The Whirlpool Galaxy, also known as Messier 51 (M51) and NGC 5194, is a grand-design spiral galaxy in Canes Venatici. The galaxy appears face-on when seen from Earth. Its designation in the New General Catalogue is NGC 5194. The galaxy lies about 23 million light years from Earth. It was named the Whirlpool because of its swirling spiral structure.
The Whirlpool Galaxy is one of the brightest and most famous galaxies in the night sky, notable for its two striking spiral arms that make M51 a grand design galaxy. The spiral arms are really lanes of stars and starburst regions interspersed with dust. They compress hydrogen gas and are responsible for creating new clusters of stars.
Messier 51 is gravitationally interacting with a smaller dwarf galaxy, NGC 5195, and the two make one of the best known pairs of interacting galaxies in the sky.
NGC 5195 has been gliding past the larger galaxy for hundreds of millions of years and its shape is highly distorted, making classification difficult. It has been identified as an amorphous or irregular galaxy and sometimes classified as a lenticular galaxy.
The face-on view and relative proximity to Earth make the two galaxies relatively easy to find and observe, even for amateur astronomers. The galaxy pair can even be seen with binoculars.
The Whirlpool Galaxy (NGC 5194) is sometimes referred to as Messier 51a, while NGC 5195 is often called Messier 51b. The designation Messier 51 is used to refer either to the Whirlpool or to both galaxies. The bigger galaxy in particular is a popular target for astronomers, who observe and study it for the insight it provides into spiral arm structure and galaxy interactions.
The striking spiral structure of the Whirlpool Galaxy is thought to be the result of its close interaction with the smaller NGC 5195. The interaction between the pair also results in compression of hydrogen gas which in turn leads to formation of starburst regions, seen in pictures as bright blue knots across the galaxy’s spiral arms.
FACTS, LOCATION AND SIZE
The Whirlpool Galaxy was first observed by Charles Messier on October 13, 1773, when he was observing a comet. He later added the object to his catalogue as Messier 51. The smaller galaxy, NGC 5195, was discovered by Messier’s colleague Pierre Méchain in 1781.
Charles Messier originally described the object as a “very faint nebula, without stars, near the eye of the Northern Greyhound, below the star Eta of 2nd magnitude of the tail of Ursa Major… One cannot see this nebula without difficulties with an ordinary 3.5 foot telescope. Near it is a star of 8th magnitude.” After the discovery of the smaller companion galaxy, he noted, “It is double, each has a bright centre, which are separated 4’35”. The two ‘atmospheres’ touch each other, the one is even fainter than the other.”
The Whirlpool galaxy was the first galaxy to be recognized as a spiral, in spring 1845, after William Parsons, the 3rd Earl of Rosse, observed it with his 72-inch reflector, the so-called Leviathan. Lord Rosse also created a very accurate painting of the object, which is why Messier 51 is sometimes also called Rosse’s Galaxy or Lord Rosse’s ‘Question Mark.’
The Whirlpool Galaxy is relatively easy to find in good observing conditions. It lies just below Alkaid, Eta Ursae Majoris, the bright star marking the tip of the handle of the Big Dipper in Ursa Major constellation. It can be found by moving the binoculars to the southwest of Alkaid, toward Cor Caroli, Alpha Canum Venaticorum, the brightest star in Canes Venatici constellation. The Whirlpool Galaxy is located 3.5° to the southeast of Alkaid. It is best viewed at a low magnification and, to make out the spiral arms, one needs at least a 4-inch telescope.
The Whirlpool Galaxy is approximately 60 light years across. It has an angular diameter of about 11.2′. The galaxy’s bright circular disk is believed to have a radius of approximately 43,000 light years. The estimated mass of M51 is about 160 billion solar masses.
The galaxy’s compact nucleus is classified as of Seyfert type 2.5. The declination of the galaxy is +47°, which makes M51 circumpolar (never setting below the horizon) for observers above 43°N latitude.
With a diameter of approximately 75,000 light years, the Whirlpool is about 25 percent smaller than our own galaxy, the Milky Way. The angular diameter of the galaxy is about 1/3rd the width of the full Moon.
In good viewing conditions, the Whirlpool Galaxy can be seen with binoculars, and with amateur telescopes it can be observed in greater detail. The outlines of the Whirlpool and its companion galaxy are visible through a 100 mm telescope, and the spiral structure can be seen though a 150 mm telescope. To see the galaxy’s spiral bands and HII regions, one needs a larger instrument, ideally one of more than 300 mm.
Three supernovae have been observed in the galaxy. SN 1994I was spotted on April 2, 1994. It peaked at magnitude 12.8 and was classified as a type Ic supernova. SN 2005cs, a type II supernova was seen on June 27, 2005, and peaked at visual magnitude 14. SN 2011dh was detected on May 31, 2011. It had an apparent visual magnitude of 14.2, peaked at magnitude 13.5, and was classified as a type II supernova.
The central region of the Whirlpool Galaxy is undergoing a period of intense star formation at a rate it will likely not be able to sustain for more than another 100 million years. In NGC 5195, however, there are almost no new stars being formed. This means that the smaller galaxy was either dust-poor even before the encounter with the Whirlpool or it has been stripped of dust as a result of the interaction with the larger galaxy.
The Whirlpool Galaxy and NGC 5195 had not been confirmed to be interacting until the first radio images of the pair were obtained. Prior to that, astronomers could not determine with any degree of certainty that the two galaxies were not just lying in the same line of sight.
Both NGC 5194 and NGC 5195 are believed to contain supermassive black holes, each emitting intense X-rays.
The current spiral structure of the larger galaxy is believed to be a result of the smaller galaxy passing through its main disk some 500 to 600 million years ago and making another crossing about 50 to 100 million years ago. NGC 5195 is now located slightly behind the Whirlpool Galaxy.
The two galaxies are gravitationally bound and approaching each other for another interaction. They will eventually merge, but not before they have made a few more passes, which will likely take hundreds of millions of years.
The Whirlpool Galaxy is the brightest member of the M51 Group, a group of galaxies that includes several notable members located in the same region of the sky: the Sunflower Galaxy (Messier 63), NGC 5023, NGC 5229, UGC 8313, and UGC 8331.
The nucleus of Messier 51 contains a cross, or an X-structure, which suggests that the galaxy has two dust rings surrounding the central black hole.
One of the dust rings stands almost perpendicular to the relatively flat spiral galaxy, and the other ring crosses the first one on a different axis. Two ionization cones stretch from the axis of the primary dust ring.
The dark “X” appears silhouetted across the M51’s nucleus and is believed to reveal the exact location of the galaxy’s central black hole and to be a result of absorption by dust. The darker bar could be a dust ring about 100 light years in diameter, appearing edge-on. The other of the two bars forming the “X” may be either another disk, also seen edge-on, or just rotating dust and gas crossing paths with the ionization cones and jets.
The X-structure was revealed in an image taken by NASA’s Hubble Space Telescope, with Dr. Holland Ford of the Johns Hopkins University and Space Telescope Science Institute and his team reporting the following observations:
“Pictures of M51 taken with the [Hubble] Planetary Camera show a striking, dark ‘X’ silhouetted across the nucleus. The ‘X’ is due to absorption by dust and marks the exact position of the nuclear black hole. If these ideas are correct, M51 provides the first direct view of a torus [ring] which both fuels a massive black hole and hides the hole from direct view from anyone outside the ionization cone [narrow cone of light emitted from the near-vicinity of the black hole].”
Whirlpool Galaxy – Messier 51 (NGC 5194)
Constellation: Canes Venatici, the Hunting Dogs
Galaxy type: SA(s)bc pec
Coordinates: 13h29m52.7s (right ascension), +47°11’43” (declination)
Distance: 23 ± 4 million light years (7.1 ± 1.2 megaparsecs)
Redshift: 463 ± 3 kilometres per second
Apparent dimensions: 11.2 x 6.9 arc minutes
Diameter: 75,000 light years
Visual magnitude: 8.4
Designations: Whirlpool Galaxy, Messier 51 (M51), Messier 51a (M51a), Rosse’s Galaxy, Question Mark Galaxy, NGC 5194, UGC 8493, Arp 85, PGC 47404, GC 3572, VV 001a, VV 403