Scientists have discovered the first binary system ever known to consist of a black hole and a 'spinning' star.

The discovery was made possible by observations from the Liverpool and Mercator telescopes at the Observatorio del Roque de los Muchachos in Canary Islands, Spain.
'Spinning' star or Be-type stars are quite common across the Universe. In our Galaxy alone more than 80 of them are known in binary systems together with neutron stars.

"Their distinctive property is their strong centrifugal force: they rotate very fast, close to their break-up speed. It's like they were cosmic spinning tops," said Jorge Casares of the Instituto de Astrofisica de Canarias (IAC) and La Laguna University (ULL).
The newly discovered black hole orbits the Be star known as MWC 656, located in the constellation Lacerta (the Lizard) - 8,500 light years from Earth.

The Be star rotates so fast that its surface speed exceeds 1 million kilometres per hour.

"We started studying this star back in 2010, when space telescopes detected transient gamma-ray emission coming from its direction," said Marc Ribo, of the Institut de Ciencies del Cosmos of Barcelona University (ICC/IEEC-UB).

"No more gamma-ray emission has subsequently been detected, but we found that the star was part of a binary system," he added.

A detailed analysis of its spectrum allowed scientists to infer the characteristics of its companion.

"It turned out to be an object with a mass between 3.8 and 6.9 solar masses. An object like that, invisible to telescopes and with such large mass, can only be a black hole, because no neutron star with more than three solar masses can exist," said Ignasi Ribas, of CSIC in the Instituto de Ciencias del Espacio (IEEC-CSIC).

Scientists believe this object to be a nearby member of a hidden population of Be stars paired with black holes.

"We think these systems are much more common than previously thought, but they're difficult to detect because their black holes are fed from gas ejected by the Be stars without producing much radiation, in a "silent" way, so to speak," said Casares.

"However, we hope to detect other similar binary systems in the Milky Way and other nearby galaxies by using bigger telescopes, such as the Gran Telescopio Canarias," he said.

The discovery is published in the journal Nature.