Spark image

Life and death of stars

Our Sun is a star. It was formed when a huge cloud of gas (mostly hydrogen) slowly condensed a few thousand million years ago. This huge cloud slowly contracted because of the pull of gravity and as it shrunk its density increased, the gravity pull increased further and the density and pressure at the centre of this contracting ball got greater.

Eventually the pressure and temperature at the centre was big enough for the nuclei of the hydrogen atoms there to start joining together to make heavier elements - (nuclear fusion), large amounts of energy were given out the Sun had begun to shine.

This all happened around ten thousand million years ago and since then the Sun has gone on shining, releasing huge amounts of energy into space. To do this it converts matter into energy at an enormous rate, its mass reducing by some four million tons every second! Don't worry there is still a lot left and the Sun won't stop shining for many millions of years.

The Earth only receives a little of this energy but is enough to support life on our planet.

As time passes more and more of the hydrogen will be converted to helium and so the core of the star will run out of hydrogen. The outer layers of the star will collapse inwards, heat up and the star will expand. The star will grow to become a Red Giant, cooling as it expands and its surface temperature will fall from 6000 oC to a little over 3000 oC. It will become a Red Giant. Red Giants are huge stars, big enough to swallow up the orbit of Mars. It is likely that the Sun will reach the Red Giant stage in something between 5000 and 8000 million years in the future. The core of the star will now start to fuse helium to make heavier elements, some of the outer layers of the star may be blown off to form a planetary nebula.

Then, as more fuel is used the Red Giant will shrink to become first a white dwarf and finally as its fuel is exhausted it will go out leaving the dark remains a Black Dwarf.

The end of massive stars

What we have just described is what will happen to a medium sized star like our Sun massive stars have a different end. The pull of gravity is so great that they begin to shrink and the internal temperature rises. Heavier and heavier elements are fused in the core and the energy output increases. Finally iron is produced and the core collapses to a diameter of about 100 km in less than a second. The shock wave of energy is so great (as much as a whole galaxy) that instead of becoming a relatively cool red giant the temperature at the surface reaches 200 thousand degrees. The star disintegrates in an enormous explosion called a supernova.

The Crab nebula in Taurus is the remains of a supernova first observed by the Chinese in 1054.

We can see gas clouds in our galaxy where new stars are being created now. Have a look at some of the superb images produced by the Hubble space telescope to see this happening very clearly.

One final thing to think about. On the Earth there are many heavy elements. The only place where such elements can have been formed is in the enormous temperatures of a supernova explosion. This means that it is likely that the Sun has had a previous existence as a much more massive star than it is today. Its life ended as a supernova explosion and the Sun we see today and all the planets were formed from the material of that explosion.

We have been here before!


© Keith Gibbs 2020