Pinwheel Galaxy

The pinwheel galaxy is a galaxy 21 million light-years away from Earth in the Constellation Ursa Major. It is also known as Messier 101, M101 or NGC 5457. It was discovered in 1781 by Pierre Méchain and was communicated that year 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 European Space Agency 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 of 51 individual exposures, plus some extra ground-based photos.

On August 24, 2011, a Type Ia supernova, SN 2011fe, was discovered in M101.

## Discovery

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 Boötes and the tail of the great Bear. 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.”

## Structure and Composition

The giant spiral disk of stars, dust, and gas is 170,000 light-years across — nearly twice the diameter of our galaxy, the Milky Way. M101 is estimated to contain at least one trillion stars. The galaxy’s spiral arms are sprinkled with large regions of star-forming nebulas. These nebulas are areas of intense star formation within giant molecular hydrogen clouds. Brilliant, young clusters of hot, blue, newborn stars trace out the spiral arms. 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 the 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 super bubbles. In a 1990 study, 1,264 H II regions were catalogued 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.

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.