Messier 91 (also known as NGC 4548 or M91) is a

barred spiral galaxy A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in about two thirds of all spiral galaxies in the local universe, and generally affect both the motions of stars and interstellar gas ...

that is found in the south of

Coma Berenices Coma Berenices is an ancient asterism in the northern sky, which has been defined as one of the 88 modern constellations. It is in the direction of the fourth galactic quadrant, between Leo and Boötes, and it is visible in both hemispher ...

. It is in the

local supercluster The Local Supercluster (LSC or LS), or Virgo Supercluster is a formerly defined supercluster containing the Virgo Cluster and Local Group, which itself contains the Milky Way and Andromeda galaxies, as well as others. At least 100 galaxy groups a ...

and is part of the

Virgo Cluster of galaxies The Virgo Cluster is a cluster of galaxies whose center is 53.8 ± 0.3 Mly (16.5 ± 0.1 Mpc) away in the Virgo constellation. Comprising approximately 1,300 (and possibly up to 2,000) member galaxies, the cluster forms the heart of the larger V ...

. It is about 63 million

light-years A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astro ...

away from our galaxy. It was the last of a group of eight "

nebula A nebula (; or nebulas) is a distinct luminescent part of interstellar medium, which can consist of ionized, neutral, or molecular hydrogen and also cosmic dust. Nebulae are often star-forming regions, such as in the Pillars of Creation in ...

e" – the term 'galaxy' only coming into use for these objects once it was realized in the 20th century that they were extragalactic – discovered by

Charles Messier Charles Messier (; 26 June 1730 – 12 April 1817) was a French astronomer. He published an astronomical catalogue consisting of 110 nebulae and star clusters, which came to be known as the ''Messier objects'', referred to with th ...

in 1781. It is the faintest object in the Messier catalog, with an apparent magnitude of 10.2. As a result of a bookkeeping error by Messier, M91 was for a long time one of the few missing entries in the Messier catalog, not matching any known object in the sky. It was not until 1969 that amateur

astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...

William C. WilliamsWilliam C. Williams
Letter
Sky and Telescope, December 1969, p. 376. Accessed 13 April 2011 realized that M91 was NGC 4548,Students for the Exploration and Development of Spac

Accessed online 14 April 2011 which was catalogued by

William Herschel Frederick William Herschel ( ; ; 15 November 1738 – 25 August 1822) was a German-British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel. Born in the Electorate of Hanover ...

in 1784. Some sources contend the nearby spiral galaxy

NGC 4571 NGC 4571 is a spiral galaxy located in the constellation of Coma Berenices that William Herschel thought was Messier 91 in Charles Messier's catalog of deep-sky objects,Students for the Exploration and Development of SpacNGC 4571 Accessed onlin ...

was considered as a candidate for this object by Herschel.

Observation history

The object was discovered in 1781 by Messier who described it as nebula without stars, fainter than M90. Messier mistakenly logged its position from

Messier 58 Messier 58 (also known as ''M58'' and ''NGC 4579'') is an intermediate barred spiral galaxy with a weak inner ring structure located within the constellation Virgo (constellation), Virgo, approximately 68 million light-years away from Earth. It ...

, where in fact it should have been

Messier 89 Messier 89 (M89 for short, also known as NGC 4552) is an elliptical galaxy in the constellation Virgo. It was discovered by Charles Messier on March 18, 1781. M89 is a member of the Virgo Cluster of galaxies. Features Current observations allo ...

. William Herschel observed the same object in 1784. In 1969 Williams solved this lost Messier object by measuring its right ascension and declination relative to those of the nearby galaxy M89 (notable reference stars angularly nearby are sparse) – rather than M58, a 9th-magnitude galaxy which Messier recorded in 1778. This amended night sky "star-hopping" reference point matches Messier's figures to 0.1 of an

arcminute A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

() in right ascension and 1 in declination, a sixtieth of a degree. M91map

Features

Inclusion of Messier 91 in the Virgo cluster was confirmed in 1997 from observing Cepheid variables which place it at million light years away. Its bar is very conspicuous – it is seen with position angle of 65 to 245 degrees when being measured from the North direction to the East. There is a countering peculiar (local) velocity toward us through the Virgo cluster of about 700 km/s within the cluster's recession velocity of about 1100 km/s, which produces its observed

recessional velocity Recessional velocity is the rate at which an extragalactic astronomical object recedes (becomes more distant) from an observer as a result of the expansion of the universe. It can be measured by observing the wavelength shifts of spectral lines e ...

of only about 400 km/s. Another source gives the latter figure as 803 km/s.Virgo cluster
/ref> Messier 91 is also classified as an

anemic galaxy An anemic galaxy is a type of spiral galaxy characterized by a low contrast between its spiral arms and its disk. Etymology The term was coined in 1976 by the Canadian astronomer Sidney van den Bergh to classify galaxies that are an intermed ...

, that is: a spiral galaxy with little star formation and gas compared with other galaxies of its type.

References and footnotes

External links

SEDS: Messier Object 91

Galaxy M91
* {{DEFAULTSORT:Messier 091 Barred spiral galaxies Virgo Cluster Coma Berenices 091 NGC objects 07753 41934 Astronomical objects discovered in 1781 Discoveries by Charles Messier

Observation history

Features

See also

References and footnotes

External links