Twenty years ago, the answer this question was ‘only one that we know’. Most astronomers believed that the Sun was no different to most stars in the universe. So if we had a star with planets and moons – a solar system – than why don’t others.
In 1992, the first planet orbiting another star – sometimes called an exoplanet – was detected. Since then, astronomers have used larger telescopes and faster computers to find more than 450 exoplanets orbiting more than 300 stars.
Astronomers measure the distance to other galaxies by looking at their colour. The more red a galaxy is, the faster it is moving from us and the further it is away.
Another way to measure distance is to look at the brightness of exploding stars – called supernova. These explosions are brighter the closer they are to us. By measuring the brightness it is possible to know how far away they, and the galaxy they live in, are from us.
Using these techniques astronomers have seen galaxies more than 13 billion light years away.
Space telescopes that measure cosmic background radiation, which are the remains of the Big Bang explosion. It is thought to be the outer edge of our Universe and is 13.7 billion light years away.
This is a difficult question to answer, because the number keep growing.
Almost every year astronomers find new moons, using better telescopes on Earth and spacecraft flying through the solar systen. Some moons are so small that that their width is smaller than most cities on Earth.
Stars come in different many different sizes and the bigger the star is, the faster it will burn its fuel.
Our sun is an average star and will take about 10 billion years to burn its fuel. The brightest star in our night sky, Sirius, is 21 times the size of our sun and may only last a billion years. Some stars will burn so brightly, they last less than 10 million years.
We can tell how old a star is by looking at its colour – what scientists call its spectra. A spectra is like a chemical fingerprint, showing what is inside the star. Young stars have simple chemical elements, such as hydrogen and helium. As the star gets older, heavier elements like carbon and oxygen appear.
The spectra of our sun tells astronomers that it is 5 billion years old, or about half way through its life.
A galaxy is a group of stars that are held close together. They come in different shapes and sizes. Some have millions of stars, others have billions.
Most galaxies look round like a ball or oval like an egg. These are called elliptical galaxies.
Our galaxy, the Milky Way, looks like a fried egg – fat in the middle and thin on the edge with arms like an octopus. This type of galaxy is called a spiral galaxy.
Some galaxies have no shape at all and are called irregular.
Astronomers believe there are 125 billion galaxies in the universe. That’s a lot of stars.