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Discovered by Pierre Méchain in 1781.

Messier 101 (M101, NGC 5457) was discovered byPierre Méchain on March 27, 1781, and added asone of the last entries inCharles Messier'scatalog. It was one of thefirst "spiral nebula" identified as such, in 1851 byWilliam Parsons, the third Earl of Rosse.

Although extended 22 arc minutes on photos and quite bright, only thecentral region of this galaxy is visible in smaller telescopes, bestat 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, thereare 9 such numbers, 3 of which go back to Herschel who has found them onApril 14, 1789, 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 themost prominent Grand Design spirals in the sky. While quite symmetric visuallyand in very short exposures which show only the central region, it is of remarkable unsymmetry, its core being considerably displaced from the centerof the disk.Halton Arp has included M101 as No. 26 in hisCatalogue of Peculiar Galaxiesas a "Spiral with One Heavy Arm".

M101 is the brightest of a group of at least 9 galaxies, called theM101 Group. The brightest companions areNGC 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), Holmberg IV (UGC 8837, Ir+, 13.1 p), and UGC 9405.

The distance of M101 has been determined by the measurement of Cepheidvariables with the Hubble Space Telescope in 1994/95 to be about 24 +/- 2 million light years, by the HSTH0 Key Project Team (paper III, 1996).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, byFeldmeier, Ciardullo and Jacoby (1996)which is 25.1 +/- 1.6 million light years.Later it was suspected that according to the recent recalibration of the Cepheid distance scale, the "true" distance of M101 could be 10 percent higher and we had longly used a value of 27 million light years.Modern value has been refined to 24 Mly again (e.g. NED), which we now adopt here.The work to adjust this webpage is still in progress.

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.

Four supernovae have been discovered in M101:

• The first one, SN 1909A, appeared on January 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.
• The third, SN 1970G, also type II, was discovered on July 30, 1970 by Michael Lovas, and reached mag 11.5. The remnant ofSupernova 1970G was later detected in X-ray light and e.g. observed with the Chandra X-ray Observatory (CXO) satellite.
• The fourth one,SN 2011fe, was the most spectacular: This type Ia supernova was found on August 24, 2011 by Palomar Transient Factory (PTF), and brightened to mag 9.9 around September 10.

• PSN J14021678+5426205 (iPTF13afz), discovered January 20, 2015 by Ciprian Dumitru Vintdevara, located 489" west and 324" north of the center of M101, at mag 16.5. This one was found to be too faint for a supernova, and is probably a Luminous Red Nova (LRN).Luminous Red Nova 2015 in M101

NGC Online data for M101

References

• Feldmeier, John J., Ciardullo, Robin, and Jacobi, George H., 1996. The Planetary Nebulae Distance to M101. Astrophysical Journal Letters, Vol. 461, p. L25-L28 (April 10, 1996) [ADS: 1996ApJ...461L..25F]
• H0 Key Project materials on M101:
  • The HST Key Project Archives - M101
  • D.D. Kelsonet.al., 1996. The Extragalactic Distance Scale Key Project. III. The Discovery of Cepheids and a New Distance to M101 Using the Hubble Space Telescope. Astrophysical Journal, Vol. 463, p. 26-59, May 20, 1996. [ADS: 1996ApJ...463...26K]

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Christine Kronberg
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