Question: what is a black hole?

Alexander Finch answered on 17 Dec 2015:

A black hole is an object that is so heavy that its gravity is strong enough to actually pull light into it. Since light cannot get away from it, it appears completely black and we cannot see “into” it.

They are formed when particularly large stars come to the end of their life and collapse down to size that’s tiny compared to the size of the star, but still as heavy. Once a black hole forms, it slowly grows by “sucking” in light and anything else that goes near it.

Because they emit no light, we can’t see them anywhere – they appear black, which compared to the black of space is hard to see! But we can guess their existence from the gravity they exert on nearby stars and even galaxies. Sometimes, they pass in front of other objects and we see the light from the object behind get “bent” towards us like light going through someone’s glasses – we call this “gravitational lensing”.

Black holes are really interesting because the laws of physics get all mixed up in them. They’re really weird – we don’t fully understand them..