M 101

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; according to the NGC and Burnham, there are 9 such numbers, 3 of which go back to Herschel, while the RNGC states that five of the others don't exist (ne); it mentions however that deVaucouleurs has them as knots: NGC 5447 (H III.787), 5449 (ne), 5450 (ne), 5451 (ne), 5453 (ne), 5455, 5458 (ne), 5461 (H III.788), 5462 (H III.789), and 5471.

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.

M101 is the brightest of a group of at least 9 galaxies, the brightest companions being NGC 5474 (type Sc, 10.85 mag vis) to the SSE and NGC 5585 (Sa, 11.49 mag; Glyn Jones and Burnham misprinted this as 5485) to the NE. Other probable group members are NGC 5204 (Ir, 11.26), NGC 5238 (SB(d)m, 13.35p), NGC 5477 (Ir+, 13.8), UGC 8508 (Ir+, 14.5 p), UGC 8837 (Ir+, 13.1 p), and UGC 9405.

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 million light years. Kenneth Glyn Jones mentions earlier Earth-bound attempts of 1986, when two Cepheids were claimed to have been detected (yielding distance estimates between 20 and 26 million light years). It is also in good agreement with a distance determined from the Planetary Nebula Luminosity function (by John J. Feldmeier, Robin Ciardullo, and George H. Jacoby, ApJ 461:L25-L28, April 10, 1996) which is 25.1 +/- 1.6 million light years. At the new distance from the HST, it has a linear diameter of over 150,000 light years and is thus among the biggest disk galaxies.

Three supernovae have been discovered in M101: The first one, SN 1909A, appeared on Jan 26, 1909 and was discovered by Max Wolf; it was of peculiar type and reached mag 12.1 (Glyn Jones reports that the discovery took place in February, and the SN reached only mag 13.5). The second supernova 1951H was of type II, occurred in September 1951 and reached mag 17.5, while the third, SN 1970G, also type II, occurred on Jul 30, 1970 and reached mag 11.5.