One of the final entries in Charles Messier's list of comet imposters is M101, the dominant member of a small group of nine galaxies. M101 is truly a giant galaxy. With a visible diameter of 170,000 light years, M101 is one of the largest disk galaxies known. Although its size exceeds that of our Milky Way its overall mass is similar to our galaxy.
A unique feature of M101 is the large number of extremely luminous HII regions that populate its spiral arms. Several of these HII regions are bright and large enough to earn individual catalog numbers (NGC 5451, 5441, 5447, 5453, 5458, 5461, 5462, 5471). In fact several are so large and bright they are referred to as Giant HII Regions (GHR). Giant HII regions have an extraordinary luminosity and are powered by star clusters with an enormous ionization flux requiring the equivalent of 100 to even 1000 O-type giants. Some of these clusters have such an exceptionally high concentration of massive stars they are referred to as super star clusters (SSC). SSC's typically possess a cumulative stellar mass of 100,000 to 1,000,000 suns. The Giant HII region NGC 5461 was believed powered by a single SSC similar to R136 within 30 Doradus. High resolution imaging by the HST showed that instead of one SSC, three distinct clusters were resolved which accounted for the cumulative light output. NGC 5471, one of the giant H II regions in M101, is 2 orders of magnitude larger and brighter than the largest H II region in our Milky Way, NGC 3603. What causes M101 to produce such bright H II regions? Studies of giant molecular clouds in M101 suggest the key to the formation of giant H II regions may be related to the efficiency of molecular gas clouds to make new stars. The size of the cloud may be less of an important factor.
Celebrated as one of the finest grand design spirals in the sky, M101 has an interesting lopsided asymmetric structure. Asymmetry is present in about 30% of spiral galaxies and can influence the number and density of HII regions and star formation. Observations of M101 show a greater number of HII regions along the denser SW arm supporting the belief that increased disk mass density leads to a higher rate of star formation.