The GALEX observation of the large galaxy in Andromeda, Messier 31, was performed during September and October 2003. The image is a mosaic of 10 separate GALEX images combining the FUV (colored blue) and NUV detector (colored red) images together to make a color image of the sky nearly six full moons across.
The Andromeda galaxy is the most massive in the local group of galaxies, which includes our Milky Way, and is the nearest large galaxy to our own.
The GALEX image shows blue regions of young, hot, high mass stars tracing out the spiral arms where star formation is occurring, and the central orange-white "bulge" of old, cooler stars formed long ago.
The red stars in this image are foreground stars in our own Milky Way galaxy.