Messier 67

Discovered before 1779 by Johann Gottfried Koehler.

Messier 67 (M67, NGC 2682) is one of the oldest known open clusters, and by far the oldest of Messier's open clusters, being aged at 3.2 billion years in the Star Catalogue 2000.0; Mallas/Kreimer quote an even higher, but probably outdated value of 10 billion years. New estimates of G. Meynet's Geneva Team indicate an age of 4.0 billion years. Note: This is still less than the age of our Solar System, but open clusters usually get destructed much faster. It has been calculated that M67 can expect to exist as a cluster for about another 5 billion years.

Only few known open clusters were found to be older, among them probably NGC 188 at about 5 billion years, longly quoted as the oldest known cluster, and NGC 6791, which is about 7 billion years old (according to Götz), and is currently the oldest known open cluster in our Milky Way galaxy.

At this later stage of evolution, the open cluster M67 shows, in its Hertzsprung-Russell diagram, a well-developed red giant branch, while the main sequence ends to the hot blue end at spectral class A or F. It contains 11 bright K-type giants of absolute magnitude +0.5 to +1.5, and several stars scattered on the horizontal branch. However, it also contains some strange stars near the bluer main sequence, representatives of the so-called Blue Stragglers, the brightest of which is of spectral class B8 or B9 and apparent mag 10, corresponding to a luminosity of 50 times that of the Sun at the distance of M67 (2,700 light years according to Glyn Jones and Götz, 2600 from the Sky Catalog 2000). The total number of stars in M67 is probably at least about 500. The Trumpler type of this cluster is given as II,2,r (Trumpler according to Glyn Jones), II,2,m (Sky Catalog 2000) or II,3,r (Götz).

According to Cecilia Payne-Gaposhkin, M67 contains nearly 200 white dwarfs.

As M67 is of an age of the same order of magnitude as our Solar System, and its stars happen to have a similar chemical composition as the Sun, this cluster is an appropriate target of observation for the study of solar-type stars. Mark Giampapa of the National Science Foundation's National Solar Observatory in Tucson, Arizona, has observed more than 100 sun-like stars in M67 and found that most of them are either significantly more, or significantly less active than our home star; anout 10-15 percent of these star expose an exceptionally quiescent levels of magnetic activity, while about 30 percent of the M67 suns are in a state of enhanced activity compared to that seen at solar maximum (see NOAO Press Release 99-07).

According to Johann Elert Bode, M67 has been discovered by Johann Gottfried Koehler (1745-1801) somewhen before 1779; it seems, however, that Koehler's instruments were so inferior that he couldn't resolve this cluster. Charles Messier independently rediscovered M67, resolved it into stars, and cataloged it on April 6, 1780.

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