Gabrielle Still, of Cashmere High School, asks :-

What would happen to the Earth if our Sun turns into a red giant?

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

The question is not 'if' but 'when' the sun turns into a red giant.

At present the sun makes its energy by turning hydrogen into helium. This thermonuclear action happens in the core of the sun where it is very hot.

Around five billion years from now the hydrogen in the core will be used up. The core will begin to contract and get hotter, as any squeezed gas gets hot. Hydrogen surrounding the core will be heated and will also `burn' faster. The hotter core will cause the outer layers of the sun to expand.

The sun will expand until it is about 200 times its present size. Its surface will be near the earth's orbit. The surface temperature will be around 3000 degrees, half its present temperature. At this temperature it will orange-red in colour; hence the 'Red Giant' label.

Though cooler, the surface will be thousands of times bigger than now. So the total energy radiated will be thousands of times greater than now. This will vapourise the earth and the other planets close to the sun.

There are several red giant stars visible to the naked eye. Betelgeuse, north of Orion's Belt ('the saucepan' or 'the pot') is in the summer sky. Antares in the Scorpion is nearly overhead in winter. Don't confuse the red planet Mars with a red giant star.