As are all stars, Rigel is hot enough internally that "free" electrons produce a continuous ("rainbow")
spectrum. As that radiation passes through the outer layers of the star, some wavelengths are absorbed by atoms and ions in the stellar
atmosphere, producing the dark lines seen in the spectrum. The hot temperature of Rigel (11,000 K) means that the continuous background
is most intense at the higher-energy "blue" end of the spectrum. The precise line pattern for Rigel is produced primarily by Hydrogen since
that element is both abundant in stars and most efficient at absorbing visible radiation for temperatures near 10,000 K.