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[Chicago Astronomer - Astro Joe]

Astronomers glimpse exploded star

A star on the brink of exploding as a spectacular supernova has been glimpsed by international astronomers.

The star flared up suddenly last February, briefly becoming 1,000 times brighter than normal.

RS Ophiuchi is close to destroying itself in a nuclear explosion called a type 1a supernova, scientists report in the journal Nature.

These are among the brightest phenomena in the Universe, radiating five billion times as much light as the Sun.

They are so bright they can be seen far across the cosmos.

They also seem to be remarkably uniform - they always appear to give off the same amount of light, so that their visibility from Earth, dimmed only by their immense distance, has been used to measure the size of the Universe.

The only problem, which is a great embarrassment to astronomers, is that they have never seen a type 1a close up - their measurements are all based on theory.

They are so rare that the last one known in our galaxy was seen in 1572 by the great Danish astronomer Tycho Brahe, who first coined the term nova, for "new star", not realising he was in fact witnessing the violent end of an unknown star.

RS Ophiuchi, in the equatorial constellation of Ophiuchus (near Libra), is just the right kind of white dwarf.

Several times in the past century it has flared up, as if in a failed supernova explosion. It is as if a thermonuclear flame has swept across the face of the star without quite catching hold.

The last time it flared up was in 1985, when astronomers' technology was not up to catching all the details.

Theory says that white dwarfs self destruct when their mass equals 1.4 Suns.

RS Oph is nearing that critical mass. It is slowly gobbling up material from a nearby giant star, gaining a millionth of a solar mass every decade.

At the moment, sitting just below the critical mass, the novae on RS Oph blow themselves out, achieving brief episodes of brilliance.

But soon, RS Oph could pass the tipping point - the nuclear flame will detonate from deep inside the star and blow it apart. How soon is not clear.

"It could be tomorrow, but most likely it'll be 1,000, 10,000, 100,000 years from now," says Jeno Sokoloski.

Whenever it happens, it will be, she says, a spectacular event, outshining the planets, so that its final glory will be visible against the day time sky.

Source: news.bbc.co.uk/2/hi/science/nature/5204676.stm

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[Maddad]

antwrp.gsfc.nasa.gov/apod/ap060726.html

Spectacular explosions keep occurring in the binary star system named RS Ophiuchi. Every 20 years or so, the red giant star dumps enough hydrogen gas onto its companion white dwarf star to set off a brilliant thermonuclear explosion on the white dwarf's surface. At about 2,000 light years distant, the resulting nova explosions cause the RS Oph system to brighten up by a huge factor and become visible to the unaided eye. The red giant star is depicted on the right of the above drawing, while the white dwarf is at the center of the bright accretion disk on the left. As the stars orbit each other, a stream of gas moves from the giant star to the white dwarf. Astronomers speculate that at some time in the next 100,000 years, enough matter will have accumulated on the white dwarf to push it over the Chandrasekhar Limit, causing a much more powerful and final explosion known as a supernova.

RS Ophiuchi belongs to an elite group of stars; there are only seven members as of May, 2000. These recurrent novae always involve a binary system. One will be a white dwarf (usually 10,000 to 20,000 kilometes in diameter), a star that has run out of fuel, typically hydrogen and helium, and has hence shrunk to the size of the Earth. While this dense remnant slowly cools, its companion looses mass which spirals in to the dwarf's surface. Once the gas has accumulated to a depth of several feet (figure from my memory, years ago, and may apply to neutron stars as opposed to white dwarfs. I'm still trying to verify that number and could use some help), it suddenly star-wide undergoes nuclear fusion - the nova. This gigantic thermonuclear bomb will release as much energy as our sun produces in 1,000 years.

The same process that works for a white dwarf can work for a neutron star, although the energy release range is maybe 500 times higher.

The supernova is a different and more powerful mechanism.