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Red Giant fails to devour Brown Dwarf companion

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Astronomers using the European Southern Observatory's Very Large telescope have identified a star system in which a brown dwarf has survived being engulfed by its companion star's Red Giant phase.

Now, both stars are planet sized, and orbit one another very closely, and very fast. The orbital period is around two hours. One is a white dwarf - a star roughly earth-sized, but around half as massive as our sun, and the other is a brown dwarf - a failed star approximately 55 times as massive as Jupiter.

But it wasn't always that way, according to Pierre Maxted, lead author of a paper in this week's Nature

"Such a system must have had a very troubled history", he said. "Its existence proves that the brown dwarf came out almost unaltered from an episode in which it was swallowed by a red giant."

Millions of years ago, the white dwarf was a star very much like our own sun, and was orbited by its then much more distant brown dwarf companion. As its supplies of hydrogen began to run out, it started to collapse. This created a little extra pressure at its core, giving it a little more energy to keep those fires burning by igniting the hydrogen that remained in its shell.

As the hydrogen in the star's shell burnt, so the shell expanded outwards and the star became a red giant and started burning helium, just for fun.

It got so large that it enveloped its companion star. Being inside a star instead of just orbiting one is a disturbing thing, and the brown dwarf was jostled into a new orbit, spiralling closer and closer to the centre of the red giant.

At some point, the Red Giant ran out of helium and collapsed in again. When the core collapsed, it released energy causing the star's envelope to blow off. And so the star shed its outer layers in a planetary nebula (nothing to do with planets), leaving its core intact, and visible to us as a white dwarf.

The brown dwarf is also intact, although orbiting the star much more closely. If it had originally been less than 20 Jupiter masses, it would have evaporated.

Eventually, the two stars will move still closer together, and their orbital period will reduce still further. In about 1.4bn years they will be so close that the white dwarf will begin siphoning material off the brown dwarf. ®

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