July 13, 1996
There are stars whose light will never reach us, worlds we will never touch, mysteries we may never solve.
M81: A Bulging Spiral Galaxy
Birthday · July 13, 1996
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APOD published July 13, 1996. Full image credits are on the original APOD entry below. A publication-date match, not a photo taken on your date. Independent of NASA.
Few stars are still forming in the old giant spiral galaxy M81. The blue regions in this picture - representing ultraviolet light - highlight regions of bright young stars and star formation and appear rare than in M74 and M33. The red regions - representing the visible light - show a large population of older, less massive stars. M81 is therefore classified as spiral galaxy type "Sab" on the Hubble Sequence of Galaxies. One distinguishing feature of these types of galaxies is the relatively large central bulge surrounding the center of the galaxy. A massive density wave circulates around the center of spiral galaxies. It is not well understood why the bulge of M81 glows as bright as it does in ultraviolet light. Speculation includes that this may be due to hot evolved stars such as those found in the ancient globular cluster Omega Centauri.