Messier 67 (also known asM67 orNGC 2682) and sometimes called theKing Cobra Cluster or theGolden Eye Cluster[5] is anopen cluster in the southern,equatorial half ofCancer. It was discovered byJohann Gottfried Koehler in 1779. Estimates of its age range between 3.2 and 5 billion years. Distance estimates are likewise varied, but typically are 800–900 parsecs (2,600–2,900 ly).[1][2][3][4] Estimates of 855, 840, and 815 pc were established viabinary star modelling andinfraredcolor-magnitude diagram fitting.[2][3][4]
M67 is not the oldest known open cluster; severalMilky Way clusters are known to be older, yet farther than M67. It is a paradigm study object instellar evolution:[6]
it is well-populated
has negligible amounts of dust obscuration
all its stars are at the same distance and age, save for approximately 30 anomalousblue stragglers
M67 is one of the most-studied open clusters, yet estimates of its physical parameters such as age, mass, and number of stars of a given type, vary substantially. Richeret al. estimate its age to be 4 billion years, its mass to be 1080 solar masses (M☉), and number itswhite dwarfs at 150.[7] Hurley et al. estimate its current mass to be 1,400 M☉ and its initial mass to be approximately 10 times as great.[8]
It has more than 100 stars similar to the Sun, and numerousred giants. The total star count has been estimated at well over 500.[9] The ages and prevalence of Sun-like stars had led some astronomers to theorize it as the possible parent cluster of theSun.[10] However, computer simulations disagree on whether the outerSolar System would have survived an ejection from M67,[11][12] and the cluster itself would probably not have survived such an ejection event.[13]
The cluster contains nomain sequence stars bluer (hotter) thanspectral type F, other than perhaps some of the blue stragglers, since the brighter stars of that age have already left the main sequence. In fact, when the stars of the cluster are plotted on theHertzsprung-Russell diagram, there is a distinct "turn-off" representing the stars which have terminated hydrogen fusion in the core and are destined to become red giants. As a cluster ages, the turn-off moves progressively down the main sequence to cooler stars.
It appears that M67 has a bias toward heavier stars. One cause of this ismass segregation, the process by which lighter stars gain speed at the expense of more massive stars during close encounters, which moves them to greater average distance from the center of the cluster or allows escape altogether.[14]
A March 2016 jointAIP/JHU study by Barneset al. on rotational periods of 20 Sun-like stars, measured by the effects of moving starspots on light curves, suggests that these approximately 4 billion-year old stars spin in about 26 days – like the Sun, which has a period at the equator of 25.38 days.[15] Measurements were carried out as part of the extendedK2 mission ofKepler space telescope. This reinforces the applicability of many key properties of the Sun to stars of the same size and age, a fundamentalprinciple of modernsolar andstellar physics.[16] The authors abbreviate this as the "solar-stellar connection".[16]
^abcYakut, K.; Zima, W.; Kalomeni, B.; Van Winckel, H.; Waelkens, C.; De Cat, P.; Bauwens, E.; Vučković, M.; Saesen, S.; Le Guillou, L.; Parmaksızoğlu, M.; Uluç, K.; Khamitov, I.; Raskin, G.; Aerts, C. (2009). "Close binary and other variable stars in the solar-age Galactic open cluster M 67".Astronomy and Astrophysics.503 (1): 165.arXiv:0906.4908.Bibcode:2009A&A...503..165Y.doi:10.1051/0004-6361/200911918.S2CID1136371.
^abcMajaess, Daniel J.; Turner, David G.; Lane, David J.; Krajci, Tom (2011). "Deep Infrared ZAMS Fits to Benchmark Open Clusters Hosting delta Scuti Stars".Journal of the American Association of Variable Star Observers (Jaavso).39 (2): 219.arXiv:1102.1705.Bibcode:2011JAVSO..39..219M.
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