Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

July 13, 1996

M81: A Bulging Spiral Galaxy
Credit: NASA, UIT

Explanation: 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 centre of the galaxy. A massive density wave circulates around the centre 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.

Tomorrow's picture: Galaxy M81 in True Colour

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Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA)
NASA Technical Rep.: Jay Norris. Specific rights apply.
A service of: LHEA at NASA/ GSFC
&: Michigan Tech. U.