|Observation data (J2000 epoch)|
|Right ascension||14h 03m 12.6s|
|Declination||+54° 20′ 57″|
|Helio radial velocity||241 ± 2 km/s|
|Distance||20.9 ± 1.8 Mly (6.4 ± 0.5 Mpc)|
|Apparent dimensions (V)||28′.8 × 26′.9|
|Apparent magnitude (V)||7.86|
|Messier 101, NGC 5457, UGC 8981, PGC 50063, Arp 26|
The Pinwheel Galaxy (also known as Messier 101, M101 or NGC 5457) is a face-on spiral galaxy distanced 21 million light-years (six megaparsecs) away in the constellation Ursa Major, first discovered by Pierre Méchain on March 27, 1781, and communicated to Charles Messier who verified its position for inclusion in the Messier Catalogue as one of its final entries.
On February 28, 2006, NASA and the ESA released a very detailed image of the Pinwheel Galaxy, which was the largest and most detailed image of a galaxy by Hubble Space Telescope at the time. The image was composed from 51 individual exposures, plus some extra ground-based photos.
Pierre Méchain, the discoverer of Messier 101, described it as a "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. It is difficult to distinguish when one lits the [grating] wires."
William Herschel noted in 1784 that "[M101] in my 7, 10, and 20-feet [focal length] reflectors shewed a mottled kind of nebulosity, which I shall call resolvable; so that I expect my present telescope will, perhaps, render the stars visible of which I suppose them to be composed."
Lord Rosse observed M101 in his 72-inch [diameter] Newtonian reflector during the second half of the 19th century. He was the first to make extensive note of the spiral structure and made several sketches.
To observe the spiral structure in modern instruments requires a fairly large instrument, very dark skies, and a low power eye piece.
Structure and composition 
M101 is a relatively large galaxy compared to the Milky Way. With a diameter of 170,000 light-years it is seventy percent larger than the Milky Way. It has a disk mass on the order of 100 billion solar masses, along with a small bulge of about 3 billion solar masses.
Another remarkable property of this galaxy is its huge and extremely bright H II regions, of which a total of about 3,000 can be seen on photographs. H II regions usually accompany the enormous clouds of high density molecular hydrogen gas contracting under their own gravitational force where stars form. H II regions are ionized by large numbers of extremely bright and hot young stars.
On photographs M101 can be seen to be asymmetrical on one side. It is thought that in the recent past (speaking in galactic terms) M101 underwent a near collision with another galaxy and the associated gravitational tidal forces caused the asymmetry. In addition, this encounter also amplified the density waves in the spiral arms of M101. The amplification of these waves leads to the compression of the interstellar hydrogen gas, which then triggers strong star formation activity.
Companion galaxies 
M101 has five prominent companion galaxies: NGC 5204, NGC 5474, NGC 5477, NGC 5585, and Holmberg IV. As stated above, the gravitational interaction between M101 and its satellites may have triggered the formation of the grand design pattern in M101. M101 has also probably distorted the companion galaxy NGC 5474. M101 and its companion galaxies comprise most or possibly all of the M101 Group.
On August 24, 2011, a Type Ia supernova, SN 2011fe, initially designated PTF 11kly, was discovered in M101. The supernova was visual magnitude 17.2 at discovery and reached magnitude 9.9 at its peak. This was the fourth supernova recorded in M101. The first, SN 1909A, was discovered by Max Wolf in January 1909 and reached magnitude 12.1. SN 1951H reached magnitude 17.5 in September 1951 and SN 1970G reached magnitude 11.5 in January 1970.
See also 
- Messier 74 – a similar face-on spiral galaxy
- Messier 83 – a similar face-on spiral galaxy that is sometimes referred to as the Southern Pinwheel Galaxy
- Messier 99 – a similar face-on spiral galaxy
- Triangulum Galaxy – another galaxy sometimes called the Pinwheel Galaxy
- A Wrinkle in Time – features Uriel, a planet in the Pinwheel Galaxy
- "NASA/IPAC Extragalactic Database". Results for Messier 101. Retrieved 2006-12-06.
- Shappee, Benjamin; Stanek, Kris (06-2011). "A New Cepheid Distance to the Giant Spiral M101 Based on Image Subtraction of Hubble Space Telescope/Advanced Camera for Surveys Observations". Astrophysical Journal 733 (2): 124. arXiv:1012.3747. Bibcode:2011ApJ...733..124S. doi:10.1088/0004-637X/733/2/124. Retrieved 26 August 2011.
- R. W. Sinnott, ed. (1988). The Complete New General Catalogue and Index Catalogue of Nebulae and Star Clusters by J. L. E. Dreyer. Sky Publishing Corporation / Cambridge University Press. ISBN 0-933346-51-4.
- "Distance Results for Messier 101". NASA/IPAC Extragalactic Database. Retrieved 2010-05-01.
- "SIMBAD-M101". SIMBAD Astronomical Database. Retrieved 2009-11-29.
- Armando, Gil de Paz; Boissier; Madore; Seibert; Boselli; et al. (2007). "The GALEX Ultraviolet Atlas of Nearby Galaxies". Astrophysical Journal Supplement 173 (2): 185–255. arXiv:0606440. Bibcode:2007ApJS..173..185G. doi:10.1086/516636.
- HubbleSite – NewsCenter – Hubble's Largest Galaxy Portrait Offers a New High-Definition View (02/28/2006) – Introduction
- SEDS Historical Notes
- Comte, G., Monnet, G., & Rosado, M.; Monnet; Rosado (1979). "An optical study of the galaxy M 101 - Derivation of a mass model from the kinematic of the gas". Astronomy and Astrophysics 72: 73–81. Bibcode:1979A&A....72...73C.
- A. Sandage, J. Bedke (1994). Carnegie Atlas of Galaxies. Carnegie Institution of Washington. ISBN 0-87279-667-1.
- R. B. Tully (1988). Nearby Galaxies Catalog. Cambridge University Press. ISBN 0-521-35299-1.
- P. Fouque, E. Gourgoulhon, P. Chamaraux, G. Paturel; Gourgoulhon; Chamaraux; Paturel (1992). "Groups of galaxies within 80 Mpc. II – The catalogue of groups and group members". Astronomy and Astrophysics Supplement (2nd ed.) 93: 211–233. Bibcode:1992A&AS...93..211F.
- A. Garcia (1993). "General study of group membership. II – Determination of nearby groups". Astronomy and Astrophysics Supplement 100: 47–90. Bibcode:1993A&AS..100...47G.
- G. Giuricin, C. Marinoni, L. Ceriani, A. Pisani (2000). "Nearby Optical Galaxies: Selection of the Sample and Identification of Groups". Astrophysical Journal 543 (1): 178–194. arXiv:astro-ph/0001140. Bibcode:2000ApJ...543..178G. doi:10.1086/317070.
- Nugent, Peter et al (24 August 2011). "Young Type Ia Supernova PTF11kly in M101". The Astronomer's Telegram. Retrieved 25 August 2011.
- Nugent, Peter et al. "Supernova Caught in the Act". Retrieved 7 September 2011.
- Hartmut Frommert and Christine Kronberg (15 Sept 2011). "Supernova 2011fe in M101". Retrieved 17 Sept 2011.
- Stoyan, Ronald Atlas of the Messier Objects, Cambridge University Press 2008 page 329
|Wikimedia Commons has media related to: Pinwheel Galaxy|
- The Pinwheel Galaxy on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images
- Messier 101, SEDS Messier pages
- Extremely detailed picture of the Pinwheel Galaxy at HubbleSite.org
- Pinwheel Galaxy at ESA/Hubble
- Spiral Galaxy Messier 101 (Pinwheel Galaxy)
- Picture of the day on APOD on April 14, 2009
- Harutyunyan, Avet; Merrifield, Mike; Dhillon, Vik. "M101 – Pinwheel Galaxy". Deep Space Videos. Brady Haran.