What allows us to observe a supernova and identify the elements that explode from it? I understand that a supernova is the only thing that can make most of the elements that are denser than oxygen. How do we know this? How do we know what makes up stars, gases etc?
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We use a spectrograph to analyze the light signal, and look for characteristic absorption or emission lines (depending on what we are looking at) that represent electron state transitions for particular elements. By comparing relative heights (strengths) of the signal, we can calculate the relative proportions of the element.
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A spectrograph or spectroscope.