M101 is a large face-on spiral galaxy located 22 million light-years away in the constellation of Ursa Major. At magnitude +7.9, it can be glimpsed in binoculars or small telescopes from dark sites. However, this galaxy suffers from low surface brightness and in bad seeing conditions or light polluted areas is sometimes difficult to spot even with 200mm (8-inch) scopes. M101 is best seen from the Northern Hemisphere during the months of March, April and May.
M101 is also known as the Pinwheel galaxy and was discovered by Pierre Méchain on March 27, 1781. He described it as "nebula without star, very obscure and pretty large, 6' to 7' in diameter, between the left hand of Boötes and the tail of the great Bear." He communicated this to Charles Messier, who verified its position and then included it in his catalogue as one of the final entries.
Locating the part of sky where M101 is positioned is easy, since it's close to the handle of the bowl that forms the Plough or Big Dipper asterism of Ursa Major. The Pinwheel galaxy is located at one corner of an equatorial triangle formed with second magnitude stars Mizar (ζ UMa - mag. +2.2) and Alkaid (η UMa - mag. +1.8). M101 is 5.5 degrees east of Mizar (the celebrated naked eye double star) and 5.5 degrees northeast of Alkaid.
The low surface brightness of M101 is a combination of its large apparent size, 29 x 27 arc minutes, and face-on appearance. In this case, and also for other similar large face-on galaxies, dark moonless skies are a must. In such seeing conditions, M101 can be spotted with 7x50 or 10x50 binoculars as a large featureless dim patch of light. Small 80mm (3.1-inch) refractors display a large nebulous haze with a brighter centre. On good nights, using a 200mm (8-inch) scope, it's possible to observe a bright-condensed core surrounded by a halo of nebulosity, which includes several knotty patches and fills an entire low power field of view. It also possible to make out a weak spiral shape.
M101 is an extremely large galaxy with a diameter of 180,000 light-years, which is almost double that of our Milky Way. It contains about 1 trillion stars and is the brightest member of a group of at least 9 galaxies, called the M101 Group. There have been 4 recorded supernovae in M101 (SN 1909A, SN 1951H, SN 1970G and SN 2011fe). The last one was discovered on August 24, 2011. This type Ia supernova reached magnitude +9.9 and was visible with small scopes.
M101 Data Table
|SAB (rs) cd
|14h 03m 12s
|+54d 20m 55s
|Apparent Size (arc mins)
|28.8 x 26.9
|Number of Stars