The Triangulum Galaxy is also called M33, because it is the 33rd object in Charles Messier’s sky catalog. The galaxy is about 2.9 million light-years away from Earth in the constellation Triangulum. The Triangulum Galaxy is a member of our Local Group, which is the small cluster of nearby galaxies that includes our Milky Way Galaxy and the Andromeda Galaxy (M31). Similarly to the Milky Way, the Triangulum Galaxy has a spiral shape. However, its visible disk is about 50,000 light-years across, almost half the diameter of the Milky Way Galaxy, and astronomers believe it has only about one-tenth the mass of the Milky Way.
The mosaic is based on several images captured by Swift’s Ultraviolet/Optical Telescope (UVOT) through three separate ultraviolet filters. The images were taken between December 23, 2007 and January 4, 2008. The mosaic’s high quality enables one to distinguish between individual star clusters and star-forming gas clouds, even in the crowded nucleus of the galaxy. Moreover, one can see the Milky Way foreground stars and stars in much more distant galaxies shining through the Triangulum Galaxy.
Young, hot stars heat up the surrounding gas clouds to such high temperatures that they radiate brightly in ultraviolet light. In the image, the giant star-forming region NGC 604 is shown. With a diameter of 1,500 light-years (40 times that of the Orion Nebula), it is the largest stellar nursery in our Local Group.
“The ultraviolet colors of star clusters tell us their ages and compositions,” says Swift team member Stephen Holland of Nasa Goddard. “With Swift’s high spatial resolution, we can zero in on the clusters themselves and separate out nearby stars and gas clouds. This will enable us to trace the star-forming history of the entire galaxy.”
More information on the Triangulum Galaxy can be found on NASA’s official press release.