The Sun

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.