Tau Canoris Majoris Cluster NGC 2362

[AusGuy]

The Tau Canoris Majoris Cluster is currently well placed for viewing in the early evening. It's such an easy target that it shows up well in my SharpCap display even when using the 2 second exposure interval that I typically employ for GoTo slewing and framing. Visually, it's not particularly large, occupying only a small part of the camera's frame. But zooming-in to the 100% resolution display level produces a very satisfactory view.

This is an interesting cluster – particularly if and when we can ever get back to public viewing at outreach events – as it has a number of talking points that can keep viewers involved. Overall it's roughly the same size as the Pleiades (15 light years). But whereas the Pleiades is only 430 light years away the distance to NGC 2362 is 5,100 light years which is why it looks so much smaller. It is very young. 4 to 5 million years seems to be its age. This is only about 4% of the age of the Pleiades. We know this because most of what we see in the cluster are massive class O or B type stars which (in astronomical terms) quickly change into red supergiants. But there are few red supergiants to be seen indicating the the cluster is not yet old enough for most of them them to have changed. One would expect to see evidence of the molecular cloud of gas and dust from which such young cluster stars were born but there is none in their immediate surroundings. This is probably because such massive stars have the energy to very quickly disperse their nascent cloud. However, that energy does seem to be energising the giant HII region Sh2-310 one degree to the east (out of frame in the image below).

Of course the star of the show (pun intended) is Tau Canoris Majoris itself. Clearly it outshines all of the other cluster members. Although it's very young, being so massive (see below) it has still had enough time to evolve away from the Main Sequence and change into a very hot and blue class O9 supergiant. But it's wrong to refer to Tau as a single star. It is in fact a very complicated multiple star system as indicated in the labelled enlarged crop from the full frame image. Initially components A to D were observed and catalogued by John Herschell in the 19^th century **. In 1906 it was discovered that component A was a spectroscopic binary (components Aa and Ab) which was eventually visibly resolved in 1951. Then in 2010 component E was discovered lurking less than 1 second away from component Aa. Even more recently Ab itself has been found to be an eclipsing binary (Ab1 and Ab2). Thus there seem to be seven members of this remarkable star. Component A is thought to have a mass 86 times that of the Sun (Aa 50 + Ab1 18 + Ab2 18). Aa has a temperature of 32,000 degrees Kelvin and shines 280,000 times as brightly as the Sun. Factor in the brightness of the other components of A and you have a truly bright and massive star. It is said to be amongst the hottest known stars in the Universe.

** Looking at the image below I wonder how it was that Herschell determined that component D was a member of the multiple star. It's not too hard to see why he catalogued components A to C. But D seems so far away from A and although slightly brighter than its surrounding companions is otherwise pretty much indistinguishable from them. I've found no source questioning his decision but still I wonder.

I had supposed that this cluster being observable from the northern hemisphere would have already had an entry in this forum but I haven't been able to find one. My apologies if that is not the case and the information given now is redundant.

Vixen R200SS; SW AZ-EQ6 Pro; ZWO ASI294MC Pro; Baader MPCC; Bortle 4.5

SharCap livestack 6 x 30 seconds at 300 gain; minor processing and labelling in Photoshop;

Edited April 6, 2021 by AusGuy
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[Mike JW]

Hi, Fascinating write up and lovely to see this cluster which is so low down for us in GB. I always enjoy multiple stars, especially when tracking them down visually. I cannot find Tau CMa in the Washington Double Star catalogue.

Mike

[RobertI]

Great report and image of an object I’ve never seen. Thanks!

[AusGuy]

Mike / Rob Thanks for your kind feedback. It is appreciated.