The galaxy and its companion,NGC 5195,[11] are easily observed byamateur astronomers, and the two galaxies may be seen withbinoculars.[12] The Whirlpool Galaxy has been extensively observed by professional astronomers, who study it and its pair with dwarf galaxy NGC 5195 to understand galaxy structure (particularly structure associated with thespiral arms) and galaxy interactions. Its pair with NGC 5195 is among the most famous and relatively close interacting systems, and thus is a favorite subject of galaxy interaction models.
The advent ofradio 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 (NGC 5194) and M51b (NGC 5195).
The image of the Whirlpool Galaxy in visible light (left) and infrared light (right)
Deep in the constellationCanes Venatici, M51 is often found by finding the easternmost star of theBig Dipper,Alkaid, and going 3.5° southwest. Its declination is, rounded, +47°, making itcircumpolar (never setting) for observers above the43rd parallel north;[a] it reaches a high altitude throughout this hemisphere making it an accessible object from the early hours in November through to the end of May, after which observation is more coincidental in modest latitudes with the risen sun (due to the Sun approaching to and receding from itsright ascension, specifically figuring inGemini, just to the north).
M51 is visible throughbinoculars under dark sky conditions, and it can be resolved in detail with modern amateur telescopes.[12] When seen through a 100 mm telescope the basic outlines of M51 (limited to 5×6') 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 (>300 mm) instruments under dark sky conditions, the various spiral bands are apparent withHII regions visible, and M51 can be seen to be attached toM51B.
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-system (IPCS)—developed jointly by the CNRS Laboratoire d'Astronomie Spatiald (L.A.S.-CNRS) and the Observatoire de Haute Provence (O.H.P.) along with the particularly nice visibility offered by the Canada-France-Hawaii-Telescope (C.F.H.T.) 3.60m Cassegrain focus on the summit of Mauna Kea in Hawaii, Hua et al. detected the double component of the very nucleus of the Whirlpool Galaxy.[16][full citation needed]
In January 2005 theHubble Heritage Project constructed a 11,477 × 7,965-pixel composite image (shown in the infobox above) of M51 using Hubble'sACS instrument. The image highlights the galaxy's spiral arms, and shows detail into some of the structures inside the arms.[17]
Whirlpool Galaxy – Observed in Various Light a) 0.4 and 0.7 μm; b) vis-blue/green and IR-red; c) 3.6, 4.5, and 8 μm; d) 24 μm
A 1992 Hubble image showing M51's active galactic nucleus, occluded by some dust
The Whirlpool Galaxy lies at a distance of 23[2] to 31 million light-years from Earth.[18] Based on the 1991 measurement by the Third Reference Catalogue of Bright Galaxies using theD25 isophote at the B-band, the Whirlpool Galaxy has a diameter of 23.58kiloparsecs (76,900light-years).[2][5] Overall the galaxy is about 88% the size of theMilky Way.[19][20][21] Itsmass is estimated to be 160 billion solar masses,[22] or around 10.3% of the mass of Milky Way Galaxy. It's believed to be an estimated 400 million years old.
Ablack 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 ofionization cones extend from the active galactic nucleus.[23]
The Whirlpool Galaxy has two, very prominent spiral arms that wind clockwise. One arm deviates from a constant angle significantly.[24] 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.[25]
As a result of the Whirlpool Galaxy's interaction with NGC 5195, a variety of tidal features have been created. The largest of these features is the so-called Northwest plume, which extends out to 43kiloparsecs (140,000light-years) from the galaxy's center. This plume is uniform in color and likely originated from the Whirlpool Galaxy itself due to having diffuse gas. Adjacent to it are two other plumes that have a slightly bluer color, referred to as the Western plumes due to their location.[26]
In 2015, a study discovered two new tidal features caused by the interaction between the Whirlpool Galaxy and NGC 5195, the "Northeast plume" and the "South plume". The study remarks that a simulation that takes into account only one passage of NGC 5195 into the Whirlpool Galaxy will fail to produce an analogue to the Northeast tail.[26] In contrast, the multiple-passage simulations made by Salo and Laurikainen et.al reproduce the northeast plume.[25][26]
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.[27]Similarly, the spiral arms are experiencing high levels of star formation, as well as the space along the arms.[28]
Threesupernovae have been observed in the Whirlpool Galaxy:[29]
In 1994,SN 1994I was observed in the Whirlpool Galaxy. It was classified astype Ic, indicating that its progenitor star was very massive and had already shed much of its mass, and its brightness peaked atapparent magnitude 12.91.[30]
In June 2005 the type II supernovaSN 2005cs was observed in the Whirlpool Galaxy, peaking at apparent magnitude 14.[31][32]
On 31 May 2011 atype II supernova was detected in the Whirlpool Galaxy, peaking at magnitude 12.1.[33] This supernova, designatedSN 2011dh, showed a spectrum much bluer than average, withP Cygni profiles, which indicate rapidly expanding material, in itshydrogen-Balmer lines.[34] The progenitor was probably ayellow supergiant[35] and not ared orblue supergiant, which are thought to be the most common supernova progenitors.
NGC 5195 (also known as Messier 51b or M51b) is a dwarf galaxy that is interacting with the Whirlpool Galaxy (also known as M51a or NGC 5194). Both galaxies are found in the constellation Canes Venatici, some 31 million light-years away. These two galaxies are among the most extensively researched pairs of interacting galaxies.
The Whirlpool Galaxy is the brightest galaxy in theM51 Group, a smallgroup of galaxies that also includes M63 (theSunflower Galaxy),NGC 5023, andNGC 5229.[41][42][43][44] This small group may actually be a subclump at the southeast end of a large, elongated group that includes theM101 Group and theNGC 5866 Group, although most group identification methods and catalogs identify the three groups as separate entities.[45]
^47 out of 90 degrees north of thecelestial equator. Thus its light emits as far south, to a good minimal cumulation of 15° above the horizon, once a day, on the28th parallel south.
^Garner, Rob (2017-10-06)."Messier 51 (The Whirlpool Galaxy)". NASA. Retrieved2021-01-20.Discovered by Charles Messier in 1773, M51 is located 31 million light-years from Earth in the constellation Canes Venatici.
^"M 51".Messier Objects 101.com. 2016-10-10. Archived fromthe original on 2016-10-28.
^Rosse revealed the spiral structure of Whirlpool galaxy (M51) at the 1845 meeting of the British Association for the Advancement of Science. Rosse's illustration of M51 was reproduced in J.P. Nichol's book of 1846.
Rosse, Earl of (1846)."On the nebula 25 Herschel, or 61 [should read: 51] of Messier's catalogue".Report of the Fifteenth Meeting of the British Association for the Advancement of Science; Held at Cambridge in June 1845 § Notices and Abstracts of Miscellaneous Communications to the Sections. Report of the ... Meeting of the British Association for the Advancement of Science (1833): 4.
^Goodwin, S. P.; Gribbin, J.; Hendry, M. A. (August 1998). "The relative size of the Milky Way".The Observatory.118:201–208.Bibcode:1998Obs...118..201G.
^"Whirlpool Galaxy".Herschel Space Observatory. Cardiff University. June 19, 2009. Archived fromthe original on January 10, 2019. RetrievedOctober 11, 2018.
