Spectra are like the barcodes used at the supermarket checkout; each pattern can be scanned to provide information about the star. The main reason spectra of different stars are different is that the stars have different surface . While we know that each leaves a different pattern in the spectrum, mostly the stars have roughly similar , with only a few exceptions. The most common element in all stars is . Astronomers use the patterns in the spectra of stars to sort all stars into categories called ; the hottest stars are given the letter __. Objects which are not hot enough for hydrogen fusion to take place in their cores are called .