20 March 2002 - Astronomers find shortest period binary system

Astronomers at the Mullard Space Science Laboratory (MSSL) together with a colleague in Finland have discovered a stellar binary system in which the two stars are orbiting around each other every 5 minutes. A seperate group in Rome also made this discovery independently at the same time. This object sets the record as the fastest known binary and beats the previous record-holder by 5 minutes.

The astronomers found this object using the Nordic Optical Telescope in the Canary Islands. Before they went there they knew the approximate position in the sky of an X-ray source which varied in brightness on a timescale of 5 minutes. They managed to locate one faint star in the same position in optical light which showed small variations in its brightness on exactly the same timescale. They came to the conclusion that this 5 minute modulation is the time that it took two stars to orbit each other. The two stars are both thought to be very dense, compact, old stars called white dwarfs. They are extremely close to each other - about 1/5 of the distance to the Moon.

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"Dashing Dwarfs"

Over time, the two stars will gradually move closer together and form one star as gravitational radiation is released. If the combined mass of the two stars is high enough, the merger will result in a supernovae. Currently it is not clear if the mass will be high enough. Such a merger is expected to occur in around a 100,000 years.

Short period binary systems of the sort detected by the group at MSSL are expected to be the strongest sources of constant gravitational radiation in the night sky. They will easily be detectable using a satellite called LISA which the European Space Agency and NASA plan to launch in 2010. The MSSL group is setting out to find more of these elusive objects.

Further details can be found in the paper by Ramsay et al 2002 and also the paper by Israel et al 2002.

Gavin Ramsay
MSSL switchboard
01483 204100
MSSL website


Illustration "Dashing Dwarfs" by Cameron Slayden. Reprinted with permission from Science 295:1997 (15 March 2002), Stellar Pair Whirls in a 5-minute dash, L Irion. Copyright 2002 American Association for the Advancement of Science.

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Last modified Friday, March 28, 2003 2:58 PM

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