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Transcript
…a quick reminder… …about the nature of light… …if we look at the light from the sun… …we will see… …mysterious black lines in the sun’s spectrum… …which can tell us what the sun… …(or any other star)… …is made of… …these specific lines tell us there is hydrogen in the sun… …in fact… …the sun is almost entirely made of hydrogen… …and there’s a few other things in there too… …now, if we look at the spectra of distant stars… …we find the lines aren’t quite where they should be… …they have moved… …towards the longer wavelength ‘red’ end of the spectrum… …we call this redshift… …and it’s caused by the stars moving away from us… …so the waves ‘spread out’ to longer wavelengths… …in fact, everything around us is moving away… …let’s try a couple of questions and see what we understand…
