Julia van Luijk, of Cashmere High School, asks :-

How do scientists know the Sun won't start turning into a red Giant now?

Alan Gilmore, an astronomer at the University of Canterbury's Mt John Observatory, responded.

We know that the sun is made mostly of hydrogen. We learn this by analysing its light in a spectrograph. From studies of how atomic nuclei combine we know that the sun makes its energy by turning hydrogen into helium. Four atoms of hydrogen make one atom of helium and a lot of energy. Hence the hydrogen bomb!

By measuring the light and heat from the sun we can calculate how much energy it is making. From that we can work out how fast the sun is `burning' its hydrogen: about 600 million tonnes per second. Four million tonnes of this becomes energy, the rest becomes helium.

From the pull of the sun's gravity, measured by the planet orbits, we know how heavy it is. And most of it is hydrogen. If the sun were to burn all its hydrogen at the current rate then it could keep going for 100 billion years. However, it doesn't work that way. The hydrogen is being burnt only in the core of the sun. The core has enough to keep it going for around 10 billion years.

When the core's hydrogen supply is exhausted then the core will compress into a smaller hotter ball. It will begin converting helium into heavier elements. The outer layers will expand away from the hot core and the sun will become a red giant.

From studies of radioactive elements in rocks and meteorites we know the planets formed about around five billion years ago. The sun began then too. So the sun is around halfway to becoming a red giant. Just five billion years to go.