Thirteen things you may not know about black holes

Today it has been announced that scientists who are part of the LIGO Collaboration have detected gravitational waves from black holes.

Black holes often make the headlines - popular fascination frequently centres on exotic effects associated with them, but they have profound effects on the galaxies that host them.

Recent research by Professor Andrew King from the Department of Physics and Astronomy has explained how black holes cannot easily grow beyond a mass of about fifty billion suns, but there are many other interesting things we know about these enigmatic objects:

1)      Not even light can escape black holes

2)      If you fell into a black hole, you would be stretched out very long and thin – and you would not enjoy the experience

3)      Time runs very differently near a black hole

4)      The centre of almost every galaxy hosts a supermassive black hole

5)      Our own Milky Way has a black hole of about 4 million sun masses in its centre

6)      But this is fairly modest – some galaxies have black holes near the likely maximum value of fifty billion suns

7)      Gas infall into a black hole is the most energetic process in the large-scale universe

8)       Black holes throw vast amounts of energy out into space as matter falls into them and grows their masses

9)       A black hole in the centre of a galaxy could put out enough energy to blow away  the entire central bulge of the galaxy, dispersing hundreds of billions of sun masses of gas and stars into space, but

10)     Until the hole’s mass grows to a certain value, almost all of its power escapes harmlessly into space as light

11)     But once the black hole mass reaches this critical value, related to the large-scale gravity of the galaxy, almost the full power it emits suddenly smashes into the galaxy as a high-speed wind of gas. This wind leaves the stars relatively untouched, but blows away all the gas which would have fuelled further growth of the hole itself. The hole mass remains stuck at the critical value, as it has starved itself of further food

12)    The architecture of almost every galaxy has been profoundly influenced by the power output of the giant black hole in its centre

13)   The only way a black hole can grow any further is if its host galaxy gains more gas itself by merging with another galaxy.

• Read about University astronomers joining the world-wide hunt for Einstein’s gravitational sirens
• Read an article by Professor Andrew King about how black holes shape galaxies