Messier 91 (also known asNGC 4548 orM91)[4][5] is abarred spiral galaxy that is found in the south ofComa Berenices. It is in thelocal supercluster and is part of theVirgo Cluster of galaxies. It is about 63 millionlight-years away fromour galaxy. It was the last of a group of eight "nebulae" – the term 'galaxy' only coming into use for these objects once it was realized in the 20th century that they were extragalactic – discovered byCharles Messier in 1781. It is the faintest object in the Messier catalog, with an apparent magnitude of 10.2.[3]
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 amateurastronomer William C. Williams[4] realized that M91 was NGC 4548,[5] which was catalogued byWilliam Herschel in 1784. Some sources contend the nearby spiral galaxyNGC 4571 was considered as a candidate for this object by Herschel.[5]
The object was discovered in 1781[a] by Messier who described it as nebula without stars, fainter than M90. Messier mistakenly logged its position fromMessier 58,[5][6] where in fact it should have beenMessier 89. William Herschel observed the same object in 1784.[b]
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[4] to 0.1 of anarcminute (′) in right ascension and 1′ in declination, a sixtieth of a degree.
Inclusion of Messier 91 in the Virgo cluster was confirmed in 1997 from observing Cepheid variables which place it at52±6 million light years away.[5]
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.[5]
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 observedrecessional velocity of only about 400 km/s.[5] Another source gives the latter figure as 803 km/s.[7]
Messier 91 is also classified as ananemic galaxy, that is: a spiral galaxy with little star formation and gas compared with other galaxies of its type.[8][9]
^Vollmer, B.; Cayatte, V.; Boselli, A.; Balkowski, C.; Duschl, W.J. (1999). "Kinematics of the anemic cluster galaxy NGC 4548. Is stripping still active?".Astronomy and Astrophysics.349:411–423.arXiv:astro-ph/9907351.Bibcode:1999A&A...349..411V.
