Object Name: Messier 67
Alternative Designations: M67, NGC 2682
Object Type: Open Galactic Star Cluster
Right Ascension: 08 : 50.4 (h:m)
Declination: +11 : 49 (deg:m)
Distance: 2.7 (kly)
Visual Brightness: 6.1 (mag)
Apparent Dimension: 30.0 (arc min)
Locating Messier 67: Finding M67 is easy in both binoculars and a telescope once you’ve identified the upside down Y shape of the constellation of Cancer. Simply take aim at the easternmost star in the Y, and you’ll find this delightful open cluster about a finger width to the west.
In binoculars and very small telescopes you’ll spy a rich concentration that will appear almost galaxy-like, while larger apertures will fully resolve this cloud of stellar points. M67 is well suited to urban skies and moderate moonlit conditions.
What You Are Looking At: At 3.2 billion years, Messier 67 is billed as one of the oldest star clusters known and the oldest of all the Messier clusters. Containing perhaps 500 stars, and about 100 stars similar to our own Sun, this cloud contains no main sequence stars bluer than spectral type F, since the brighter stars of that age have already left the main sequence – no matter how it may appear! Among its 150 white dwarf stars, there’s only about 30 blue stragglers…
“We present results of a time-series CCD photometry of two blue stragglers in the open cluster M67 that are also oscillating variables, S1280 and S1284. The observations obtained on 11 nights confirmed the Delta Scuti-like variability of the two stars. Four and five main pulsating frequencies are detected for S1280 and S1284, respectively, through a power spectral analysis.” says Xiao-Bin Zhan (et al), “A preliminary mode identification indicates that the two stars are both in radial oscillation. Based on the nature of oscillation, the physical parameters of the two stars are determined, and their evolutionary status discussed.”
As you look at this old open cluster, realize that it’s a great study field for stellar evolution. “The old open cluster M67 is an ideal testbed for current cluster evolution models because of its dynamically evolved structure and rich stellar populations that show clear signs of interaction between stellar, binary and cluster evolution. Here we present the first truly direct N-body model for M67, evolved from zero age to 4 Gyr taking full account of cluster dynamics as well as stellar and binary evolution. Our preferred model starts with 12000 single stars and 12000 binaries placed in a Galactic tidal field at 8.0 kpc from the Galactic Centre. Our choices for the initial conditions and for the primordial binary population are explained in detail. At 4 Gyr, the age of M67, the total mass has reduced by 90% as a result of mass loss and stellar escapes. The mass and half-mass radius of luminous stars in the cluster are a good match to observations although the model is more centrally concentrated than observations indicate. The stellar mass and luminosity functions are significantly flattened by preferential escape of low-mass stars. We find that M67 is dynamically old enough that information about the initial mass function is lost, both from the current luminosity function and from the current mass fraction in white dwarfs.” says Jarrod R. Hurley (et al), “The model contains 20 blue stragglers at 4 Gyr which is slightly less than the 28 observed in M67. Nine are in binaries. The blue stragglers were formed by a variety of means and we find formation paths for the whole variety observed in M67. Both the primordial binary population and the dynamical cluster environment play an essential role in shaping the population. A substantial population of short-period primordial binaries (with periods less than a few days) is needed to explain the observed number of blue stragglers in M67.”
History: According to Johann Elert Bode, M67 was originally discovered by Johann Gottfried Koehler before the year 1779, but his telescope was so primitive that little more than the light could be made out. According to historical records his listed it as his object nineteen, describing it as, “A rather conspicuous nebula in elongated figure, near Alpha of Cancer.”
Charles Messier independently rediscovered M67, resolved it into stars, and cataloged it on April 6, 1780: “Cluster of small stars with nebulosity, below the southern claw of Cancer.” It was observed again by Caroline Herschel, and many times by Sir William, ending up getting its General Catalog designation for John Herschel. Of all the folks in history who described it… Dreyer said it best when he said it was a “Remarkable; cluster; very bright; very large; extremely rich.”
Enjoy the magnificent M67 yourself!
Top M67 image credit, Palomar Observatory courtesy of Caltech, Messier 67 2MASS image, M67 courtesy of the Sloan Digital Sky Survey and M67 color image courtesy of Nigel Sharp, Mark Hanna/NOAO/AURA/NSF.