Milky Way centre teeming with black holes

Astronomers have discovered that the centre of our galaxy is teeming with black holes.

The Milky Way's huge Sagittarius A black hole is circled by many smaller ones, scientists say. (AAP)

The centre of our galaxy is teeming with black holes, astronomers have discovered.

For decades, scientists theorised that circling in the centre of galaxies were lots of stellar black holes, collapsed giant stars where the gravity is so strong even light doesn't get out.

But they hadn't seen evidence of them in the Milky Way core until now.

Astronomers poring over old x-ray observations have found signs of a dozen black holes in the inner circle of the Milky Way.

Since most black holes can't even be spotted that way, they calculate there are likely thousands of them there.

They estimate it could be about 10,000, maybe more, according to a study in Wednesday's journal Nature .

"There's lots of action going on there," said study lead author Chuck Hailey, a Columbia University astrophysicist. "The galactic centre is a strange place. That's why people like to study it."

The stellar black holes are in addition to - and essentially circling - the already known supermassive black hole, called Sagittarius A , that's parked at the centre of the Milky Way.

In the rest of the massive Milky Way, scientists have only spotted about five dozen black holes so far, Hailey said.

The newly discovered black holes are within about 31 trillion kilometres of the supermassive black hole at the centre. So there's still a lot of empty space and gas amid all those black holes. But if you took the equivalent space around Earth there would be zero black holes, not thousands, Hailey said.

The newly confirmed black holes are about 10 times the mass of our sun, as opposed to the central supermassive black hole, which has the mass of 4 million suns.

Also the ones spotted are only the type that are binary , where a black hole has partnered with another star and together they emit large amount of x-rays as the star's outer layer is sucked into the black hole. Those x-rays are what astronomers observe.