Josh Dent, of Balclutha Primary School, asks :-

How did the sun get so hot and stay hot?

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

The second part of the question is the easiest to answer. The sun stays hot because it has a giant hydrogen bomb in its centre. The bomb doesn't blow up the sun because the sun is very heavy. The outside part of the sun squeeze down on the centre. So the 'bomb' is squeezed and stays the same size.

The sun is made mostly of hydrogen. It keeps shining from energy got from turning hydrogen into helium. Every second the 657 million tonnes of hydrogen is converted to 652.5 million tonnes of helium in the sun's core. The missing 4.5 million tonnes is converted into energy. This keeps the core of the sun at a temperature of 14 million degrees C. This heat gradually spreads out to the surface of the sun. It keeps the sun shining. The sun's surface temperature is 5500 C.

It sounds like a lot of hydrogen is being used up, but the sun has plenty. It is very heavy. It weighs 2,000,000,000,000,000,000,000,000,000 tonnes. It started with enough hydrogen in its core to keep it hot for 11 billion years. Since it is now 4 billion years old, it has enough left for another 7 billion years.

The sun, like all stars, began in a big cloud of gas and dust in space. You can see these clouds looking like holes in the Milky Way. The cloud gradually pulls itself together by its own gravity. It breaks up into smaller clouds. Each of them shrink down. Squeezed gas gets hot. As gravity squeezes a blob of gas its centre gets hot. Eventually it is hot enough for hydrogen to become helium. Then the blob becomes a star. This is how the sun got hot.