Tau Canis Majoris and its vivid cluster

2016.Feb20tau cma cluster small
Star Tau Canis Majoris and its cluster NGC 2362. Sketch and copyright Harry Roberts ©, all rights reserved.


Regular solar observer & correspondent Harry Roberts reports on the Tau Canis Majoris star cluster


Bayer’s System. Stars names like this were created by Bayer in 1603, in an attempt to rationalize the prevailing chaos.  At the time names were mostly of ancient Persian or Indian origins – a rich mix of constellation legend and seasonal events: it couldn’t persist into the age of science. Or could it?  Owners of GOTO ‘scopes are expected to know archaic names in setting up their hi-tech gear – and name lists are once again circulating! The 1603 change was mostly due to old astrology being supplanted by the new science of astronomy.

Bayer’s elegant system blended Greek letters and constellation names. For example: ‘Alpha Canis Majoris’ means – star alpha (lower case: α), the brightest, in the constellation of the Great Dog: its ancient name is Sirius. By using the Greek alphabet and the Latin he managed to tag stars down to naked-eye limits.

His system told the user where in the sky the named star resided and gave its order of brightness: two useful properties that the archaic namings ignored. It was a good system but failed to cope with big changes over the next 300 years. When telescopes came along (soon after in 1610) the hosts of ‘new’ stars visible led to more naming systems; but Bayer’s survived for most brighter stars.

Thus it is that we find a beautiful bright star tau (τ) in the Great Dog, some ten degrees south of Sirius, a faint star easily seen in binoculars. In any telescope, a ‘four inch’ or bigger, τ CanMaj is a wonderful surprise!

Cluster NGC2362. At first glance tau is a bright blue star, seemingly embedded in a faint nebula. Use more magnification and the ‘haze’ turns into a swarm of smaller stars: the rich galactic cluster NGC2362 – a breathtaking sight! The Catalogue says there are sixty cluster members, the whole having a brightness of 4th mg. – with an age of 25My: a young cluster.

Tau is the one star of the cluster that has evolved away from the main sequence into a spectral type O supergiant: its mass is 50 times the Sun, and its brightness an astounding 280,000 times the Sun’s! Visually it is 4.4 mg.

Tau is also a double star, ADS 5977, with component B, a 10th mg. star, at 8.2arcsec separation – as well as 11th mg. C at 14.5arcsec:  a very nice binary, it is shown in the INSET. Sketched in an urban backyard with a ten inch Dobson, it is a beautiful sight too in a 4” telescope.

North American amateurs (says Wiki) call star tau the “Mexican Jumping Star”.  I know what they mean: if you gaze at the central star the cluster seems to vanish; focus on the whole field – and the cluster reappears, while the central star seems to fade! It’s an unusual effect. Take a close look at tau CanMaj.  What do you see?

Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.


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