R. J. Trumplerclassified the cluster appearance as II2r, indicating a rich cluster with little central concentration and a medium range in the brightness of the stars.[8] This was later revised to I2r, denoting a dense core.[6] The cluster has a core radius of 2.97±0.46ly (0.91±0.14pc) and a tidal radius of 42.7±7.2ly (13.1±2.2pc).[4] It has an estimated age of 158.5million years[1] and a mass of 1,200M☉.[4]
↑ Akbulut, B.; Ak, S.; Yontan, T.; Bilir, S.; Ak, T.; Banks, T.; Kaan Ulgen, E.; Paunzen, E. (2021). "A study of the Czernik 2 and NGC 7654 open clusters using CCD UBV photometric and Gaia EDR3 data". Astrophysics and Space Science. 366 (7). arXiv:2107.03462. doi:10.1007/s10509-021-03975-x. S2CID235765696.
↑ Bond, Howard E. (August 1973), "Be Stars in the Galactic Cluster M 52", Publications of the Astronomical Society of the Pacific, 85 (506): 405, Bibcode:1973PASP...85..405B, doi:10.1086/129477.
The Butterfly Cluster is an open cluster of stars in the southern constellation of Scorpius. Its name derives from the vague resemblance of its shape to a butterfly. The Trumpler classification of II 3 r encodes it is rich in stars, ranks II out of IV for disparateness and greatly mixes bright with faint components. It is 3.5° to the northwest of Messier 7, both north of the tail of Scorpius.
Messier 21 or M21, also designated NGC 6531 or Webb's Cross, is an open cluster of stars located to the north-east of Sagittarius in the night sky, close to the Messier objects M20 to M25. It was discovered and catalogued by Charles Messier on June 5, 1764. This cluster is relatively young and tightly packed. A few blue giant stars have been identified in the cluster, but Messier 21 is composed mainly of small dim stars. With a magnitude of 6.5, M21 is not visible to the naked eye; however, with the smallest binoculars it can be easily spotted on a dark night. The cluster is positioned near the Trifid nebula, but is not associated with that nebulosity. It forms part of the Sagittarius OB1 association.
Messier 23, also known as NGC 6494, is an open cluster of stars in the northwest of the southern constellation of Sagittarius. It was discovered by Charles Messier in 1764. It can be found in good conditions with binoculars or a modestly sized telescope. It is in front of "an extensive gas and dust network", which there may be no inter-association. It is within 5° the sun's position so can be occulted by the moon.
Messier 29 or M29, also known as NGC 6913, is a quite small, bright open cluster of stars just south of the central bright star Gamma Cygni of a northerly zone of the sky, Cygnus. It was discovered by Charles Messier in 1764, and can be seen from Earth by using binoculars.
Messier 18 or M18, also designated NGC 6613, is an open cluster of stars in the constellation Sagittarius. It was discovered by Charles Messier in 1764 and included in his list of comet-like objects. From the perspective of Earth, M18 is situated between the Omega Nebula (M17) and the Small Sagittarius Star Cloud (M24).
Messier 35 or M35, also known as NGC 2168, is a relatively close open cluster of stars in the west of Gemini, at about the declination of the sun when the latter is at June solstice. It was discovered by Philippe Loys de Chéseaux around 1745 and independently discovered by John Bevis before 1750. It is scattered over part of the sky almost the size of the full moon and is 2,970 light-years away. The compact open cluster NGC 2158 lies directly southwest of it.
Messier 36 or M36, also known as NGC 1960, is an open cluster of stars in the somewhat northern Auriga constellation. It was discovered by Giovanni Batista Hodierna before 1654, who described it as a nebulous patch. The cluster was independently re-discovered by Guillaume Le Gentil in 1749, then Charles Messier observed it in 1764 and added it to his catalogue. It is about 1,330 pc away from Earth. The cluster is very similar to the Pleiades cluster (M45), and if as far away it would be of similar apparent magnitude.
Messier 46 or M46, also known as NGC 2437, is an open cluster of stars in the slightly southern constellation of Puppis. It was discovered by Charles Messier in 1771. Dreyer described it as "very bright, very rich, very large." It is about 5,000 light-years away. There are an estimated 500 stars in the cluster with a combined mass of 453 M☉, and it is thought to be a mid-range estimate of 251.2 million years old.
Messier 50 or M 50, also known as NGC 2323, is an open cluster of stars in the constellation Monoceros. It was recorded by G. D. Cassini before 1711 and independently discovered by Charles Messier in 1772 while observing Biela's Comet. It is sometimes described as a 'heart-shaped' figure or a blunt arrowhead.
Messier 68 is a globular cluster found in the east south-east of Hydra, away from its precisely equatorial part. It was discovered by Charles Messier in 1780. William Herschel described it as "a beautiful cluster of stars, extremely rich, and so compressed that most of the stars are blended together". His son John noted that it was "all clearly resolved into stars of 12th magnitude, very loose and ragged at the borders".
Messier 69 or M69, also known NGC 6637, is a globular cluster in the southern constellation of Sagittarius. It can be found 2.5° to the northeast of the star Epsilon Sagittarii and is dimly visible in 50 mm aperture binoculars. The cluster was discovered by Charles Messier on August 31, 1780, the same night he discovered M70. At the time, he was searching for an object described by Nicolas-Louis de Lacaille in 1751–2 and thought he had rediscovered it, but it is unclear if Lacaille actually described M69.
Messier 75 or M75, also known as NGC 6864, is a giant globular cluster of stars in the southern constellation Sagittarius. It was discovered by Pierre Méchain in 1780 and included in Charles Messier's catalog of comet-like objects that same year.
Messier 93 or M93, also known as NGC 2447, is an open cluster in the modestly southern constellation Puppis, the imagined poop deck of the legendary Argo.
NGC 381 is an open cluster of stars in the northern constellation of Cassiopeia, located at a distance of approximately 3,120 light-years from the Sun. Credit for the discovery of this cluster was given to Caroline Herschel by her brother William in 1787, although she may never have actually seen it.
NGC 6709 is an open cluster of stars in the equatorial constellation of Aquila, some 5° to the southwest of the star Zeta Aquilae. It is situated toward the center of the galaxy at a distance of 3,510 light-years.
NGC 3293 is an open cluster in the Carina constellation. It was discovered by Nicolas-Louis de Lacaille in 1751. It consists of more than 100 stars brighter than 14th magnitude in a 10 arc minute field, the brightest of which are blue supergiants of apparent magnitude 6.5 and 6.7. There is also a 7th magnitude pulsating red supergiant, V361 Carinae.
NGC 637 is an open cluster of stars in the northern constellation of Cassiopeia, positioned about 1.5° to the WNW of the star Epsilon Cassiopeiae. The cluster was discovered on 9 November 1787 by German-born English astronomer William Herschel. It is located in the Perseus Arm of the Milky Way, at a distance of approximately 7.045 kilolight-years from the Sun. The cluster is small but compact, and is readily visible in a small telescope.
NGC 6604 is a young open cluster of stars in the equatorial constellation of Serpens, positioned about 1.5° north of the Eagle Nebula. The cluster was discovered by William Herschel on July 15, 1784. It is located at a distance of 4,580 light years from the Sun, about 210 ly (65 pc) above the galactic plane. NGC 6604 forms the densest part of the Ser OB2 association of co-moving stars.
NGC 5662 is an open cluster in the constellation Centaurus. It was discovered by Nicolas Louis de Lacaille on May 17, 1752 from South Africa. James Dunlop observed it on July 10, 1826 from Parramatta, Australia and added it to his catalog as No. 342.
NGC 3228 is an open cluster in Vela. It was discovered by Nicolas Louis de Lacaille in 1751–1752, while he was in South Africa and catalogued it as Lac II.7. It is small but bright and can be observed easily with binoculars in sufficiently dark skies.
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