This is done primarily by analyzing the light that
comes from the star. You can try this simple
experiment at home but first ask your parents to
supervise. Turn on a burner on the stove. Notice
the nice blue glow of the flame. Now take a pinch
of salt - just a little - put it on a spoon and
put the NaCl into the blue flame. What happens??
The flame should turn very yellow!! Why? Because
when sodium (salt= NaCl) is heated, the electrons
in the outer shell jump around to different
levels. When they do this they emit light of a
certain color. In this case its yellow. But this
same technique we can figure out the composition
of different stars
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