As are all stars, Antares 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 "low" temperature of Antares (3000 K) means that the continuous background
is most intense at the lower-energy "red" end of the spectrum. The precise line pattern for Antares is produced primarily by neutral metals (e.g. Na)
and even some molecules (particularly TiO) since those are most efficient at absorbing visible radiation for the temperature conditions of this star.
Spectrum of Antares