Two new kinds of black hole have been discovered - medium-sized ones and "quiet" ones which do not give off much radiation.

Astronomers announced the discovery at the American Astronomical Society meeting in Charleston.

Black holes can come in many sizes.

Galaxy NGC 1313 conceals an intermediate black hole

They can be monsters, thousands of times the size of our Sun, sitting at the centre of galaxies devouring stars and gas clouds.

Around the black hole can be a swirling mass of superhot gas giving off high-energy radiation.

As the black hole's irresistible gravity draws gas into it, cataclysmic amounts of energy are given off. It then enters the black hole and disappears from our universe altogether.

Jets of matter often spurt away from the polar regions above the black hole. These jets can stream through space for billions of miles.

Small but powerful

Or black holes can be much smaller.

Sometimes they can be just a few tens of miles across. This can happen after a star has exploded and compressed a mass equivalent to our Sun into a radius of a few miles.

Until now, astronomers have detected evidence for both types of objects but not for black holes whose size and mass is between the two extremes.

Now they have filled the gap with the discovery of intermediate-sized black holes between 100 and 10,000 times the mass of our Sun and about the size of our Moon.

Come together

The new class of black holes was identified through the X-ray radiation given off by material being pulled into them. This radiation is different to that associated with the supermassive black holes.

Scientists from Nasa and Carnegie Mellon University studied 39 relatively nearby galaxies. They found that over billions of years the smaller black holes collide and coalesce into intermediate-size holes.

Astronomers have also detected supermassive black holes at the centre of galaxies even though they are not giving of vast amounts of radiation.

This could be the case if they do not have any stars in their vicinity to swallow.