The Whirlpool Galaxy, also known as Messier 51a, M51a, and NGC 5194, is an interactinggrand-designspiral galaxy with a Seyfert 2active galactic nucleus. It lies in the constellationCanes Venatici, and was the first galaxy to be classified as a spiral galaxy. Its distance is estimated to be 23 million light-years away from Earth. The galaxy and its companion, NGC 5195, are easily observed by amateur astronomers, and the two galaxies may be seen with binoculars. The Whirlpool Galaxy has been extensively observed by professional astronomers, who study it to understand galaxy structure and galaxy interactions.
Discovery
What later became known as the Whirlpool Galaxy was discovered on October 13, 1773, by Charles Messier while hunting for objects that could confuse comet hunters, and was designated in Messier's catalogue as M51. Its companion galaxy, NGC 5195, was discovered in 1781 by Pierre Méchain, although it was not known whether it was interacting or merely another galaxy passing at a distance. In 1845, William Parsons, 3rd Earl of Rosse, employing a 72-inch reflecting telescope at Birr Castle, Ireland, found that the Whirlpool possessed a spiral structure, the first "nebula" to be known to have one. These "spiral nebulae" were not recognized as galaxies until Edwin Hubble was able to observe Cepheid variables in some of these spiral nebulae, which provided evidence that they were so far away that they must be entirely separate galaxies even though they are seen close together The advent of radio astronomy and subsequent radio images of M51 unequivocally demonstrated that the Whirlpool and its companion galaxy are indeed interacting. Sometimes the designation M51 is used to refer to the pair of galaxies, in which case the individual galaxies may be referred to as M51a and M51b.
Visual appearance
Located within the constellation Canes Venatici, M51 is found by following the easternmost star of the Big Dipper, Eta Ursae Majoris, and going 3.5° southwest. Its declination is +47°, making it a circumpolar for observers located above 43°N latitude; it reaches high altitudes throughout the northern hemisphere making it an accessible object from the early hours in winter through the end of spring season, after which observation is hindered in lower latitudes. M51 is visible through binoculars under dark sky conditions, and it can be resolved in detail with modern amateur telescopes. When seen through a 100 mm telescope the basic outlines of M51 and its companion are visible. Under dark skies, and with a moderate eyepiece through a 150 mm telescope, M51's intrinsic spiral structure can be detected. With larger instruments under dark sky conditions, the various spiral bands are apparent with HII regions visible, and M51 can be seen to be attached to M51B. As is usual for galaxies, the true extent of its structure can only be gathered from inspecting photographs; long exposures reveal a large nebula extending beyond the visible circular appearance. In 1984, thanks to the high-speed detector - the so-called image-photon-counting- IPCS - system - developed jointly by the CNRS Laboratoire d'Astronomie Spatiald and the Observatoire de Haute Provence along with the particularly nice seeing offered by the Canada-France-Hawaii-Telescope 3.60m Cassegrain focus at Mauna Kea summit in Hawaii, Hua et al. detected the double component of the very nucleus of the Whirlpool galaxy. In January 2005 the Hubble Heritage Project constructed a 11477 × 7965-pixel composite image of M51 using Hubble's ACS instrument. The image highlights the galaxy's spiral arms, and shows detail into some of the structures inside the arms.
Properties
Whirlpool Galaxy lies 23 million light years from Earth and has an estimated diameter of 76,000 light years. Overall the galaxy is about 43% the size of the Milky Way. Its mass is estimated to be 160 billion solar masses. or around 10.3% of the mass of Milky Way Galaxy. A black hole, once thought to be surrounded by a ring of dust, but now believed to be partially occluded by dust instead, exists at the heart of the spiral. A pair of ionization cones extend from the active galactic nucleus.
Spiral structure
The pronounced spiral structure of the Whirlpool Galaxy is believed to be the result of the close interaction between it and its companion galaxy NGC 5195, which may have passed through the main disk of M51 about 500 to 600 million years ago. In this proposed scenario, NGC 5195 came from behind M51 through the disk towards the observer and made another disk crossing as recently as 50 to 100 million years ago until it is where we observe it to be now, slightly behind M51.
Star formation
The central region of M51 appears to be undergoing a period of enhanced star formation. The present efficiency of star formation, defined as the ratio of mass of new stars to the mass of star-forming gas, is only ~1%, quite comparable to the global value for the Milky Way and other galaxies. It is estimated that the current high rate of star formation can last no more than another 100 million years or so.
Transient events
Three supernovae have been observed in the Whirlpool Galaxy: In 1994, SN 1994I was observed in the Whirlpool Galaxy. It was classified as type Ic, indicating that its progenitor star was very massive and had already shed much of its mass, and its brightness peaked at apparent magnitude 12.91. In June 2005 the type II supernovaSN 2005cs was observed in the Whirlpool Galaxy, peaking at apparent magnitude 14. On 31 May 2011 a type II supernova was detected in the Whirlpool Galaxy, peaking at magnitude 12.1. This supernova, designated SN 2011dh, showed a spectrum much bluer than average, with P Cygni profiles, which indicate rapidly expanding material, in its hydrogen-Balmer lines. The progenitor was probably a yellow supergiant and not a red or blue supergiant, which are thought to be the most common supernova progenitors. On 22 January 2019, a supernova impostor, designated AT2019abn, was discovered in Messier 51. The transient was later identified as a luminous red nova. The progenitor star was detected in archival Spitzer Space Telescope infrared images. No object could be seen at the position of the transient in archival Hubble images, indicating that the progenitor star was heavily obstructed by interstellar dust. 2019abn peaked at magnitude 17, reaching an intrinsic brightness of.
Companion
is a dwarf galaxy that is interacting with the Whirlpool Galaxy. Both galaxies are located approximately 25 million light-years away in the constellation Canes Venatici. Together, the two galaxies are one of the most widely studied interacting galaxy pairs.
The Whirlpool Galaxy is the brightest galaxy in the M51 Group, a small group of galaxies that also includes M63, NGC 5023, and NGC 5229. This small group may actually be a subclump at the southeast end of a large, elongated group that includes the M101 Group and the NGC 5866 Group, although most group identification methods and catalogs identify the three groups as separate entities.
