Pinwheel Galaxy

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Lore & Background

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.' 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.' Lord Rosse observed the galaxy with 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. Though the galaxy can be detected with binoculars 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.

In Their Own Story

M101 is a large galaxy, with a diameter of 252,000 light-years. By comparison, the Milky Way has a diameter of 87,400 light-years. It has around a trillion stars. 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. Its characteristics can be compared to those of Andromeda Galaxy. M101 has a high population of H 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 where stars form. H II regions are ionized by large numbers of extremely bright and hot young stars; those in M101 are capable of creating hot superbubbles. In a 1990 study, 1,264 H II regions were cataloged in the galaxy. Three are prominent enough to receive New General Catalogue numbers—NGC 5461, NGC 5462, and NGC 5471. M101 is asymmetrical due to the tidal forces from interactions with its companion galaxies. These gravitational interactions compress interstellar hydrogen gas, which then triggers strong star formation activity in M101's spiral arms that can be detected in ultraviolet images.

Reader's Guide

In 2001, the X-ray source P98, located in M101, was identified as an ultra-luminous X-ray source—a source more powerful than any single star but less powerful than a whole galaxy—using the Chandra X-ray Observatory. It received the designation M101 ULX-1. In 2005, Hubble and XMM-Newton observations showed the presence of an optical counterpart, strongly indicating that M101 ULX-1 is an X-ray binary. Further observations showed that the system deviated from expected models—the black hole is just 20 to 30 solar masses, and consumes material (including captured stellar wind) at a higher rate than theory suggests. It is estimated that M101 has about 150 globular clusters, the same as the number of the Milky Way's globular clusters. M101 has six prominent companion galaxies: NGC 5204, NGC 5474, NGC 5477, NGC 5585, UGC 8837 and UGC 9405. 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. The list comprises most or all of the M101 Group. Six supernovae have been recorded in M101: SN 1909A (type unknown, mag. 10) discovered by Max Wolf on 21 February 1909; SN 1951H (type unknown, mag. 17.5) discovered by Milton Humason on 3 February 1951; SN 1970G (Type II, mag. 11.5) discovered by Miklós Lovas on 30 July 1970; SN 2011fe (Type Ia, mag. 17.2) discovered by the Palomar Transient Factory on 24 August 2011; M101 OT2015-1 (also known as AT 2015dl), a luminous red nova, discovered by Dumitru Ciprian Vîntdevară on 10 February 2015; SN 2023ixf (Type II, mag. 14.9) discovered by Kōichi Itagaki on 19 May 2023.

Did You Know?

• M101 has a diameter of 252,000 light-years, nearly three times that of the Milky Way (87,400 light-years).
• In a 1990 study, 1,264 H II regions were cataloged in the galaxy; three are prominent enough to have NGC numbers: NGC 5461, NGC 5462, and NGC 5471.
• The ultra-luminous X-ray source M101 ULX-1 contains a black hole of just 20 to 30 solar masses that consumes material at a higher rate than theory suggests.
• Six supernovae have been recorded in M101, including SN 2011fe which reached visual magnitude 9.9, making it the brightest supernova of 2011.
• M101 has about 150 globular clusters, the same number as the Milky Way.