Question: how does a black whole form

Andrew McKinley answered on 18 Nov 2014:

A star has a very very dense core which usually tends to be made from iron. It is though highly compressed iron from the sheer weight of material pressing down on it – think of how your ears pop as you dive in a swimming pool – this is the pressure of the water pushing on your ears. The density of this iron core then is much greater than the iron that we know at the Earth’s surface.

A star is basically a bomb, held together by two forces: the heat of the nuclear fusion wanting to throw all the gases out into the cosmos, and the gravity of the core pulling it all back in. When a star runs out of fuel though, the heat force diminishes, and the gravity starts to win. The star can suddenly collapse and, if the star is massive enough, this triggers a different explosive chain reaction and it will explode in a supernova, releasing all that energy. However, if the core is massive enough, it will capture all this material and pull it back to the centre, compressing it under huge gravity and eventually becoming a black hole from which nothing – not even light – can escape.

They are seriously weird things, and it is unfortunately not possible to observe them directly because nothing can escape!