M101 is a large face-on spiral galaxy located 22 million light-years away in the constellation of Ursa Major. At mag. +7.9, it can be glimpsed in binoculars or small telescopes from dark sites but suffers from low surface brightness and in bad seeing conditions or light polluted areas, the galaxy can be difficult to spot even with a 200mm (8-inch) scope. It's 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 Bootes 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 when viewed from our line of sight. In this case, and also for other similar large face-on galaxies, dark moonless skies are a must! In such seeing conditions M101 can even be spotted with 7x50 or 10x50 binoculars as a large featureless dim patch of light. A small 80mm (3.1-inch) telescope displays a large nebulous haze with a brighter centre. On good nights using a 200mm (8-inch) scope it's possible to observe the bright-condensed core surrounded by a halo of nebulosity, which includes several knotty patches and fills an entire low power field of view. It may be possible to make out a weak spiral shape.
M101 is an extremely large galaxy with a diameter of 180,000 light-years; almost double that of our Milky Way galaxy. 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 with the last one discovered on August 24, 2011. This type Ia supernova was the brightest of the four reaching magnitude +9.9, making it was visible with small telescopes.
M101 Data Table
|Object Type||Spiral galaxy|
|Classification||SAB (rs) cd|
|RA (J2000)||14h 03m 12s|
|DEC (J2000)||54d 20m 55s|
|Apparent Size (arc mins)||28.8 x 26.9|
|Number of Stars||1 Trillion|