The Sun

The Sun Today
The current Sun from YOHKOH

Where did the world come from? And what is its ultimate fate? Philosophers and scientists have struggled with these questions for millennia, most recently arriving at the Big Bang Theory, which explains how the Universe might have been made from nothing. The problem is that a Big Bang would only create hydrogen and helium, the simplest and lightest elements. What could have produced carbon, oxygen and everything else so essential to life?

Hydrogen can be converted into other elements by thermonuclear fusion, the process that powers stars and keeps the Sun shining, so it has long been suggested that we were formed from elements made inside stars. Unfortunately, the only element produced by the Sun so far is helium --- largely irrelevant to the Earth --- and most of that is still trapped in the Solar core. The only explanation is that we live in a second generation solar system, the elements now on Earth scattered by prehistoric explosions. The Hertzsprung-Russell diagrams, shown here, help us to understand how this happened.

In the diagram opposite, Hertzsprung and Russell have plotted many stars' brightness against temperature, which we know from the star's colour: "white hot" means a higher temperature than "red hot", for example. Most seem to lie on a diagonal line across the diagram, which they called the "main sequence", but there are also two distinct groups towards the corners, descriptively named "red giants" and "white dwarfs". There seem to be more red giants, but this is only because they are so bright and thus easier for us to see.
The main sequence represents stars such as the Sun, which are fusing hydrogen into helium: most stars sit here for most of their active lives. A star's position on the line is related to its size: the bigger and heavier the star, the greater the pressure at its core and the more violent its nuclear reactions.

Hertzsprung-Russell Diagram showing the Main Sequence


The two corner clusters can be explained by charting the life of a star, for example the Sun, as plotted below. After billions of years on the main sequence, the Sun will have converted most of its core hydrogen into helium, and in turn start fusing helium into elements such as carbon and oxygen. The star expands, so it gets brighter but colder: a red giant. When the Sun reaches this stage it will grow so big that it engulfs all the inner planets, including Earth.

HR diagram showing the Evolution of the Sun

The largest giants do not last very long: the nuclear reactions at the star's core Become so intense that it explodes in a supernova, spreading carbon, oxygen, etc. over a wide volume of space, with many more rare and heavy elements such as gold and uranium produced during the explosion itself. Eventually this material combines with clouds of interstellar hydrogen to form new stars, planets and perhaps life-forms.

So what of the Sun? The pressure at its core isn't great enough to ignite fusion of much more than helium, so once that has been used up it will stop burning and shrink under its own gravity to a very hot, dense state: a white dwarf. Most stars are white dwarfs, slowly cooling over eternity to become red then black dwarfs: dead lumps of carbon ash. We are dust, and to dust we shall return.

Andy Dornan

Last Revised: 2010 January 29th