Messier 101, NGC 5457 - Pinwheel Galaxy

Messier 101 is a very large, relatively nearby, face-on spiral galaxy in the constellation Ursa Major. It is also known as the Pinwheel Galaxy.

Discovery and History

Messier 101 was discovered by Pierre Mechain in 1781, and Charles Messier added it as one of the last entries to his catalog later that year. Méchain described M 101 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."

William Herschel noted in 1784 that "[M101] in my 7, 10, and 20-feet 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."

M 101 was one of the first "spiral nebulae" identified by William Parsons, the third Earl of Rosse. Lord Rosse observed M 101 in his 72-inch Newtonian reflector in 1851, and made several sketches; he was the first to note its spiral structure.

Halton Arp included M 101 as No. 26 in his Catalogue of Peculiar Galaxies, with the description "Spiral with One Heavy Arm."

Amateur Observation

Although quite bright, at magnitude 7.9, M 101's large angular extent - some 22' across - gives it a low surface brightness, requiring dark skies to see. Only the central region of this galaxy is visible in smaller telescopes. Suggestions of the spiral arms can be glimpsed as nebulous patches in telescopes starting at 4 inches' aperture. A fairly large instrument, very dark skies, and a low power eyepiece are required to observe the spiral structurewell.

Larger telescopes show M 101 to be a magnificent 20' x 15' NNE-SSW spiral galaxy, with conspicuous mottled arms. Several bright, isolated bright patches stand out: 8' SW, 4.5' ESE, and 6' ENE of the galaxy's center; the SW patch lies just south of a 13th-magnitude foreground star. These patches were assigned their own catalog numbers by William Herschel and later added to the NGC as NGC 5447 (H III.787), NGC 5461 (H III.788), and NGC 5462 (H III.789).

M 101 is remarkably asymmetric, with its core considerably displaced from the center of its disk. The most prominent spiral arm springs from the southeast side of the galaxy's 2' diameter core, and arcs east and then north out of the central region: a bright knot is embedded in this arm NE of the nucleus. A fainter arm begins at the west side of the core and arcs south and southeast; it contains bright knots southwest of the nucleus.

M 101 is the brightest of a group of at least nine galaxies, including NGC 5204, NGC 5238, NGC 5474, NGC 5477, NGC 5585, UGC 8508, Holmberg IV (UGC 8837), and UGC 9405. The M 101 Group lies physically close to the larger M 51 Group, and the two are often included together as one large group.

Properties and Evolution

The distance to M 101 has been determined to be about 27 million light years, by the observation of Cepheid variables with the Hubble Space Telescope. At this distance, M 101 has a linear diameter of over 170,000 light years - nearly twice the size of the Milky Way - and is thus among the largest spiral galaxies. M 101 has an absolute magnitude of -21.5, or a luminosity of about 33 billion suns. Its disk contains on the order of 100 billion solar masses.

M 101 produced three supernovae in the 20th century: SN 1909A, 1951H, and SN 1970G. Another remarkable property of this galaxy is its large number of star-forming H II regions; a total of about 3000 can be seen on photographs. H II regions are enormous clouds of high density molecular hydrogen gas, ionized by large numbers of hot, bright, young stars forming within them.

The gravitational interaction between M 101 and its satellite galaxies may have triggered the formation of M 101's "grand design" spiral pattern. It is thought that in the recent galactic past, M 101 underwent a near collision with another galaxy. This encounter amplified the density waves in the spiral arms of M 101, and also caused their asymmetry. M101 has also probably distorted its companion galaxy, NGC 5474.