Also, Mike, to do with
spectroscopy, a star is mostly hydrogen and helium, but it will also have other
elements in its make up as well. Depending on how hot the star is, some of these elements will appear in the
spectra of the stars, others won't. There are two types of spectra a star can have,
absorption and
emission. The type you normally see in the astronomy books is an
absorption spectrum, where you see a rainbow of colours with dark lines in the rainbow. Those dark lines are caused by the gases in the star absorbing some of the light that the star is shining. Each element absorbs the light at particular
wavelengths (like UV, visible or infrared) and from this you can tell what elements are in the star, how hot the star is and quite a few other things about the star. Sometimes an element
can have more than one dark absorption line present in the spectrum, because the element absorbs more than one particular wavelength of light. However, the line that is the strongest (easiest to see) is the wavelength which the element is absorbing the light the most in. As I said before, astronomers can use this to tell how hot a star is.
Hope that has helped
