What is a Black Hole exactly??

Question: What is a Black Hole exactly??

Asked by zackaroo222 to Meeks, Pete, Stephen, Steve, Tom on 21 Jun 2010 in Categories: General. This question was also asked by lucyc, isabella.
- Marieke Navin answered on 18 Jun 2010:
  
  I always describe a black hole as a region of space from which nothing, not even light can escape. Did you know that there is a black hole at the centre of our galaxy the Milky Way?
- Stephen Curry answered on 19 Jun 2010:
  
  You had to add the word ‘exactly’, didn’t you? 😉
  
  Doesn’t *anyone* know what it is ‘exactly’, I wonder?
  
  Black holes are believed to be massively heavy objects (usually created after the burn-out and collapse of a large star), so massive that their gravitational field even prevents light escaping. (Though I gather that Stephen Hawking’s work shows that they can nonetheless give rise to lots of radiation — Hawking radiation? — that makes them bright objects at times.)
- Tom Hartley answered on 19 Jun 2010:
  
  OK I think I know this, so I am going to try to answer from memory (rather than looking it up on wikipedia or in my Stephen Hawking book) – I like to do this because it makes I’m A Scientist more fun for me – if I get it wrong, the other scientists will put me right, I am sure.
  
  When a star gets very old it grows larger and more massive. At some stage it begins to run out of “fuel” and produces less heat. The heat that normally keeps it expanded like a balloon, as the fuel runs out it can no longer support its weight and collapses in on itself, all the mass is compressed into a much smaller space. If it has enough mass it will be so dense that even light cannot escape it’s gravitational pull. So anything (including light) entering the black hole cannot get out (making it appear dark) and (almost) nothing comes out.
  
  So that’s my answer, there could be a few mistakes in there. If so, I lose points. However if I am right I deserve a bonus because I did it without looking it up, and it is really not my specialist subject.
- Pete Edwards answered on 21 Jun 2010:
  
  A black hole is a tiny region of space into which a huge amount of matter has been packed. This leaves nothing that we can see but we can feel the effects of the enormous force due to its gravity reaching out into the space around it.
  Black holes are the corpses of giant stars, born at least ten times bigger than our Sun, that have run out of fuel and died. A star shines by “burning” hydrogen, fusing it into helium . Although the star may contain enough hydrogen to burn for billions of years eventually it will run out of gas. When this happens the core of the star cools, the core pressure drops and the force acting to support the outer layers of the star is reduced. The perfect balance of internal forces is now disturbed and the inward force due to the gravity of the stars gas overwhelms the outward pressure and the outer layers of the star collapse. It’s like somebody has pulled the floor away. As the outer layers rush inwards they compress the core of the star, squeezing it tighter and tighter, until it can’t be squashed any further. The inrushing gas then hits this solid core and bounces off in a massive explosion we call a supernova.
  This explosion is so powerful that the force of it can compress the core of the star even further. This concentrates the mass of the core into such a small space that the force due to its gravity is so strong that it swallows itself! Most of the mass of the star is squeezed into a single point, creating a black hole. It is hard to picture the immense forces at work during their creation, but you would need to squash the entire mass of the Earth, that’s around 6000000000000000000000 tonnes, down to the size of a garden pea for it to turn into a black hole!

Question: What is a Black Hole exactly??

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