This is the chant hymn for vespers for the feasts of the Immaculate Conception and Assumption.
The name Mary is said to mean “Star of the Sea” = stella maris.
See a list of other popular chant themes here.
Charles Tournemire – Ave Maris Stella – Ste. Clotilde, Paris, France *original audio recorded by Tournemire with slide show of pictures for video