Messier 51: Whirlpool Galaxy

Messier 51

Messier 51 (M51), better known as the Whirlpool Galaxy, is a well-known spiral galaxy located in the constellation Canes Venatici. The Whirlpool Galaxy has an apparent magnitude of 8.4 and is located about 23 million light years from Earth. This galaxy has the designation NGC 5194 in the New General Catalog.

Messier 51 is one of the easiest Messier objects to find, as it is located around the asterism of the Big Dipper. This galaxy is located just 3.5 degrees southwest of Alkaid, Eta Ursae Majoris, the star that marks the tip of the Big Dipper's handle, or the tail end of the Big Bear. An imaginary line drawn from Alkaid towards Cor Caroli, the brightest star in Canes Venatici, leads directly to M51. Under good conditions, the Whirlpool Galaxy can be seen with binoculars. It is bright enough and looks direct, which makes it a popular target among amateur astronomers and astrophotographers. M51 is also the brightest example of a spiral galaxy interacting in the sky.

Messier 51 appears as a patch of light in 10x50 binoculars, while small telescopes show a more diffuse patch of light with a bright central region. The galaxy's bright core appears more clearly in the 8-inch instrument, which also reveals the galaxy's massive halo and bits of dark dust lanes and spiral arms. M51's smaller companion galaxy, NGC 5195, is also visible, but the bridge connecting the two can only be detected in the larger instruments. Telescopes  inches and larger reveal numerous spiral bands and extensive H II regions, as well as bands of light connecting the Whirlpool Galaxy to its smaller neighbours. The best time of year to observe M51 is March, April and May.

The Whirlpool Galaxy is also known as the Question Mark Galaxy or the Rosse Galaxy, after William Parsons, 3rd Earl of Rosse, who first recognized the spiral nature of this "nebula" in 1845. Lord Rosse was using his 72-inch reflector at Birr Castle in Ireland to observe M51 when he made the discovery. He also made very accurate drawings of "spiral nebulae" and, for this reason, the galaxy is sometimes known as Lord Rosse's Question Mark. It wasn't until the 1920s, when Edwin Hubble proved that "spiral nebulae" were actually distant galaxies, that the Whirlpool galaxies and other objects of this type were recognized as independent galaxies and not nebulae within the Milky Way.

The Whirlpool Galaxy is the brightest member of the M51 Group, a relatively small group of galaxies that also includes the well-known Sunflower Galaxy (M63) and the dimmer spirals NGC 5023 and NGC 5229.

Messier 51 is classified as the Seyfert 2 galaxy, an active galaxy with a quasar-like nucleus, very high surface brightness, and a distinctively bright core, which appears very bright at infrared wavelengths.

The Whirlpool Galaxy interacts with its smaller companion, NGC 5195 (Messier 51b), a dwarf galaxy connected to its larger neighbor via a tidal dust bridge. The bridge is visible in this pair of silhouetted images in the central region of the smaller galaxy. Tidal interaction with NGC 5195, which was not discovered until the advent of radio astronomy, has greatly enhanced the spiral structure of the Whirlpool Galaxy. Astronomers believe that it is also triggering a wave of new star formation. The interaction leads to the compression of hydrogen gas which, in turn, leads to the formation of a star nursery. The two galaxies will eventually merge, but it will take several more passes to complete the merger.

Three supernovae have been observed in M51 to date: SN 1994I (type Ic) in April 1994, SN 2005cs (type II) in June 2005, and SN 2011dh on 31 May 2011. The latter is a 14.2 magnitude type II supernova and it helps astronomers estimate the distance to the galaxy at 23 million light years.

A supernova was also observed at NGC 5195 on April 8, 1945. Designated SN 1945A, it reached magnitude 14. The supernova event also helped scientists estimate the angular diameter of the Whirlpool Galaxy at 11.2 arc minutes and the spatial radius of its bright circular disk to be about 43,000 light years. Messier 51 is about 35 percent the size of our own galaxy, the Milky Way, and has an estimated mass of 160 billion solar masses. The Whirlpool Galaxy is believed to contain a central black hole surrounded by a ring of dust. The other rings cross the first on a different axis, giving the appearance of a cross at the galactic core. The dark cross marks the exact location of the galactic black hole's center. The central region is currently showing evidence of increased star formation, which will not last for another 100 million years at the current rate.

The Whirlpool Galaxy's companion, NGC 5195, is believed to have passed by i the main disk of M51 around 500 to 600 million years ago, which resulted in the larger galaxy developing a very prominent spiral structure. The smaller galaxy most likely came from behind, passed through the larger galactic disk and then crossed it again about 50 to 100 million years ago. Now, NGC 5195 appears to be slightly behind the Whirlpool Galaxy from our perspective. Messier 51 contains a large number of X-ray sources, most of which are X-ray binary systems, systems consisting of two objects – a neutron star or black hole and an orbiting companion star – with the first object capturing material from the companion. The stolen material is accelerated by the powerful gravitational fields of a neutron star or black hole and heated to very high temperatures, producing a source of highly luminous X-rays. At least 10 of the X-ray binary systems found in M51 are bright enough to harbor black holes, and most of these black holes most likely stole material from stars much more massive than the Sun. A survey of the Whirlpool Galaxy reveals nearly 500 X-ray sources. About 400 of these are believed to be within the galaxy, while others are in front of or behind M51.

The Whirlpool Galaxy was one of Charles Messier's original discoveries. Messier first observed the object on October 13, 1773. In January 1774, he gave the following description:

A very dim nebula, starless, near the eyes of a Northern Greyhound, under the 2nd magnitude tail star Eta Ursa Major: M. Messier discovered this nebula on October 13, 1773, when he was observing the comet visible at the time . One cannot see this nebula without difficulty with an ordinary 3.5-foot [FL] telescope: Nearby is a star with magnitude 8.M. Messier reports his position on the Comet Charts observed in 1773 & 1774. Memoirs of the Academy 1774, plate III. Dual in shape, each has a bright center, which is 4'35” apart. The two “atmospheres” touch each other, one even dimmer than the other. Repeated several times. NGC 5195 was discovered by Messier's friend and colleague, Pierre Méchain, on March 20, 1781. In his 1781 catalog, Messier did not fully explain whether the object he referred to as M51 was simply a larger galaxy or an interacting pair. As a result, the Whirlpool Galaxy (NGC 5194) is sometimes referred to as Messier 51a and the smaller NGC 5195 as Messier 51b.

William Herschel cataloged the object as H I.186 on May 12, 1787, with the following note: “Exactly bright. Large enough. Round or slightly elongated. Gradually lighter towards the center. 3′ north before [NW] 51 Connoissance des Temps [M51].

John Herschel cataloged M51 as h 1622 in April 1830, noting "a very bright spherical core surrounded by faint rings at a distance." He later added the object to the General Catalog as GC 3574.

Admiral William Henry Smyth observed M51 in September 1836 and gave the following description:

A pair of clear white nebulae, each with a clear core, with their nebulosities colliding with each other, as if under the influence of the power of condensation. They were by the ear of Asterion, the northern hunting dog; and the smaller nebula, or northern nebula one, has the brightest nucleus, distinguished by a micrometer of wire; they are 3 degrees southwest of Alkaid, where it is indicated by a line from Dubhe to Megrez, extending almost twice that distance in a southeast direction. There are three telescopic stars that follow, and a bright 7th magnitude as far as they are from the nebula, but the previous part of the field is quite clear. Sir John Herschel has given a splendid representation of this extraordinary object, [fig.] no. 25 , in his 1830 Catalog illustration. This subtle field was discovered by Messier in 1772 [actually 1773], and was described as a faint double nebula whose centers are 4'35 apart", but with "touching boundaries". singular, having a bright center surrounded by luminosity, resembling the ghost of Saturn, with its rings in a vertical position They form Nos. 1622 and 1623 of [J.] H.'s catalogue, which calls the southern nebula, or halo nebula, the most astonishing object, perhaps a system similar to ours, the halo representing the Galaxy.

If it consisted of stars, the appearance it would give to an onlooker stationed on one of the accompanying planets, which are eccentrically located towards a quarter np of the central mass, would be exactly the same as ours. The Milky Way, crosses, in an exactly analogous way, the firmament of stars the large ng, on which the central cluster would have projected, and (because of its greater distance) appeared, as it were, composed of stars much smaller than any other part of the sky. Is it possible that we have here a fraternal system, which bears a marked physical resemblance and strong structural analogy to ours?

We then have an object which displays a magnificent display of unbridled almighty energy, the contemplation of which forces reason and awe to yield to awe. At the outer threshold of the telescopic range, we see a universe of stars similar to our own, whose vast amplitudes are undoubtedly inhabited by countless beings of perception; for those beautiful spheres cannot be considered as mere masses of inert matter. And it is interesting to know that, if there were intelligent existence, an astronomer looking at our distant universe, would see it, with a good telescope, exactly under the lateral aspect shown by them to us. But after all what did we see? Both the beautiful universe, our own, and all that optical aids have revealed to us, may be mere outliers of a much more numerous cluster. The millions of suns that we perceive cannot consist of the Creator's Universe. There is no limit to infinity; and the elder Herschel's boldest look simply puts us in command of a ken some 35,000 times as far away as Sirius is from us. May the dying Laplace explain: “What we know is little; what we don't know is huge."

Halton Arp has entered the Whirlpool Galaxy as his No. 85 in the Curious Galaxies Catalog, listing it as “Spiral with Large High Surface Brightness Companion.”

Facts and Data

Object: Galaxy

Type: Spirals

Class: SA(s)bc pec

Designations: Messier 51, M51, M51a, NGC 5194, Whirlpool Galaxy, Rosse Galaxy, Question Mark Galaxy, PGC 47404, UGC 8493, Arp 85, GC 3572, VV 001a, VV 403, BD+47 2063, IRAS 13277+4727 , 1RXS J132953 .8+471143, TC 827

Features: Interact with the smaller galaxy NGC 5195

Constellation: Canes Venatici

Right ascension: 13 hours 29 minutes 52.7 seconds

Declination: +47°11'43"

Distance: 23 million light years (7.1 megaparsecs)

Number of stars: > 100 billion

Apparent magnitude: +8.4

Sighting dimensions: 11′.2 x 6′.9

Diameter: 60,000 light years

The first sketch of a spiral "nebula" (i.e., galaxy), as published in 1850 by Lord Rosse.

Supernova SN2005cs in M51

The Whirlpool Galaxy Core (M51). The location of the black hole at the center of the galaxy. The cross was first interpreted as two rings of dust surrounding the nucleus. It was then determined to just be a silhouetted foreground dust lane by the active nucleus. Bright ionization cones can be seen extending perpendicular to the largest dust features.

X-ray data from NASA's Chandra X-ray Observatory reveal point-like sources (purple) that are black holes and neutron stars in binary star systems. Chandra also detected hot gas jets spreading in interstellar space. Optical data from the Hubble Space Telescope (green) and infrared emissions from the Spitzer Space Telescope (red) highlight long paths in the spiral arms of stars and gas sprinkled with dust.
Image credit: NASA/CXC/Wesleyan Univ./R.Kilgard et al; NASA/JPL-Caltech

The 51st entry in Charles Messier's famous catalog is possibly a genuine spiral nebula – a large galaxy with a well-defined spiral structure that is also cataloged as NGC 5194. More than 60,000 light years away, M51's spiral arms and dust lanes clearly sweep ahead of its counterpart. galaxy (above), NGC 5195. Image data from Hubble's Advanced Camera for Surveys has been reprocessed to produce an alternative portrait of this famous interacting galaxy pair. This processing further sharpens details and enhances color and contrast in dim areas, bringing out long trails of dust and streams that criss-cross the tiny companion, along with features all around and at the heart of the M51 itself. The pair is about 31 million light years away. Not far in the sky from the handle of the Big Dipper, they are officially located within the confines of the minor constellation Canes Venatici.
Image credit: S. Beckwith (STScI), Hubble Heritage Team, (STScI/AURA), ESA, NASA, Additional Processing: Robert Gendler

This Hubble composite image shows visible starlight as well as the glow from the glowing hydrogen glow associated with the brightest young stars in the spiral arms. The Whirlpool Galaxy, also known as M51 or NGC 5194, is having a close encounter with its closest companion galaxy, NGC 5195, right at the top edge of this image. The companion's gravitational pull fuels star formation in the primary galaxy, as seen in great detail by the many glowing young and energetic star clusters. The bright clusters are highlighted in red by the associated emission of glowing hydrogen gas. This Wide Field Planetary Camera 2 image allowed researchers to clearly define the structure of the cold dust clouds and hot hydrogen and link individual clusters to their parent dust clouds. The intricate structures are also visible for the first time in the dust cloud. Along the spiral arms, dust "spurs" are seen branching almost perpendicularly to the main spiral arms. This regularity and large number of features suggest to astronomers that the previous "two-armed" model of spiral galaxies may need to be revisited. The new images also reveal disks of dust within the nucleus, which may provide fuel for nuclear black holes.
Image credit: NASA and Hubble Heritage Team (STScI/AURA)

These images by NASA's Hubble Space Telescope show off two very different direct views of the spiral galaxy M51, which has been dubbed the Whirlpool Galaxy. The image at left, taken in visible light, highlights the signature features of a spiral galaxy, including its gracefully curved arms, pink star-forming regions, and strands of brilliant blue star clusters. In the image on the right, most of the starlight has been removed, revealing the dusty structure of the Whirlpool galaxy skeleton, as seen in near-infrared light. This new image is the sharpest look yet at the solid dust on the M51. The narrow column of dust revealed by Hubble reflects the galactic moniker, the Whirlpool Galaxy, as if they were spiraling towards the galactic core. To map the structure of the galaxy's dust, the researchers collected the light from the galaxy's stars by combining images taken in the visible and near infrared. Visible light images only capture part of the light; the rest is covered in dust. However, the near-infrared view reveals more starlight because the near-infrared light penetrates the dust. The researchers then subtracted the total amount of starlight from the two images to look at the structure of the galaxy's dust. The red color in the near-infrared image traces the dust, which is punctuated by hundreds of tiny clumps of stars, each 65 light-years wide. These stars have never been seen before. Star clusters cannot be seen in visible light because of the thick dust that surrounds them. The image reveals details as small as 35 light years away.
Image credits – infrared: NASA, ESA, M. Regan & B. Whitmore (STScI), & R. Chandar (U. Toledo); Optical: NASA, ESA, S. Beckwith (STScI), & Hubble Heritage Team (STScI/AURA)

NASA's Spitzer Space Telescope has captured this infrared image of the "Whirlpool Galaxy," revealing strange structures bridging gaps between the dust-rich spiral arms, and tracking populations of dust, gas and stars in the bright spiral galaxy and its companions. The Spitzer image is a composite of four colors of invisible light, showing emission from the wavelengths of 3.6 microns (blue), 4.5 microns (green), 5.8 microns (orange), and 8.0 microns (red). This wavelength is approximately 10 times longer than that seen by the human eye. The visible-light image came from the Kitt Peak National Observatory's 2.1m telescope, and is the same orientation and size as the Spitzer infrared image, measuring 9.9 by 13.7 arc minutes (north to top). Also a four-color composite, the visible light image shows emission from 0.4 to 0.7 microns, including the nebular feature H-alpha (red in image). The light seen in the image comes from a very different source. At shorter wavelengths (in the visible band, and in the infrared from 3.6 to 4.5 microns), light mostly comes from stars. This starlight fades at longer wavelengths (5.8 to 8.0 microns), where we see the glow from clouds of interstellar dust. This dust is primarily composed of various carbon-based organic molecules known collectively as polycyclic aromatic hydrocarbons. Wherever these compounds are found, there will also be dust grains and gas, providing reservoirs of raw materials for future star formation. Most puzzling are the large numbers of thin filaments of red emission seen in the infrared data between the arms of a large spiral galaxy. In contrast to the beady nature of the dust emission seen in the arms themselves, these bar-like features are thin and regular, and widely scattered in fissures across the galaxy's surface. Also of interest is the contrast in the distribution of dust and stars between the spiral and its dim counterpart. While the spiral is rich in dust, bright in the longer infrared bands, and actively forming new stars, its blue partner shows little infrared emission and harbors a population of older stars. The spectacular whirlpool structure and star formation on M51 is thought to have been triggered by an ongoing collision with its companion. Understanding the impact of star formation by galactic interactions is one of the goals of these observations. Spitzer's observations of M51 are part of a major 500-hour science project, known as the Spitzer Infrared Nearby Galaxy Survey, which will comprehensively study the 75 nearby galaxies by infrared imaging and spectroscopy. From this data, astronomers will investigate the physical processes that link star formation to the properties of galaxies. This information will provide a vital data foundation, diagnostic tool, and astrophysical input to understanding the distant universe, ultraluminous galaxies, and galaxy formation and evolution.

Image credit: NASA/JPL-Caltech/R. Kennicutt (University of Arizona)/DSS

Far-infrared tricolor image of M51, the Whirlpool Galaxy. The red, green and blue correspond to the 160 micron, 100 micron and 70 micron wavelength bands from the HerschelÕs Photoconductor Array Camera and Spectrometer, PACS instrument. The glow from the clouds of dust and gas around and between the stars is clearly visible. These clouds are the reservoir of raw materials for the ongoing star formation in this galaxy. Blue indicates areas of warm dust heated by young stars, while cooler dust is red.

Image credit: ESA and the PACS Consortium

The Whirlpool Galaxy (M51) in X-rays. X-rays from a rare type of supernova in the Whirlpool Galaxy were recently observed, thanks to the fine resolution of NASA's Chandra X-ray Observatory. The research team also detected a large number of point-like X-ray sources caused by black holes and neutron stars in binary star systems. The Chandra image highlights the energetic central region of two interacting galaxies, NGC 5194 (center) and its smaller companion (top left) NGC 5195.
Image credit: NASA/CXC/UMD/A. Wilson et al.

Location of Messier 51.
Image credits: IAU and Sky & Telescope Magazine (Roger Sinnott & Rick Fienberg)