How can a star's composition be inferred by analyzing its spectral pattern?

I need to know how can a star's composition be inferred by analyzing its spectral pattern.

Please help!How can a star's composition be inferred by analyzing its spectral pattern?"Spectrometry (spectroscopy) is the essential tool used by astronomers since the 1800s to measure the composition, color and temperature of stars through analyses of emitted light spectra. Each chemical element reveals a distinct banding pattern in a star's absorption spectrum. When light from a star splits by prism or grating into a spectrum of wavelengths, the spectral pattern reflects the star's composition. While all stars are 95 percent hydrogen, variations in composition reveal age, luminosity and origin. Spectroscopy was out of reach for most amateur astronomers until the recent development of reasonably priced spectroscopes costing hundreds to thousands of dollars."How can a star's composition be inferred by analyzing its spectral pattern?It's even stranger than you suppose. Helium, for instance, was found on the sun using spectrometry before it was found on Earth. When Newton looked at sunlight through prisms, he didn't notice the dark lines in the spectrum. Kirchoff was probably the first to marry spectroscopy to astronomy, finding evidence of sodium in the sun. His banker, as a banker would, told him that gold in the sun would be of no use, since there's no way to collect it, but when Kirchoff won a prize for his discovery, he deposited the amount in that bank, noting that he was depositing gold from the sun.