Scientists solve mystery of massive, not-so-bright large star

London, Oct 22: Astronomers have been baffled over an exceptionally massive black hole that is travelling around a massive star in an unusually tight orbit. Also odd, the star is not as bright as it should be.

Now a Northwestern University research team has produced a model of the system's evolutionary history and formation that explains all of the system's observational characteristics.

"We were attracted to this system because it has one of the most massive black holes to have formed from a star, and yet the rest of its characteristics, especially the mass of its companion star and its orbit, did not make any sense from an evolutionary point of view," said Vicky Kalogera at the Weinberg College of Arts and Sciences.

M33 X-7 is one of the few known X-ray binary systems containing a black hole outside our galaxy, and its star is the most massive star ever discovered in such a system.

The researchers' evolutionary model of M33 X-7 starts with two stars in a binary system (or in orbit one around the other). One star is 100 solar masses (100 times the Sun's mass), and the other is 30 solar masses.

The star, which is now 70 solar masses, is not as luminous as stars of similar mass partially because of the way it gained its mass and partially because of the inclination of the system with respect to us.

On one hand, the star accreted matter so quickly from its interaction with the other star (now a black hole) that it could not adjust fast enough to its new, greater mass. Therefore, the star does not burn as bright as an undisturbed star of this greater mass would.

"Solitary black holes are very difficult to observe, but X-ray binary systems, such as M33 X-7, make black holes visible to us," said Francesca Valsecchi.

The study is published in the journal Nature.

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