Thursday 16 March 2017

A pear-shaped cluster

I hope that it's just this star cluster being pear-shaped and not my observation... :-) The reason why I say this is because M67, a bright star cluster in the constellation of Cancer, is extremely old. Estimates vary between 3,5 and 5 billion years and this is extraordinary. As you remember, most star clusters are soon ripped apart by the gravitational pull of our Milky Way and the individual stars each go their own way some hundreds of thousands of years after they're born. But M67 seems to resist till the bitter end. Most of its stars are therefore middle-aged and in many ways comparable to our own Sun. Actually, it was once thought that our Sun also originated from this cluster, but that seems very unlikely now. You may also notice some reddish stars, which are the most massive ones in the group. Because of their great mass they've already run out of hydrogen and have swollen to red giants. Yet, I've also observed several young, blue stars in it, hence why I was having doubts about my observation. But a quick check revealed that quite a few young stragglers have been absorbed by the cluster. The more new stars the cluster absorbs, the greater its overall mass becomes and the more it manages to resist the gravitational pull of our galactic centre. Over 500 cluster members have been classified so M67's one of the largest clusters in our vicinity... well... it's only 2.600 lightyears away. It's total mass is estimated at 1.400 solar masses and originally it could have been even ten times greater! Imagine the gas cloud needed to produce 5.000 stars... it would have dwarfed the Orion Nebula!

Because of its proximity and the great variety of stars it contains due to its age, M67 has been observed extensively for the study of stellar evolution. Having such a strange cluster so close that you can already observe it quite easily with ordinary binoculars is much more convenient of course than having to peer at some of the other very old clusters like NGC2158 or worse... Be19, which are extremely far away.  

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