Keith Ross, of Taumarunui, asks :-

Has the Sun's diameter changed over the past 100 million years and is this a likely problem for the Earth?

Christopher Tout, an astrophysicist at the University of Cambridge (UK), responded.

The Sun is now about four and a half thousand million years old.

It has been growing since it first ignited hydrogen fusion in its core and has reached a rate of about 2.5cm per year. In the last hundred million years it has grown by two and a half thousand kilometres, enough to get a fair way across Europe.

Since birth the Sun has grown in radius by almost 80,000km but this is only about a tenth of it's total radius of 700,000km. As it ages the Sun will grow at an increasingly faster rate.

When it exhausts hydrogen fuel in its centre, at an age of about ten thousand million years, it will be 36 per cent bigger than it is now. Then the evolution of the Sun will accelerate. It will become a red giant and grow to 200 times its current radius, reaching almost to the Earth's present orbit, in the subsequent 2,000 million years.

However the Sun is losing mass in its stellar wind which will get much stronger as it grows. At its maximum size the Sun will have lost as much as two fifths of its mass. The weakened gravitational attraction means that our orbit will have grown by two thirds and the Earth will safely escape the Sun. Not that this will help very much! The temperature will have risen to over a thousand degrees and we would need to be as far away as Jupiter to find a habitable region of the solar system.