• Question: black holes are caused by stars exploding due to their fuel but how does the fuel decrease but cause it exploding?

    Asked by anon-206378 to Zoe, Kai, Jose Eliel, Hannah, Hamid, Claire on 9 Mar 2019.
    • Photo: Hannah Collingwood

      Hannah Collingwood answered on 9 Mar 2019: last edited 9 Mar 2019 10:29 am


      I’ll try to answer this one. This is going to get rather technical, so bear with me!

      I’ll start with gravity. Stars form from clouds of dust and gas (usually hydrogen) in space. The dust and gas have mass which means that other bits of dust and gas move towards each other, the same way that anything we drop moves towards the Earth. If the cloud of dust and gas is big enough, it will get very hot and very dense and nuclear reactions will start occurring.

      Stars produce light (and heat and lots of other things) through the process of nuclear fusion. This is where light elements (mainly hydrogen) are combined to make heavier elements (like helium) and this releases a lot of energy in the process.

      We have two forces here: gravity holding the star together, and radiation pressure which acts in the opposite direction. When the star has plenty of fuel, these two forces are balanced. Eventually the star will run out of fuel, and the radiation pressure won’t be able to balance gravity. This is where heavier elements (carbon, oxygen, iron, etc) get produced.

      This is where it gets exciting. If gravity is stronger than the radiation pressure, the star will start to contract and become smaller. But there’s another force called electron degeneracy pressure which tries to stop the star collapsing further. It’s a little like a balloon, where the elasticity of the rubber tries to make the balloon collapse (like gravity) but it can’t due to the air inside (like electron degeneracy pressure). Small stars turn into white dwarfs where the gravitational force is balanced by the electron degeneracy pressure. But bigger stars have a much greater gravitational force so keep collapsing. If the star is big enough, it can get dense enough to have a gravitational field so strong that not even light can escape. This is what we call a black hole.

      Where do the explosions come in? There are different types of supernovae (exploding stars). Some of these supernovae used to be white dwarfs. These white dwarfs still have mass so dust and gas continue to move towards the star due to gravity, and this extra mass might be just enough to allow the star to ignite any fuel it has left. Other supernovae are caused by the collapse of the core of the much heavier stars. Heavier elements are produced when the star starts to run out of fuel and the core of the star contains the heavier elements. The core then contracts due to gravity. When the gravitational force becomes greater than the electron degeneracy pressure, the core collapses which creates the supernova. The core might then go on to become a black hole if the star was big enough.

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