The Expanding Universe
We live in a galaxy called the Milky Way. It contains billions of stars. But there are billions of galaxies in the Universe so it is hard to imagine the total number of stars.
Red Shift - if a source of waves is moving relative to an observer, the wavelength and feequancy 'seen' by the observer will have changed or shifted from the original produced by the source.This effect can be heard with different sound wave. For example the sound of an ambulance siren will sound different depending on whether it is moving towards you (pitch is higher) or away from you (pitch is lower.) The effect occurs with light waves. Light observed from distant galaxies has been 'shifted' towards the red end of the spectrum. This means the frequancy has decreased. The further away the galaxy the bigger the red shift.
This suggests that distant galaxies are moving away from us, and most distant galaxies are moving the fastest. This is true of galaxies no matter which direction you look.
Galaxies are collections of billions of stars. Our Sun is one of the stars in the Milky Way. The Universe is made up of billions of galaxies.
The Big Bang
Red shift shows us that distant galaxies are moving away from us and the furthest ones are moving fastest. This gives us evidence that the Universe is expanding outwards in all directions.
We can try and imagine back in time to see where the Universe came from. If it is now expanding outwards, this suggests that it started with a massive explosion at a very small initail point. This is the Big Bang theory.
Looking Into Space
Scientists use telescopes to collect the visable light coming from the starts, and also to see them. They can also use telescopes that collect radaition from other parts of the electromagnetic spectrum such as X-Rays and radio waves. This also allows them to see distant stars.
The atmosphere is a layer of gases surrounding the Earth.
Telescopes on satellites are able to recive all types of electromagnetic radiation from space, without it being distroted or aborbed by Earths atosphere. Because there is no distortion, the pictures produced by satellites are much clearer and have more detail. So it is possible for us to observe stars that are further away.