Discovered by Pierre Méchain in 1781.
Messier 101 (M101, NGC 5457) was discovered by Pierre Méchain on March 27, 1781, and added as one of the last entries in Charles Messier's catalog. It was one of the first "spiral nebula" identified as such, in 1851 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. Several of these patches (i.e., spiral arm fragments) were assigned their own catalog numbers by William Herschel and later observers.
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. According to the recent recalibration of the Cepheid distance scale, the "true" distance of M101 should be closer to a 10 percent higher value (27 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.
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