Andromeda Galaxy in the Ultraviolet

This is an ultraviolet image of Andromeda from NASA's Galaxy Evolution Explorer. Image credit: Galex, NASA/JPL-Caltech

The Andromeda Galaxy, M31, is the nearest major galaxy to our own Milky Way. It is about 780 kiloparsecs or 2.5 million light years away, as measured by it's brightest Cepheid variable stars. It is rather similar to our own galaxy in structure. It shows spiral arms and is about 80 kiloparsecs or 260000 light years across. This image is a composite of 11 image segments. This view is two-color composite, where blue represents far-ultraviolet light, and orange is near-ultraviolet light. The individual stars in the image are foreground stars in our own galaxy.

From NASA's description: "The bands of blue-white making up the galaxy's striking rings are neighborhoods that harbor hot, young, massive stars. Dark blue-grey lanes of cooler dust show up starkly against these bright rings, tracing the regions where star formation is currently taking place in dense cloudy cocoons. Eventually, these dusty lanes will be blown away by strong stellar winds, as the forming stars ignite nuclear fusion in their cores. Meanwhile, the central orange-white ball reveals a congregation of cooler, old stars that formed long ago."

"When observed in visible light, Andromeda's rings look more like spiral arms. The ultraviolet view shows that these arms more closely resemble the ring-like structure previously observed in infrared wavelengths with NASA's Spitzer Space Telescope. Astronomers using Spitzer interpreted these rings as evidence that the galaxy was involved in a direct collision with its neighbor, M32, more than 200 million years ago."

"Andromeda is so bright and close to us that it is one of only ten galaxies that can be spotted from Earth with the naked eye. "

References:
NASA mission pages