Sarah Newbould, of Port Chalmers School, asks :-

How many stars are there?

Saskia Besier, an astronomy research student at the University of New South Wales, responded.

We can make a fairly good estimate by looking at how many stars there are in galaxies and how many galaxies there are in the universe.

There are different types and sizes of galaxies in the universe. We live in a spiral galaxy - the Milky Way - which is a bigger than average spiral galaxy and has about one hundred thousand million stars in it. Other 'spirals' may have more stars than this but most have less. The largest type of galaxy can have up to ten million million stars in them.

To give you a better idea of how many this is, if every star was the size of a rice grain, the stars in the Milky Way would totally fill the local Cathedral.

Like stars, galaxies come in groups which we call 'clusters'. Small clusters contain a few galaxies, our cluster contains about 20 galaxies and large clusters contain many thousands of galaxies. We also see clusters of clusters which form the largest structures in the universe These `superclusters' each contain millions of galaxies.

From looking at the size of clusters and superclusters we can say that there are approximately one hundred million million galaxies in the universe which adds up to about ten thousand million million million stars (1 with 22 zeros after it!), the same number as the number of grains of rice needed to fill about one hundred thousand million Cathedrals!