Stellar associations in the spiral arm S4 of the Andromeda galaxy.

Plates taken with the 2-m Ritchey-Chretien telescope of the Bulgarian National Observatory yield color-magnitude diagrams for several OB associations in spiral arm S4 of the Andromeda Nebula, enabling the luminous OB stars and red supergiants in the galaxy to be distinguished from foreground objects. The age of each association is estimated from the magnitude of its brightest blue member star. Age structure can be discerned along the main axis of M31, as well as along and across the S4 arm.