Stars and galaxies
Stars can be classified as different types depending on their brightness and colour. For many stars, the spectrumspectrum: A series of similar waves, arranged in order of wavelength or frequency. of light given out is an indication of how bright they are. Two stars with exactly the same spectrum should be of similar brightness. If one is much dimmer, that is because it is further away. This can be used to estimate the distance of stars that are too far away to show much parallax movement over the year.
Very distant stars gave out the light we see them by a long time ago. The light reaching us from a star which is 10,000 light-years away left that star 10,000 years ago. This fact, together with the theories of nuclear fusion in the cores of stars, has allowed astronomers to work out how stars are produced and how they eventually die out.
Stars of different sizes have different life cycles. This slideshow shows some possible life cycle patterns for different types of stars.
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