Messier 39 or M39, also known as NGC 7092, is an open cluster of stars in the constellation of Cygnus, sometimes referred to as the Pyramid Cluster. It is positioned two degrees south of the star Pi Cygni[7] and around 9° east-northeast of Deneb.[8] The cluster was discovered by Guillaume Le Gentil in 1749, then Charles Messier added it to his catalogue in 1764.[7][9] When observed in a small telescope at low power the cluster shows around two dozen members[7] but is best observed with binoculars.[7] It has a total integrated magnitude (brightness) of 4.6 and spans an angular diameter of 29 arcminutes[4] – about the size of the full Moon.[8] It is centered about 1,010 light-years (311 parsecs) away.[2]
^ abcdStreicher, Magda (October 2015), "Sky Delights: Another Cross", Monthly Notes of the Astronomical Society of Southern Africa, 74 (10): 243–248, Bibcode:2015MNSSA..74..243S.
^ abBone, Neil (August 2005), "Sky notes for 2005 August and September", Journal of the British Astronomical Association, 115 (4): 230, Bibcode:2005JBAA..115..230B.
^Zakirov, M. M.; et al. (July 1995), "Variability of HD205117 in the Open Cluster M39 (NGC 7092)", Information Bulletin on Variable Stars, 4220: 1, Bibcode:1995IBVS.4220....1Z.
^Gray, R. O.; Corbally, C. J. (August 2002), "A Spectroscopic Search for λ Bootis and Other Peculiar A-Type Stars in Intermediate-Age Open Clusters", The Astronomical Journal, 124 (2): 989–1000, Bibcode:2002AJ....124..989G, doi:10.1086/341609.
^Schuff, Sarah; et al. (December 2006), "A Search for Variable Stars in the Field of NGC 7092 (M39)", Bulletin of the American Astronomical Society, 38: 1135, Bibcode:2006AAS...20916514S.
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