Messier 67, NGC 2682

Messier 67, also catalogued as NGC 2682, is an open star cluster in Cancer, and one of the most ancient star clusters known.

Messier 67 was discovered by Johann Gottfried Koehler sometime before 1779; however, Koehler's instruments were unable to resolve this cluster. Charles Messier independently rediscovered it, resolved it into stars, and cataloged it as M 67 in April of 1780.

Amateur Observation

In small instruments, Messier 67 is a fine, bright cluster! With a total visual magnitude of 6.1 and overall dimensions of 30' - the size of the full Moon - M 67 makes an excellent target for binoculars or small telescopes. It is quite rich, with nearly a hundred 10th to 14th magnitude stars. Its members are distributed in several clumps, one of which is on the cluster's southern edge near the brighter stars. At 150x, Messier 67 shows some beautiful star chains, and several obvious dark lanes that meander like rivers through the cluster. A magnitude 7.5 star appears near its northeast edge.

Most open clusters are distributed along or near the plane of our Galaxy's spiral disk; but M 67 is so old that it has worked its way 1,500 light-years off the plane of the Milky Way, out to the fringes of its spiral disk where there is little obscuring gas and dust.

Properties and Evolution

Messier 67 is about 2,600 light years away, about 12 light years in diameter, and is by far the oldest of Messier's open clusters. Its age is estimated at between 3.2 and 5 billion years. The most recent estimate of four billion years - just slightly younger than our Sun! - appears to be the most reliable. M 67 is not the oldest known open cluster, but very few in the galaxy are known to be older (among them: NGC 188 at about 5 billion years; and NGC 6791, about 7 billion years old).

M 67 is the nearest old open cluster, and thus has become a standard example for studies of stellar evolution. Estimates of its mass vary from 1080 to 1400 solar masses; and it may have initially been ten times greater. M 67 contains more than 100 stars similar to the Sun, eleven bright K-type giants, many red giants, and nearly 200 white dwarfs. Its total membership has been estimated at over 500 stars.

M 67 also contains some strange "blue straggler" stars; the brightest is of spectral class B8, and has a luminosity of 50 Suns. This is puzzling because such brighter stars of the cluster's age should have already left the main sequence; thus M 67 should not contain any main sequence stars bluer than spectral type F.

It appears that M 67 does not contain an unbiased sample of stars. One cause of this is mass segregation, the process by which lighter star systems gain speed at the expense of more massive stars during close encounters. This causes the lighter stars to be at a greater average distance from the center of the cluster, or to escape altogether. It has been calculated that M 67 can expect to exist as a cluster for about another 5 billion years.