Spanning more than one degree of sky, the Great Pinwheel of Triangulum, is a fine example of an Sc-type spiral galaxy. Being a member of our very own Local Group of Galaxies, M 33 resolves into a myriad of fascinating structure for the modern astrophotographer. Of particular interest are the various regions of the galaxy as defined by their coloration. For example, the central disc or hub of the galaxy has a yellowish hue from the relative concentration of older Population II stars. Meanwhile, the arms spiraling outward from the galactic center are littered with much younger Population I stars, which appear blue due to their higher temperatures. The copious energy found in the vicinity of such hot stars is also made evident by the multitude of glowing HII regions also found along the spiral arms. In order for a mass of simple hydrogen to become ionized and emit it's pinkish light, a nearby star with a surface teperature of atleast 28,000 Kelvin is required. Many of the HII regions found throughout M 33 are actually enormous hollow "bubbles" or shells of gas much like the nearby Barnard's Loop found in our own stellar neighborhood. The most prominent HII region in this image, however, is NGC 604 located to the upper left from the galaxy's core. NGC 604 is an brilliant example of a "giant extragalactic HII region", which simply means that it is much more massive and brighter than any such area found in our own galaxy. Indeed, if NGC 604 were as close to us as the Orion Nebula, it would cover most of the constellation of Orion! A much more dramatic view of this particular object by Hubble Space Telescope can be found by clicking here. Many other groupings of young blue stars (known as OB associations) can also be resolved in this image, including OB112 which is located just below the very blue 8th magnitude star near the bottom of the galaxy. Image taken with homemade 8-inch f/5.4 astrograph and SBIG STL-1100XM. LRGB image composed of 20 minutes R,G,B and 60 minutes L. Please click on the above image for the high resolution version.