What are black holes?

Question: What are black holes?
- Keywords:
  black hole
Asked by Space... to Col Op, Elie, Floris, Jenn, RocketRich on 15 Mar 2016. This question was also asked by Matty, RealDaringNote62, Lebron James.
- Columbus Operations answered on 15 Mar 2016:
  
  Black holes are simply a very very large object. Even larger than our Sun. And the bigger an object is, the stronger gravity it has. For black holes, their gravity is so strong, this effect is so strong that even light can not go out. In because of that, we can actually not “see” directly a black hole, there is no light coming from it. We can only see it’s affects in the other objects near it or passing behind it. Like here:
  
  If you have seen Interstellar, that is a very close image of what a black hole would look like.
- Elie Allouis answered on 15 Mar 2016:
  
  Hi All,
  I will add a bit on Colombus Ops description …and correct one or two things 🙂
  Black holes are known as gravity singularity. This means that at this point in space the gravity is large enough to even prevent light from escaping.
  It’s like the densest thing (ever!), making a big dent in the fabric of space-time…but they don’t need to be big !
  They are formed by the collapse of matter on itself, like a star that ran out of fuel. They are typically classed by mass and the unit of mass is…our sun!
  Let’s start from the bottom:
  – you can have micro black holes, literally 0.1mm across, but as dense as the Moon. That means that you compress the whole mass of the Moon in just 0.1mm and voila…black hole !
  – Next one up, the Stellar black hole, about 30km across and a mass of 10 Suns
  – Intermediate mass black hole – about the radius of the Earth, but with a mass of 1000 suns
  – …and finally, the crowd pleaser, the supermassive black hole, up to 400 times the distance from earth to the Sun with a mass of billions of Suns
  
  Observations would appear to show that at the centre of a lot of galaxy we have super-massive black holes, so they may be way more common than we initially thought… and the funny thing is that over time (a long time), they slowly “evaporate”