ThePinwheel Galaxy (also known asMessier 101,M101 orNGC 5457) is a face-on, counterclockwiseIntermediate spiral galaxy located 21 millionlight-years (6.4megaparsecs)^[5] from Earth in theconstellationUrsa Major. It was discovered byPierre Méchain in 1781^[a] and was communicated that year toCharles Messier, who verified its position for inclusion in theMessier Catalogue as one of its final entries.

Pinwheel Galaxy
The Pinwheel Galaxy, as taken byHubble Space Telescope
Observation data (J2000epoch)
Constellation	Ursa Major
Right ascension	14h 03m 12.6s
Declination	+54° 20′ 57″
Redshift	0.000804
Heliocentric radial velocity	241 ± 2km/s
Distance	20.9 ± 1.8 Mly (6.4 ± 0.5 Mpc)
Apparent magnitude (V)	7.9[1]
Characteristics
Type	SAB(rs)cd
Number of stars	1 trillion (1012)
Size	51.91 kpc (169,300 ly) (diameter; 25.0 mag/arcsec2 B-band isophote)[2][3]
Apparent size (V)	28′.8 × 26′.9
Other designations
Messier 101, M101,NGC 5457,UGC 8981,PGC 50063,Arp 26
References:[4][5][6][7][8][9]

On February 28, 2006,NASA and theEuropean Space Agency released a very detailed image of the Pinwheel Galaxy, which was the largest and most detailed image of a galaxy byHubble Space Telescope at the time.^[10] The image was composed of 51 individual exposures, plus some extra ground-based photos.

Pierre Méchain, the discoverer of the galaxy, 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 illuminates the [grating] wires."^[11]

William Herschel wrote in 1784 that the galaxy was one of several which "...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."^[11]

Lord Rosse observed the galaxy in his 72-inch-diameterNewtonian 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.^[11]

Though the galaxy can be detected withbinoculars or a small telescope, to observe the spiral structure in a telescope without a camera requires a fairly large instrument, very dark skies, and a low-power eyepiece.^[12]

M101 is a large galaxy, with a diameter of 170,000 light-years. By comparison, theMilky Way has a diameter of 87,400 light-years.^[13] It has around a trillion stars.^[14] It has a disk mass on the order of 100 billion solar masses, along with a small central bulge of about 3 billion solar masses.^[15] Its characteristics can be compared to those ofAndromeda Galaxy.

M101 has a high population ofH II regions, many of which are very large and bright. H II regions usually accompany the enormous clouds of high density molecular hydrogen gas contracting under their own gravitational force wherestars form. H II regions are ionized by large numbers of extremely bright and hot young stars; those in M101 are capable of creating hotsuperbubbles.^[16] In a 1990 study, 1,264 H II regions were cataloged in the galaxy.^[17] Three are prominent enough to receiveNew General Catalogue numbers—NGC 5461, NGC 5462, and NGC 5471.^[18]

M101 is asymmetrical due to the tidal forces from interactions with its companion galaxies. These gravitational interactions compressinterstellar hydrogen gas, which then triggers strongstar formation activity in M101's spiral arms that can be detected in ultraviolet images.^[19]

In 2001, the X-ray source P98, located in M101, was identified as anultra-luminous X-ray source—a source more powerful than any single star but less powerful than a whole galaxy—using theChandra X-ray Observatory. It received the designation M101 ULX-1. In 2005,Hubble andXMM-Newton observations showed the presence of an optical counterpart, strongly indicating that M101 ULX-1 is anX-ray binary.^[20] Further observations showed that the system deviated from expected models—the black hole is just 20 to 30solar masses, and consumes material (including captured stellar wind) at a higher rate than theory suggests.^[21]

It is estimated that M101 has about 150globular clusters,^[22] the same as the number of theMilky Way's globular clusters.

M101 has six prominent companion galaxies:NGC 5204,NGC 5474,NGC 5477,NGC 5585,UGC 8837 andUGC 9405.^[23] As stated above, the gravitational interaction between it and its satellites may have spawned its grand design pattern. The galaxy has probably distorted the second-listed companion.^[23] The list comprises most or all of theM101 Group.^{[24][25][26][27]}

Six internal supernovae have been recorded:

• SN 1909A was discovered byMax Wolf in January 1909 and reached magnitude 12.1.^[28][29]
• SN 1951H was discovered byMilton Humason on 1 September 1951 and reached magnitude 17.5.^[30]
• SN 1970G (type II, mag. 11.5) was discovered byMiklós Lovas on 30 July 1970.^[31][32][33]
• On August 24, 2011, aType Ia supernova,SN 2011fe, initially designated PTF 11kly, was discovered in M101. It hadvisual magnitude 17.2 at discovery and reached 9.9 at its peak.^[34][35][36]
• On February 10, 2015, aluminous red nova, known asM101 OT2015-1 was discovered in the Pinwheel Galaxy.^[37]
• On May 19, 2023,SN 2023ixf was discovered in M101, and immediately classified as aType II supernova.^[38]

•  
  Type Ia supernovaSN 2011fe from August 2011
•  
  Type II supernovaSN 2023ixf from May 2023

• List of Messier objects
• Messier 74 – Face-on spiral galaxy in the constellation Pisces – a similar face-on spiral galaxy
• Messier 83 – Barred spiral galaxy in the constellation Hydra – a similar face-on spiral galaxy that is sometimes called the Southern Pinwheel Galaxy
• Messier 99 – Spiral galaxy in the constellation Coma Berenices – a similar face-on spiral galaxy
• Triangulum Galaxy – Spiral galaxy in the constellation Triangulum – another galaxy sometimes called the Pinwheel Galaxy

• SEDS: Spiral Galaxy M101
• The Pinwheel Galaxy onWikiSky:DSS2,SDSS,GALEX,IRAS,Hydrogen α,X-Ray,Astrophoto,Sky Map,Articles and images
• Harutyunyan, Avet; Merrifield, Mike; Dhillon, Vik."M101 – Pinwheel Galaxy".Deep Sky Videos.Brady Haran.
• Nemiroff, R.; Bonnell, J., eds. (14 April 2009)."M101: The Pinwheel Galaxy".Astronomy Picture of the Day.NASA. Retrieved4 March 2018.
• "Pinwheel Galaxy – Messier 101".Constellation Guide. 31 May 2013.
• Yoshida, Takayuki."Spiral Galaxy, M101".Astrophotography by Takayuki Yoshida.