Night Sky – The Universe
How big is it?
Scientists think that there are more stars in the universe than there are grains of sand on all the beaches on Earth – hard to imagine!
Earth is part of our Solar System, and our Solar System is a small part of the Milky Way galaxy. The Milky Way has 200-400 billion stars, with a supermassive black hole in the middle.
A galaxy is a collection of gas and dust and there are billions of stars in each galaxy. Scientists believe that there could be as many as one hundred billion galaxies in the universe.
How old is it?
Scientists believe that the universe is around 13,800,000,000 years old—that’s 13.8 billion years.
How did it begin?
13.8 billion years ago the universe was very small, very hot and very dense. So small, that it was thousands of times smaller than the head of a pin.
Then came the Big Bang – a massive explosion that started the universe. At first there were just tiny, heated particles, energy and light. As the universe started to cool down, the particles grouped together and formed atoms. Eventually the atoms grouped together to form stars and galaxies.
How do we know this?
In 1929 an astronomer named Edwin Hubble realised that the faint patches in the night sky were other galaxies and because they were faint, they had to be very far away. It would take billions of years for the light from those galaxies to reach us, so the universe had to be billions of years old. He also discovered that all those galaxies were moving away from us – so the universe had to be expanding. If they are moving apart, then they must have been closer together at some point, and from the speed of this separation, the age of the universe can be determined.
The Hubble Telescope was named after Edwin Hubble.
What is it made of?
So, the universe is made up of galaxies, planets and stars – black holes, comets and stuff – right? In fact, that is only 8% correct.
Scientists know that our universe is expanding, and it needs energy to expand. Scientists know that energy is there, but they do not know what it is or where it comes from. They call it dark energy and it makes up 65% of the universe.
There is also something that has gravity because scientists can see its effect on stars and galaxies. Scientists call this dark matter, but they do not know much about it. It makes up 27% of the universe.