Messier 67

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Messier 67

Observation data: J2000.0 epoch
Class:
Constellation:	Cancer
Right ascension:	08h 51.4m
Declination:	+11° 49′
Distance:	2.7 kly (830 Pc)
Apparent magnitude (V):	6.1
Apparent dimensions (V):	30.0′
Physical characteristics
Mass:	( M☉)
Radius:	10 ly
VHB:	-
Estimated age:	3.2 to 5 billion years
Notable features:	-
Other designations:	NGC 2682
See also: Open cluster, List of open clusters

Messier 67 (also known as M 67 or NGC 2682) is an open cluster, or galactic cluster, in the constellation Cancer. M67's Trumpler class is variously given as II 2 r, II 2 m, or II 3 r. It was discovered by Johann Gottfried Koehler in 1779. Its age is variously estimated at between 3.2 and 5 billion years. The most recently estimated age of four billion years appears to be the most reliable; thus the stars of M67 are most likely slightly younger than the Sun.

M67 is not the oldest known open cluster, but there are very few older ones in the galaxy. Most open clusters have ages less than one billion years, since open clusters are typically distrupted by loss of mass in their youth when hot young supergiants heat up gas which then escapes, by tidal forces, by field stars passing through the cluster, and by escape of stars whose orbits are changed by encounters by other cluster members. M67 is an important laboratory for studying stellar evolution, since all its stars are at the same distance, and also of the same age, except for approximately 30 anomalous blue stragglers, whose origin is not fully understood.

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. Richer et al. ^[1] estimate its age to be 4 Gyr, its mass to be 1080 solar masses, and the number of white dwarfs to be 150. Hurley et al. ^[2] estimate its current mass to be 1400 solar masses and its initial mass to be approximately 10 times as great. Zhang et al. ^[3] estimate the number of blue stragglers to be approximately 30.

M67 has more than 100 stars similar to the Sun, and many red giants. The total star count has been estimated at over 500. The cluster contains no main sequence stars bluer than spectral 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 the Hertzsprung-Russell diagram, there is a distinct "turn-off" representing the stars which are just about to leave the main sequence and become red giants. As the cluster ages, the turn-off will move down the main sequence.

It appears that M67 does not contain an unbiased sample of stars. One cause of this is mass segregation, the process by which lighter stars (actually, systems) gain speed at the expense of more massive stars during close encounters, which causes the lighter stars to be at a greater average distance from the center of the cluster or to escape altogether^[4]. Another possible cause is that a supernova explosion which leaves a remnant white dwarf could be asymmetric, giving the white dwarf a "kick" which causes it to escape the cluster. This is suggested as an explanation for the observed paucity of white dwarfs in M67.

[edit] External links

• Messier 67, SEDS Messier pages

[edit] References