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 31 million light-years (9.5 megaparsecs/Mpc) away and 23.58 kiloparsecs (76,900 ly) in diameter. The galaxy and its interaction companion, NGC 5195, are easily observed by amateur astronomers; the two galaxies may be seen with binoculars. The Whirlpool Galaxy and NGC 5195 have been extensively observed by professional astronomers, who study the pair to understand galaxy structure (particularly structure associated with the spiral arms) and galaxy interactions. This pair 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. Radiowaves permitted to map the distribution of gas in the two galaxies, understanding that they are physically connected . 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 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. 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.
In 2022, the James Webb Space Telescope observed the galaxy using MIRI for the Feedback in Emerging extrAgalactic Star clusTers (JWST-FEAST) project.
Properties The Whirlpool Galaxy lies at a distance of 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. It's believed to be an estimated 400 million years old. 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 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.
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