M101 - The Pinwheel Galaxy in Ursa Major


M101 in Ursa Major
RA: 14 03'
Dec: +54 21'
Visual Magnitude 7.9

Note also: ngc5474 at upper left, ngc5473 at upper right, ngc5471 a small nebula top just right of centre.
There is a faint galaxy at the very top right but I cannot find it's designation.


M101 was discovered by Pierre Méchain on March 27, 1781, and added as one of the last entries in Charles Messier's catalog. It was the first "spiral nebula" identified as such by William Parsons, the third Earl of Rosse.

Although extended 22 arc minutes on photos and quite bright, only the central region of this galaxy is visible in smaller telescopes, best at low powers. Suggestions of the spiral arms can be glimpsed in telescopes starting from 4 inch as nebulous patches.

On photographs, however, the Pinwheel Galaxy M101 is revealed as one of the most prominent Grand Design spirals in the sky. While quite symmetric visually and in very short exposures which show only the central region, it is of remarkable unsymmetry, its core being considerably displaced from the center of the disk. Halton Arp has included M101 as No. 26 in his Catalogue of Peculiar Galaxies as a "Spiral with One Heavy Arm".

The distance of M101 has been determined by the measurement of Cepheid variables with the Hubble Space Telescope in 1994/95 to be about 24 +/- 2 million light years. 

At the new distance from the HST and Hipparcos, it has a linear diameter of over 170,000 light years and is thus among the biggest disk galaxies, and its total apparent visual brightness of 7.9 mag corresponds to an absolute brightness of -21.6 magnitudes, or a luminosity of about 30 billion (3*10^10) times that of our sun.


ST-10XME / Tak FSQ / AP900GTO / Astrodon LRGB
Red / Green / Blue 9 x 5 min / Luminance is 25 x 4 min
Flesherton, Ontario, Canada
May 13, 2007
Ambient temp 5C; camera temp -20C; No Moon
Images captured in CCDSoft v5.0; Darks and Flats applied in CCDSoft v5.0
Aligned, debloomed and individual channels combined in Maxim
Levels and selective sharpening applied in Photoshop CS2
Gradient removal in PS CS2 using Russ Croman's GradientXTerminator