is still around in one million year's time to appreciate it!