Spark image

Forces in stars

A medium sized star like our Sun is actually an enormous object. The Sun has a diameter of 1400 thousand km (over 100 times that of the Earth) and a mass of 2 million million million million million kg (about 300 000 times that of the Earth).

This enormous mass means a very high gravitational pull a person weighing 600 N on the surface of the Earth would have the colossal weight of 16400N if they stood on the 'surface' of the Sun. As much as a large car on Earth!

The nuclear fusion reactions going on within the Sun generate huge amounts of energy in the form of radiation and this streams upwards through the Sun until it eventually leaves the surface and is radiated out into space.

If the star is stable the gravitational forces acting inwards to the centre of the star just balance the pressure of the radiation streaming out and stop the star from blowing itself apart.

As the star gets older there are two possibilities:
(a) The nuclear fusion reactions get less as the radiation flooding outwards gets less. In this case the gravitational forces overcome the radiation pressure and the star collapses in on itself

(b) The nuclear fusion reactions increase and so the gravitational forces cannot hold the star together and it blows up in an enormous explosion a supernova.


© Keith Gibbs 2020