If you collect the light coming off of a glowing sample, you can tell what the sample is made of simply by seeing how bright the sample is in each color. Elements and molecules absorb and emit different wavelengths (i.e. colors) of light depending on their internal structure.

For example, if you shine light through a sodium gas, the sodium will absorbs a particular shade of yellow light. So if we look at a star, and we see that this shade of yellow is missing, we can be pretty sure that the star has sodium in its atmosphere.

Question: how would I try to figure out whether in the above example, there wasn't a big cloud of sodium gas between me and the star, instead?

It turns out that each element (such as hydrogen, helium, oxygen, and carbon) can emit, absorb, or reflect radiation in a way that gives something like a fingerprint.By looking at how much radiation we get from an object at a range of wavelengths we can get an idea of what something is made of. We have to use our experience from what we know about various types of matter on Earth to make a best guess.

If you look at the light from stars you can see what are called "emission lines" You see, all matter has some frequencies of light that it emits more of. This just means that you may see a little blue in something where in another substance you see a different color. For example, if you look at "neon" signs you see a very distinct color based on what gas is inside... try to notice these kind of things, if you can.

Do you remember anything that has such a color. Try looking at a "neon" light or a sodium lamp (such as the orange streetlights). Where do the emission lines come from? As for planets, the same thing goes, but in their case it is light that is changed after being reflected from the sun.