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Messier 91
Messier 91 (also known as NGC 4548 or M91)〔〔 is a barred spiral galaxy located in the Coma Berenices constellation and is part of the Virgo Cluster of galaxies. M91 is about 63 million light-years away from the earth. It was the last of a group of eight nebulae discovered by Charles Messier in 1781. Originally M91 was a missing Messier object in the catalogue as the result a bookkeeping mistake by Messier. It was not until 1969 that amateur astronomer William C. Williams〔William 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 Space (Messier 91 ), Accessed online 14 April 2011〕 which was documented by William Herschel in 1784 (according to other sources, however, the nearby spiral galaxy NGC 4571 was also considered as a candidate for Messier 91 by him.〔)
==Observation history==
Messier 91 was discovered on the night of March 18, 1781, Charles Messier described it as Nebula without stars, fainter than M90. Messier mistakenly logged its position from Messier 58,〔〔(【引用サイトリンク】url=http://www.universetoday.com/48349/messier-91/ )〕 where in fact it should have been Messier 89. William Herschel observed the same galaxy on April 8, 1784. Williams solved the missing Messier object by measuring its right ascension and declination relative to those of the nearby galaxy M89 (there are no suitable reference stars in the vicinity). Williams applied the observed differences of M91 with M58, a 9th-magnitude galaxy which Messier recorded in 1778. The calculation reproduces the Messier position〔 to 0.1' in right ascension and 1' in declination.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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