The Pinwheel Galaxy (also known as Messier 101, M101 or NGC 5457) is a face-on spiral galaxy away from Earth in the constellation

Ursa Major Ursa Major (; also known as the Great Bear) is a constellation in the northern sky, whose associated mythology likely dates back into prehistory. Its Latin name means "greater (or larger) bear," referring to and contrasting it with nearby Ursa ...

. It was discovered by

Pierre Méchain Pierre François André Méchain (; 16 August 1744 – 20 September 1804) was a French astronomer and surveyor who, with Charles Messier, was a major contributor to the early study of deep-sky objects and comets. Life Pierre Méchain was born i ...

in 1781 and was communicated that year to

Charles Messier Charles Messier (; 26 June 1730 – 12 April 1817) was a French astronomer. He published an astronomical catalogue consisting of 110 nebulae and star clusters, which came to be known as the ''Messier objects''. Messier's purpose ...

, who verified its position for inclusion in the ''

Messier Catalogue The Messier objects are a set of 110 astronomical objects catalogued by the French astronomer Charles Messier in his ''Catalogue des Nébuleuses et des Amas d'Étoiles'' (''Catalogue of Nebulae and Star Clusters''). Because Messier was only in ...

'' as one of its final entries. On February 28, 2006,

NASA The National Aeronautics and Space Administration (NASA ) is an independent agencies of the United States government, independent agency of the US federal government responsible for the civil List of government space agencies, space program ...

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 The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most vers ...

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 Bootes and the tail of the great Bear. It is difficult to distinguish when one lits the ratingwires."

William Herschel Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline ...

wrote in 1784 that the galaxy was one of several which "...in my 7-, 10-, and 20-feet ocal lengthreflectors 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 in his 72-inch-diameter

Newtonian reflector The Newtonian telescope, also called the Newtonian reflector or just a Newtonian, is a type of reflecting telescope invented by the English scientist Sir Isaac Newton, using a concave primary mirror and a flat diagonal secondary mirror. Newton' ...

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 eyepiece.

Structure and composition

M101 is a large galaxy, with a diameter of 170,000 light-years. By comparison, the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

has a diameter of between 100,000 and 120,000 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 The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gal ...

. M101 has a high population of

H II region An H II region or HII region is a region of interstellar atomic hydrogen that is ionized. It is typically in a molecular cloud of partially ionized gas in which star formation has recently taken place, with a size ranging from one to hundreds ...

s, 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. 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 The Chandra X-ray Observatory (CXO), previously known as the Advanced X-ray Astrophysics Facility (AXAF), is a Flagship-class space telescope launched aboard the during STS-93 by NASA on July 23, 1999. Chandra is sensitive to X-ray sources 1 ...

. It received the designation M101 ULX-1. In 2005, Hubble and

XMM-Newton ''XMM-Newton'', also known as the High Throughput X-ray Spectroscopy Mission and the X-ray Multi-Mirror Mission, is an X-ray space observatory launched by the European Space Agency in December 1999 on an Ariane 5 rocket. It is the second cornerst ...

observations showed the presence of an optical counterpart, strongly indicating that M101 ULX-1 is an

X-ray binary X-ray binaries are a class of binary stars that are luminous in X-rays. The X-rays are produced by matter falling from one component, called the ''donor'' (usually a relatively normal star), to the other component, called the ''accretor'', whi ...

. 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

's globular clusters.

Companion galaxies

M101 has six prominent companion galaxies:

NGC 5204 NCG 5204 is a Magellanic spiral galaxy located about 14.5 million light-years away from EarthThe distance from Earth is . in the constellation of Ursa Major and is a member of the M101 Group of galaxies. It has a galaxy morphological classificat ...

, 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.

Supernovae and luminous red nova

Four internal supernovae have been recorded: *SN 1909A, was discovered by Max Wolf in January 1909 and reached magnitude 12.1. *SN 1951H reached magnitude 17.5 in September 1951. *SN 1970G reached magnitude 11.5 in January 1970. *On August 24, 2011, a Type Ia supernova, SN 2011fe, initially designated PTF 11kly, was discovered in M101. It had

visual magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's lig ...

17.2 at discovery and reached 9.9 at its peak. On February 10, 2015, a

luminous red nova A luminous red nova (abbr. ''LRN'', pl. ''luminous red novae'', pl.abbr. ''LRNe'') is a stellar explosion thought to be caused by the merging of two stars. They are characterised by a distinct red colour, and a light curve that fades slowly with ...

, known as M101 OT2015-1 was discovered in the Pinwheel Galaxy.

References

External links