Washington, Feb 28 (IANS): The outer reaches of a 'supermassive' black hole, more than two million miles across, or eight times the earth-moon distance, is spinning at nearly the speed of light.

The gigantic object is at the centre of the spiral galaxy NGC 1365. Astronomers measured its jaw-dropping spin rate using new data from the Nuclear Spectroscopic Telescope Array, or NuSTAR, and the European Space Agency's XMM-Newton X-ray satellites.

"This is the first time anyone has accurately measured the spin of a supermassive black hole," Guido Risaliti of the Harvard-Smithsonian Centre for Astrophysics (CfA) and INAF - Arcetri Observatory, who led the study, was reported as saying by the journal Nature.

Astronomers want to know the black hole's spin for several reasons.

The first is physical - only two numbers define a black hole: mass and spin. By learning those two numbers, you learn everything there is to know about the black hole. Most importantly, the black hole's spin gives clues to its past and by extension the evolution of its host galaxy.

"The black hole's spin is a memory, a record, of the past history of the galaxy as a whole," explained Risaliti, according to the Harvard-Smithsonian statement.

Although the black hole in NGC 1365 is currently as massive as several million suns, it wasn't born that big. It grew over billions of years by accreting stars and gas, and by merging with other black holes.

Similarly, if the black hole grew randomly by pulling in matter from all directions, its spin would be low. Since its spin is so close to the maximum possible, the black hole in NGC 1365 must have grown through "ordered accretion" rather than multiple random events.

Studying a supermassive black hole also allows theorists to test Einstein's theory of general relativity in extreme conditions.

Relativity describes how gravity affects the structure of space-time, and nowhere is space-time more distorted than in the immediate vicinity of a black hole.