New Delhi, Jan. 10: A back-of-the-envelope calculation by an astronomer in Bangalore has helped scientists solve the mystery surrounding a peculiar class of stars called extreme helium supergiants.

While the surface material in most ordinary stars is mainly hydrogen, extreme helium stars have unusually large amounts of helium, but virtually no hydrogen. Astronomers have catalogued 21 extreme helium supergiant stars in the galaxy over the past five decades, but their origins had remained a mystery.

Now, researchers at the Indian Institute of Astrophysics (IIA) in Bangalore and their colleagues in the UK and US have shown that these stars are produced through the merger of ageing stars called “white dwarfs”. The white dwarfs represent a phase in the life of a star when it has expended its stock of hydrogen fuel and is cooling.

IIA scientist Gajendra Pandey and his colleagues have shown that extreme helium stars are produced through the merger of two different types of white dwarfs. Their findings will appear in the February issue of The Astrophysical Journal. “We believe this is the most plausible explanation,” Pandey said.

He and his senior IIA colleague, N. Kameshwara Rao, teamed up with Simon Jeffreys at the Armagh Observatory in Northern Ireland and David Lambert at the University of Texas, to observe seven extreme helium stars in the galaxy.

The white dwarf merger scenario to explain extreme helium stars had been first suggested by US-based astronomer Ronald Webbink in 1984. But there was little observational support for the idea.

The new study has also ruled out the hypothesis that extreme helium stars are produced when a shell of helium in the inner regions of an ageing star flashes and burns away hydrogen outside it, creating a surface rich in helium and deficient in hydrogen. “Our observations of light emitted by the stars do not support this helium shell flash scenario,” Pandey said.

The study also involved computer simulations ? taking off from paper calculations by Pandey five years ago ? to mimic the effects of the mergers of white dwarfs. Both the paper calculations and the computer simulations indicate that when two types of white dwarfs merge, the resultant star has a composition very similar to that of extreme helium supergiants. “The merger of two stars isn’t really an unexpected event,” Pandey said.

Observations indicate that many stars are part of multiple or binary star systems. Sometimes stars are so close that they might merge into one.