Astronomers identify purest, most massive brown dwarf

London, March 25 : A record breaking brown dwarf -- a star too small for nuclear fusion -- with the purest composition and the highest mass yet known has been identified by an international team of astronomers.

The object, known as SDSS J0104+1535, is a member of the so-called halo - the outermost reaches of our Galaxy -- and is made up of the most ancient stars, the scientists reported in the Monthly Notices of the Royal Astronomical Society.

"We really didn't expect to see brown dwarfs that are this pure. Having found one though often suggests a much larger hitherto undiscovered population - I'd be very surprised if there aren't many more similar objects out there waiting to be found," said lead authors ZengHua Zhang from the Institute of Astrophysics in the Canary Islands in Spain.

Located 750 light years away in the constellation of Pisces, SDSS J0104+1535 is made of gas that is around 250 times purer than the Sun, so consists of more than 99.99 per cent hydrogen and helium. 

It is estimated to have formed about 10 billion years ago and measurements also suggest it has a mass equivalent to 90 times that of Jupiter, making it the most massive brown dwarf found to date.

SDSS J0104+1535 has been classified as an L type ultra-subdwarf using its optical and near-infrared spectrum, measured using the European Southern Observatory's "Very Large Telescope" (VLT). This classification was based on a scheme very recently established by Zhang.

Brown dwarfs are intermediate between planets and fully-fledged stars. Their mass is too small for full nuclear fusion of hydrogen to helium (with a consequent release of energy) to take place, but they are usually significantly more massive than planets.

It was previously not known if brown dwarfs could form from such primordial gas, and the discovery points the way to a larger undiscovered population of extremely pure brown dwarfs from our Galaxy's ancient past.