Friday, 7 April 2017

A cloud of invisible stars

Number 48 on Messier's list is an easy object. It's a very bright and large star cluster that's appreciated best in binoculars or a small telescope. Larger instruments will magnify too much and as such peer "through" the cluster rather than fill the field of view with it. But there's no need to despair! Just point your scope slightly to the right, towards the outer edge of the dim but noteworthy constellation of Monoceros, the unicorn. There you'll find this cluster which scientists refer to as NGC2506. It's significantly dimmer and it appears much smaller than M48 - being over 11.000 lightyears away (!) - but it's certainly much more interesting as well. 

For starters, NGC2506's much richer than its apparent neighbour. M48 counts about 80 stars whereas there are several hundred members that belong to NGC2506. The latter's much older too, at least 1,1 billion years against 300 million for M48. You can tell because unlike most clusters which merely consist of young and hot blue stars, I identified quite a few old orangy ones among the blue-white majority. This is quite surprising because, as I told you before, most star clusters fall apart after a couple of hundred thousand years under the gravitational pull of our galaxy. Similar to the even much older M67, this distant cluster seems to resist the tide quite well. That being said, my observation gave me the feeling that NGC2506's slowly giving in and will not hold out as long as M67. I noticed an overall flattened ring sort of shape which may indicate that the stars are being pulled away from the cluster's core, ready to be hurled into empty space. 

Being so incredibly distant also means that this cluster's difficult to resolve into individual stars. Modest telescopes will only show a nebulous patch with a couple of little stars in front and even with my big binoscope I wasn't able to distinguish every single star. Many remained hidden in the faint cloud. Nonetheless this is a beautiful and challenging object that I simply had to share with you.

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