Star Light The spectrum of light a star emits can tell us what type of atom is present both inside the core and in its atmosphere. The pattern of dark lines (absorption lines) is a fingerprint of each chemical element. The light from stars can tell us a lot more information than just the chemicals that are present inside the star. The position of the absorption lines can tell us how the star is moving relative to us on Earth.

Doppler Effect The Doppler effect is used to explain the change in pitch of the sound of a speeding car as it races past. When the car is coming towards us we hear a higher pitch. When the car is going away from us we hear a lower pitch. The sound waves have been squashed together causing the wavelength to decrease and frequency to increase. The sound waves have been stretched causing the wavelength to increase and the frequency to decrease. The greater the speed of the car the greater the change in the normal pitch of the sound of the car.

Red shift of Light Blue shift Red shift A similar thing happens to the light emitted from a moving light source. However, it is much more difficult to observe. The change in the wavelength or frequency is only noticeable for very fast moving objects. In the example below a very fast moving yellow light appears to have its light changed when observed. Blue shift Red shift Light moved to blue end of spectrum: Higher Frequency - Shorter Wavelength Light moved to red end of spectrum: - Lower Frequency - Longer wavelength

Shift in Star light Our understanding of the Universe has changed considerably in the last few hundred years. Theories come and go in science, for example, people used to think that the Earth was flat and was supported on the back of a turtle!!!! However, our picture of the Universe fundamentally changed when we look for red shift and blue shifts in star light. The shift in the dark lines in the light emitted by stars tells us how the star is moving. - Red shifted dark lines - the star is moving away from us. - Blue shifted dark lines - the star is moving towards us.

Receding Galaxies If we look at the light emitted by distant galaxies then we see the same effect, the dark lines have been shifted to longer wavelengths. Red End Blue End Our Galaxy Distant Galaxy The light has been red shifted – the distant galaxy is moving away from us! The amount of red shift tells us how fast it is moving: - the greater the red shift the faster it is moving away from us. This has very important implications to our theory of how the universe began.

Expanding Universe Edwin Hubble carried out a survey of hundreds of galaxies and discovered something that would tell us how the universe began. He found out that: All galaxies show a red shift in their light. This means all the galaxies are moving away from each other. More distant galaxies have a larger red shift and are therefore moving away faster than closer galaxies.

What is Expanding? Imagine the galaxies are like dots placed onto the surface of a balloon. As the balloon is inflated the dots move further apart just like the galaxies in our universe. However, it is the fabric of space which is expanding and causing the galaxies to spread out. As the waves of light travel through space it stretches the waves and makes the wavelength longer.

The Big Bang The Hubble Space Telescope has allowed us to get very detailed images of very distant galaxies in order to refine this theory. If the universe is expanding and all the galaxies are moving apart then, at some point earlier in time, they must have started from a common point in space. Astronomers think that the universe started with a huge explosion now known as the BIG BANG! Recent estimates of the age of the universe put it somewhere between 10 and 15 Billion years ago. So, let us say that 12, 000 Million years ago the Big Bang occurred which created all of space and time.

Timeline of the Universe