Astronomers have discovered the coolest star known in the galaxy with a surface only as hot as a freshly made cup of tea. The alien sun, termed a brown dwarf, is thought to be a star that failed to trigger the nuclear reactions needed to shine brightly.

By comparison, our own Sun has a surface temperature at a searing 5,500° Celsius.

The cool dwarf star, in mutual orbit around a similar but warmer star just 75 light years from Earth, is dubbed CFBDSIR 1458+10B. The likelihood must be therefore that there are many similar stars as cool or cooler still further out in the Milky Way.

CFBDSIR 1458+10B was identified by an international team, including European astronomers, using three of the world’s biggest telescopes.

A powerful spectrograph instrument on the European Southern Observatory’s Very Large Telescope in Chile was used to probe the double star’s temperature.

It found that the dimmer star has a temperature of around 100° Celsius – the boiling point of water. It beats the previous record holder which had a temperature of 350° C.

Co-discoverer Philippe Delorme of the Planetary and Astrophysics Institute at Grenoble, Switzerland, said: “We were very excited to see that this object had such a low temperature, but we couldn’t have guessed that it would turn out to be a double system and have an even more interesting, even colder component.”

Experts say the discovery blurs the boundaries between the coolest stars and giant gas planets that have been detected around other stars.

“At such temperatures we expect the brown dwarf to have properties that are different from previously known brown dwarfs and much closer to those of giant exoplanets — it could even have water clouds in its atmosphere,” said Michael Liu of the University of Hawaii’s Institute for Astronomy.

CFBDSIR 1458+10 was first discovered to be a binary system the Keck II Telescope in Hawaii and its distance measured using the neighbouring Canada–France–Hawaii Telescope. The discovery is revealed in the Astrophysical Journal.

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©PAUL SUTHERLAND, Skymania.com

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