M67 - An Unusually Ancient Open Cluster

Open clusters are not generally long-lived objects. They are (in cosmic timescales) relatively quickly torn apart by gravitational influences as they wander through the galaxy. So after a few scant hundreds of millions of years, their individual stellar components live out the remainder of their lives as solitary stars (just like our own sun, and nearly every other star in the Milky Way Galaxy), and their parent cluster is only a memory.

But every now and again, an open cluster will manage to hold itself together, even after multiple passes through the galactic plane (the zone of maximum disruption). Messier 67 is a prime example of this rara avis. Estimated to be somewhere between 3.2 and 5 billion years old, M67 is quite the oldster. Composed of approximately 500 stars, of which about 100 closely resemble our own Sun, the cluster contains the mass of more than 1000 Suns. The consensus amongst astronomers is that M67's initial mass, some 4 billion years ago, was more than 10 times what it has managed to retain. What a sight that would have been!

But however glorious it may have looked in the distant past, M67 remains quite the eye pleaser. At apparent magnitude 6.1, it verges on being a dark sky naked eye object. Through my 8X56 binos, it is a distinct fuzzy patch just to the right of Alpha Cancri. In my 90mm Stellarvue refractor, I can make out dozens of stars enmeshed in a faux nebulosity, which in reality is the remainder of the cluster's stars which lie beyond my scope's capability to resolve. To my eye, M67 looks like nothing so much as a giant comma lying on its side. Either that, or a cat seen from behind, with its tail prominently displayed.

M67 lies somewhere between 2600 and 2900 light years from the Earth. Practically every type of star can by found within the cluster, from white dwarfs to blue stragglers, from Sun-like main sequence stars, to red giants.

M67 is not hard to find. Start by locating the dim constellation of Cancer, between bright Gemini and dramatic Leo. The Main Attraction in Cancer is of course the Beehive, or M44. The Beehive (also known as the Manger) is to the upper right of Delta Cancri, which lies smack on the ecliptic. (Fun Fact: Delta Cancri's proper name is my favorite of all the stars in the sky, the Babylonian word  Arkushanangarushashhutu - and no, I am not going to try to pronounce that!) Below and to the left of Delta Cancri is (despite its "Alpha" designation, the somewhat fainter star) Alpha Cancri. M67 is located not far to the right of Alpha Cancri.

Not so long ago, astronomers seriously wondered whether M67 was our own Sun's parent star cluster, principally due to the large number of Sun-like stars within it. But recent computer simulations of the non-intersecting paths taken by M67 and Sol have ruled such a thing to be extremely improbable.