Luke Fifield, of Cashmere High School, asks :-
How many stars can we see on a clear night?
Saskia Besier, an astronomy research student at the University of New South Wales, responded.
We can make a fairly good guess by counting them. This might sound easy but it is in fact very difficult because it takes SO much time. As astronomers, we cheat: we don't count every single star, but divide the sky up in to little patches. We count how many stars there are in a few patches, work out the average amount of stars in a patch and then multiply this number by the number of patches we can fit in the sky.
Try this yourself. This is a lot easier and quicker and maybe you would like to try it some time. The trick is to chose the right size patch to count stars in. You could make a small frame from wire or cardboard to hold up against the sky at arms length to count the stars within. Or try using an area the size of your hand or thumbnail at arm's length.
Eventually you should find there are about 2 or 3 thousand stars to see by eye at night but this number depends on a lot of things. City lighting and the light from the moon scattered off the atmosphere hide the weaker stars. If you live in a very large city you might be able to count every single individual star you see - there will only be about 250! People lucky enough to live in the country will see the best view of the night sky.
If we use a telescope to look at the sky we can see a lot more stars. The larger the telescope the more light gathering power it has so we see fainter and/or more distant stars. But we also see a smaller fraction of the sky. The main survey of the night sky has registered about a billion stars.
There are more stars to be seen in the southern hemisphere because we are looking towards the middle of our galaxy, the Milky Way. There are about a hundred thousand million stars in our galaxy.
If we multiply the number of galaxy clusters by the average number of galaxies per cluster and by the average number of stars per galaxy we estimate that there are about ten thousand million million million stars, about the same number as the number of grains of rice needed to fill about one hundred thousand million cathedrals. So there are plenty of stars for everyone!