How are black holes formed?

Question: How are black holes formed?
- Keywords:
  black holes
Asked by elin.xx to Jim, Adrian, Gaia, Scott, Vicky on 9 Mar 2016. This question was also asked by pughy.
- Jim Barrett answered on 9 Mar 2016:
  
  Black holes are formed when really really big stars run out of fuel to burn and collapse in on themselves.
  
  Really big black holes are made when two smaller black holes crash into each other and combine.
- Adrian Buzatu answered on 14 Mar 2016:
  
  Black holes are made when a star explodes at the end of its lifetime. All its mass is then amassed in a single point. A black hole has therefore an infinite density of mass. It’s mass is finite. But it’s volume is just a point. Black holes have a property of having a sphere around them that if you go inside that sphere, you can never escape. That is why they are called black hole. They absorb all light and matter that is thrown at them.
  
  Black holes are also formed by merging other black holes.
  
  Typically every centre of a galaxy contains a very very huge black hole.
  
  They are not be feared though, as stars and planets orbit around the centre of a galaxy, and not fall into it. The same with planets continue to orbit a black hole that was a star before, and do not fall into the black hole.
- Scott Lawrie answered on 14 Mar 2016:
  
  A black hole happens when a star is so huge and old that it has used up all its fuel and cannot push against gravity anymore. Gravity then can finally squeeze the star up into a tiny ball with nothing to ever stop it getting smaller! You then have a bit of space with several stars’-worth of mass squeezed into something the size of a city! The gravity of this tiny thing is enormous – it sucks everything in and they cannot ever escape. Even light – the fastest thing there is – cannot move fast enough to escape! That’s why they’re called Black Holes ?

Question: How are black holes formed?

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