Messier Index/M73

Messier 73 300px w:J2000 Open cluster 20 58 54SIMBAD Astronomical Database Results for Messier 73 http://simbad.u-strasbg.fr/Simbad 2006-12-21 -12 38 2,500 ly 766.5 pc 9.0 2.8′ - ly - million yrs Aquarius M 73, NGC 6994 Messier 73 (also known as M73 and NGC 6994) is an asterism of four w:stars in the constellation of Aquarius. An asterism is comprised of physically unconnected stars that appear close to each other in the sky as seen from Earth. M73 is one of the best-known asterisms in the sky, and it has been carefully studied scientifically. History M73 was discovered by w:Charles Messier on w:October 4, w:1780, who originally described the object as a cluster of four stars with some nebulosity. Subsequent observations by w:John Herschel, however, failed to reveal any nebulosity. Moreover, Herschel noted that the designation of M73 as a cluster was questionable. Nonetheless, Herschel included M73 in his General Catalogue of clusters, w:nebulae, and w:galaxies, and John Dreyer included M73 when he compiled the w:New General CatalogueK. G. Jones 1991 Messier's Nebulae and Star Clusters 2nd Cambridge University Press Cambridge 0-521-37079-5 . Scientific Research: Asterism or Open Cluster? M73 was once treated as a potential sparsely populated w:open cluster, which consists of stars that are physically associated in space as well as on the sky. The question of whether the stars were an asterism or an open cluster generated a small, interesting debate. In 2000, L. P. Bassino, S. Waldhausen, and R. E. Martinez published an analysis of the colors and luminosities of the stars in and around M73. They concluded that the four bright central stars and some other nearby stars followed the color-luminosity relation that is also followed by stars in open clusters (as seen in a w:Hertzsprung-Russell diagram). Their conclusion was that M73 was an old open cluster that was 9 arcmin wideL. P. Bassino, S. Waldhausen, & R. E. Martinez CCD photometry in the region of NGC 6994: The remains of an old open cluster Astronomy & Astrophysics 2000 355 138–144 http://adsabs.harvard.edu/abs/2000A&A...355..138B . G. Carraro, however, published results in 2000 based on a similar analysis and concluded that the stars did not follow any color-luminosity relation. Carraro's conclusion was that M73 was an asterismG. Carraro NGC 6994: An open cluster which is not an open cluster Astronomy & Astrophysics 2000 357 145–148 http://adsabs.harvard.edu/abs/2000A&A...357..145C . Adding to the controversy, E. Bica and collaborators concluded that the chance alignment of the four bright stars seen in the center of M73 as well as one other nearby star was highly unlikely, so M73 was probably a sparse open clusterE. Bica, B. X. Santiago, C. M. Dutra, H. Dottori, M. R. de Oliveira, & D. Pavani Dissolving star cluster candidates Astronomy & Astrophysics 2001 366 827–833 http://adsabs.harvard.edu/abs/2001A&A...366..827B 10.1051/0004-6361:20000248 . The controversy was solved in 2002, when M. Odenkirchen and C. Soubiran published an analysis of the high resolution w:spectra of the six brightest stars within 6 arcmin of the central position of M73. Odenkirchen and Soubiran demonstrated that the distances from the Earth to the six stars were very different from each other, and the stars were moving in different directions. Therefore, they concluded that the stars were only an asterismM. Odenkirchen & C. Soubiran NGC 6994: - clearly not a physical stellar ensemble Astronomy & Astrophysics 2002 383 163–170 http://adsabs.harvard.edu/abs/2002A&A...383..163O 10.1051/0004-6361:20011730 . Although M73 was determined to be only a chance alignment of stars, further analysis of asterisms is still important for the identification of sparsely populated open clusters. Such clusters can be important for demonstrating how open clusters are ripped apart by the gravitational forces in the w:Milky Way. Location The asterism's location in the sky is shown in the following map of the constellation Aquarius: thumb|none|250px|Constellation of Aquarius External links Messier 73, SEDS Messier pages Messier 73, LRGB CCD image based on two-hours total exposure References