Question

Explain how we use a spectrometer to help us determine what stars and planets are made of?

Answer 1

@iambatman @dan815

Answer 2

Ah spectrometers, what a wonderful invention :)

Answer 3

Spectrometers help us find the wavelength/ frequency what ever, focusing on the light and the colours we see, we know they are composed of certain gasses like if we see neon, we will see a reddish color and we can get the wavelength of such a light. This might be a terrible explanation haha, try reading this:
http://bwtek.com/spectrometer-introduction/
http://en.wikipedia.org/wiki/Spectrometer

Answer 4

TLDR version (sorry batman not trying to steal your thunder :P ) : Every element in the universe has it's own light signature. In other words, it gives off radiation at SPECIFIC wavelengths, and ONLY those wavelengths. So this can be used to identify what kinds of elements are present in whatever the light is coming from.