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Tuesday, May 25, 1999 Published at 18:15 GMT 19:15 UK


New Star in Southern Skies

By BBC News Online Science Editor Dr David Whitehouse

A bright new star has exploded into visibility in the southern skies. It is so bright it can be seen with the unaided eye.

Astronomers say it is a once in a decade event.

Nova Velorum 1999, named after the constellation in which it was found, was detected independently on 22 May by Peter Williams in Australia and Alan Gilmore of Mt John University Observatory in New Zealand.

Previously, it was too dim to be seen with the naked eye. But last week, it exploded and became one of the brightest objects in the sky.

It is now estimated to have a brightness of magnitude 2, making it more luminous than many famous bright stars.

Giant explosion

It is what astronomers call a nova - an explosion that occurs in a system of two orbiting stars, a red giant and a white dwarf star. A white dwarf is a star at the end of its normal life. It is incredibly dense, cramming roughly the mass of our Sun into the size of the Earth.

The white dwarf sucks material from the atmosphere of its larger companion. Its intense gravity compresses the gas as it falls. Because of this, its temperature rises to over a million degrees starting a runaway thermonuclear explosion.

The ejected gas gets even hotter. It is like a star turned inside out. The nuclear reactions that normally occur deep within a star take place in the superheated gas.

But the material cannot expand and cool and radiation cannot carry away the heat fast enough.

One million degrees

The first few minutes of a nova explosion have never been observed. Computer simulations suggest that the surface temperature of the white dwarf can exceed one million degrees and that the hot gases are blown away at more than 5,000 kilometres (3,000 miles) per second.

The last nova as bright as Velorum was Nova Cygni 1975, which peaked just brighter than magnitude 2.

How the nova will appear over the next few weeks is uncertain, but the exploding debris will probably fade beyond detectability over the next few years.

Photograph courtesy of Gordon Garradd. The cross-hair-like spikes that appear around the nova were caused by the photographing telescope and camera.

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