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Whirlpool Galaxy
Whirlpool Galaxy (M51a), as taken by European Space Agency, the smaller object in the upper right is NGC 5195 (M51b) (NASA/ESA)
Observation data (J2000.0 epoch)
Constellation	Canes Venatici
Right ascension	13 29 52.7
Declination	+47° 11′ 43″
Redshift	0.001534±0.000007
Distance	7.22 ± 2.13 megaparsecs (23.5 ± 6.95 million light-years)
Apparent magnitude (V)	8.4
Characteristics
Type	SA(s)bc pec
Size	23.58 kpc (76,900 ly) (diameter; 25.0 mag/arcsec B-band isophote)
Apparent size (V)	11.2′ × 6.9′
Notable features	Interacting with NGC 5195
Other designations
Question Mark Galaxy, Rosse's Galaxy, M51a, NGC 5194, UGC 8493, PGC 47404, VV 001a, VV 403, Arp 85, GC 3572

The Whirlpool Galaxy, also known as Messier 51a (M51a) or NGC 5194, is an interacting grand-design spiral galaxy with a Seyfert 2 active galactic nucleus. It lies in the constellation Canes Venatici, and was the first galaxy to be classified as a spiral galaxy. It is 7.22 megaparsecs (23.5 million light-years) away and 23.58 kiloparsecs (76,900 ly) in diameter.

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 and its pair with NGC 5195 to understand galaxy structure (particularly structure associated with the spiral 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.

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. William Parsons, 3rd Earl of Rosse, employing a 72-inch (1.8 m) 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.

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 (NGC 5194) and M51b (NGC 5195).

Visual appearance

Deep in the constellation Canes Venatici, M51 is often found by finding the easternmost star of the Big Dipper, Alkaid, and going 3.5° southwest. Its declination is, rounded, +47°, making it circumpolar (never setting) for observers above the 43rd parallel north; 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 its right ascension, specifically figuring in Gemini, just to the north).

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 (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 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 system (IPCS)—developed jointly by the CNRS Laboratoire d'Astronomie Spatiald (L.A.S.-CNRS) and the very nucleus of the Whirlpool galaxy.

In January 2005 the Hubble Heritage Project constructed a 11,477 × 7,965-pixel composite image (shown in the infobox above) 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

The Whirlpool Galaxy lies at a distance of about 23 to 31 million light-years from Earth. Based on the 1991 measurement by the Third Reference Catalogue of Bright Galaxies using the D₂₅ isophote at the B-band, the Whirlpool Galaxy has a diameter of 23.58 kiloparsecs (76,900 light-years). Overall the galaxy is about 88% 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 Whirlpool Galaxy has two, very prominent spiral arms that wind clockwise. One arm deviates from a constant angle significantly. 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.

Tidal features

As a result of the Whirpool 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 43 kiloparsecs (140,000 light-years) from the galaxy's center. This plume is uniform in color and likely originated from the Whirpool 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.

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 Whirpool Galaxy will fail to produce an analogue to the Northeast tail. In contrast, the multiple-passage simulations made by Salo and Laurikainen et.al reproduce the northeast plume.

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. Similarly, the spiral arms are experiencing high levels of star formation, as well as the space along the arms.

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 supernova SN 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 ${\displaystyle M_{r}=-14.9}$.

Planet candidate

In September 2020, the detection by the Chandra X-ray Observatory of a candidate exoplanet, named M51-ULS-1b, orbiting the high-mass X-ray binary M51-ULS-1 in this galaxy was announced. If confirmed, it would be the first known instance of an extragalactic planet, a planet outside the Milky Way Galaxy. The planet candidate was detected by eclipses of the X-ray source (XRS), which consists of a stellar remnant (either a neutron star or a black hole) and a massive star, likely a B-type supergiant. The planet would be slightly smaller than Saturn and orbit at a distance of some tens of astronomical units.

Companion

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 located approximately 31 million light-years away in the constellation Canes Venatici. Together, the two galaxies are one of the most widely studied interacting galaxy pairs.

Galaxy group information

The Whirlpool Galaxy is the brightest galaxy in the M51 Group, a small group of galaxies that also includes M63 (the Sunflower Galaxy), 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.

See also

• Messier 101 – another grand-design spiral galaxy
• List of Messier objects
• List of galaxies

Notes

1. 47 out of 90 degrees north of the celestial equator. Thus its light emits as far south, to a good minimal cumulation of 15° above the horizon, once a day, on the 28th parallel south.

References

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External links

• SEDS: Spiral Galaxy M51
• NASA Astronomy Picture of the Day: M51: The Whirlpool Galaxy in Dust and Stars (10 April 2001)
• Whirlpool Galaxy at ESA/Hubble
• The Whirlpool Galaxy (Messier 51(a)/NGC 5194)
• M51 The Whirlpool Galaxy
• The Whirlpool Galaxy at Constellation Guide
• The Whirlpool Galaxy on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Astrophoto, Sky Map, Articles and images
• The Whirlpool Galaxy in the Staracle Messier catalog

v t e Messier objects
List	M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M11 M12 M13 M14 M15 M16 M17 M18 M19 M20 M21 M22 M23 M24 M25 M26 M27 M28 M29 M30 M31 M32 M33 M34 M35 M36 M37 M38 M39 M40 M41 M42 M43 M44 M45 M46 M47 M48 M49 M50 M51 M52 M53 M54 M55 M56 M57 M58 M59 M60 M61 M62 M63 M64 M65 M66 M67 M68 M69 M70 M71 M72 M73 M74 M75 M76 M77 M78 M79 M80 M81 M82 M83 M84 M85 M86 M87 M88 M89 M90 M91 M92 M93 M94 M95 M96 M97 M98 M99 M100 M101 M102 M103 Added M104 M105 M106 M107 M108 M109 M110
See also	Caldwell catalogue Catalogue of Nebulae and Clusters of Stars Herschel 400 Catalogue Index Catalogue New General Catalogue Revised New General Catalogue
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v t e Constellation of Canes Venatici
Canes Venatici in Chinese astronomy Giant Void List of stars in Canes Venatici
Stars	Bayer α α (Cor Caroli) β (Chara) Flamsteed 4 5 6 10 19 20 21 24 25 Variable R Y (La Superba) RS AM AW BH DG HR 4997 Other Gliese 521 HAT-P-12 HAT-P-36 (Tuiren) CWISE J1249+3621
Exoplanets HAT-P-12b
Star clusters Messier 3
Galaxies	Messier 51 a (Whirlpool Galaxy) b 63 94 106 NGC 4111 4138 4145 4151 4163 4183 4214 4217 4242 4244 4395 4399 4400 4401 4449 4485 4490 4534 4618 4625 4627 4631 4656 4657 4707 4800 4861 5002 5003 5005 5023 5033 5112 5223 5229 5238 5273 5371 5394 Numbered I II Other 3C 285 3C 286 3C 288 DDO 169 IC 883 IC 4182 IC 4263 IC 4271 I Zwicky 36 KKR 03 4C +41.26 TON 618
Galaxy clusters Canes II Group M51 Group M94 Group NGC 4631 Group
Astronomical events GRB 090429B SN 1994I SN 2005cs SN 2008ax
Category

• Astronomy
• Stars
• Outer space

• Unbarred spiral galaxies
• Interacting galaxies
• Peculiar galaxies
• M51 Group
• Canes Venatici
• Messier objects
• NGC objects
• UGC objects
• Principal Galaxies Catalogue objects
• Arp objects
• Seyfert galaxies
• Astronomical objects discovered in 1773
• Discoveries by Charles Messier