Close examination of the spectra from the Sun and other stars reveals that the rainbow of colors has many dark lines in it, called absorption lines. They are produced by the cooler thin gas in the upper layers of the stars absorbing certain colors of light produced by the hotter dense lower layers. You can also see them in the reflected light spectrum from planets. Some of the colors in the sunlight reflecting off the planets are absorbed by the molecules on the planet's surface or in its atmosphere. The spectra of hot, thin (low density) gas clouds are a series of bright lines called emission lines. In both of these types of spectra you see spectral features at certain, discrete wavelengths (or colors) and no where else.
The type of spectrum you see depends on the temperature of the thin gas. If the thin gas is cooler than the thermal source in the background, you see absorption lines. Since the spectra of stars show absorption lines, it tells you that the density and temperature of the upper layers of a star is lower than the deeper layers. In a few cases you can see emission lines on top of the thermal spectrum. This is produced by thin gas that is hotter than the thermal source in the background. Unlike the case for absorption lines, though, the production of emission lines does NOT require a thermal source be in the background. The spectrum of a hydrogen-emission nebula (``nebula'' = gas or dust cloud) is just a series of emission lines without any thermal spectrum because there are no stars visible behind the hot nebula. Some objects produce spectra that is a combination of a thermal spectrum, emission lines, and absorption lines simultaneously!
What is very useful about discrete spectra is that the pattern of lines you see depends on the chemical composition of the thin gas. Each element or molecule produces a distinct pattern of lines---each element or molecule has a ``fingerprint'' you can use to identify it. This allows you to remotely determine what stars, planets, nebulae, etc. are made of!
The composition canNOT be found from just one line because one element may have one spectral line at the same wavelength as another element's spectral line. However, an element's pattern of lines is unique. Using a single line to identify a gas would be like identifying the name of someone using just one letter of their name---many people will have that same letter in their name, but the pattern of letters (which letters and how they are arranged) is unique to that one person. Of course, stars, planets, nebulae, etc. are made of more than one type of material, so you see the discrete spectra of many elements and molecules superimposed on each other---all of the spectral lines add together. An experienced astronomer can disentangle all the different patterns and sort out the elements and molecules (but it does take time!).
A couple of nice interactives to try matching spectral lines to chemical composition are
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last updated: September 18, 2005