Gingerich, O., "Messier and His Catalogue II", Sky and Telescope vol. 12 (for September 1953), 1953, p. 289.[1]
ui.adsabs.harvard.edu
Herschel, J., "Observations of Nebulae and Clusters of Stars made at Slough, with a Twenty-feet Reflector, between the years 1825 and 1833", the Philosophical Transactions of the Royal Society of London, Vol. CXXIII, London, 1833.[2]
wikipedia.org
en.wikipedia.org
Gingerich, O., "Messier and His Catalogue II", Sky and Telescope vol. 12 (for September 1953), 1953, p. 289.[1]
