I’ve talked about black holes previously, and only in our own Galaxy, and only the big one in the middle, Sag A*. When I speak with the public and with kids about Black Holes, most people never really understand that there aren’t just one or two kicking around, but potentially there are as many as a Billion Black Holes in our own Galaxy!
The problem is, we are not very good at finding them. It makes sense, they give off no light, and we can only find them through indirectly measuring their effects on the surrounding environment. We can sometimes see their gravitational influence on nearby objects, and other times we see the bright x-rays from gas swirling into them. Using these methods we’ve found a very trim number in the Galaxy.
The reason we have such a disconnect between the number we’ve found and the number we estimate is that most are very hard to notice. Most of them are the result of long-past supernovae, and are just floating around in Space with nothing else near them.
But don’t they have strong gravity? I mean light can’t even escape!
They have very strong gravity, but its all because of their density. Black holes on average should only be a few times as massive as the sun (Which is pretty heavy, don’t get me wrong) but all that mass is packed into a very small space. If all the mass of the sun was packed into a black hole, it would be 6 Km wide! This is tiny! Even a black hole in a dense globular cluster would behave gravitationally just like a star, and it would be very difficult to pick out its gravity from the thousands of other stars packed within the cluster.
So Millions of small black holes are out there floating around in Space, rarely getting close enough to anything to be noticed, like sea monsters deep below the ocean rarely seen.
This leads us to a question I am often asked by kids: Will a black hole ever swallow up the Earth?
The Answer: It’s definitely possible, though highly unlikely.
This is always a good chance to explain the distance between stars, and just how vast it is. It’s much more likely that one of the 200+ Billion other stars in the Milky Way will approach Earth than one of the Millions of Black Holes will. And if a star did come close, we would definitely see it coming.
So all in all we are pretty safe.
But how can we find more Black Holes? A team of Scientists from Cardiff University have found a way for us to detect the gravitational wave signatures of Black Holes that are in the process of merging. Next year, once the Laser Interferometer Gravitational Wave Observatory (LIGO) comes online, it will make what is hopefully the first ever gravitational wave detection. Using specific signatures of gravity waves, the scientists have come up with a model to identify these merging black holes.
Will this help detect the Billions of potential Black Holes in the Galaxy? Probably not. Mergers are rare. But even with the small number of mergers, the group expects to find hundreds of new black holes.
A pretty good improvement on 37.