Explanation:
Bright Nebulae of Galaxy M33.
The magnificent spiral galaxy M33 seems to have a particularly large number of bright emission nebulae. This sharp and beautiful image, combining narrow-band and wide-band data, presents the loose spiral arms winding around the galactic core, scattered with red emission nebulae and HII regions where stars are being born. Historically, galaxy M33 has received much attention from astronomers because its HII regions are among the largest of such star-forming regions, and it is the birthplace of many very short-lived massive stars. Intense ultraviolet radiation from these massive bright stars ionizes the surrounding gas, producing a characteristic red glow. Spanning over 50,000 light-years, galaxy M33 is also known as the Triangulum Galaxy. It is one of the large members of the Local Group and is located about three million light-years away from us.
Translation from: http://www.skylook.org/apod/astropix.html
NASA Website:
http://antwrp.gsfc.nasa.gov/apod/astropix.html
http://antwrp.gsfc.nasa.gov/apod/ap091017.html
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