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NGC and Tau Canis Majoris

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#1 gustave

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Posted 01 March 2014 - 09:50 AM

I have recently moved to Florida at latitude 27 and last night had my first dob sweep of the sky in the mid - 20's dec and came across NGC 2362. What a gem. This morning I "think" I confirmed that I was viewing the central star Tau Canis Majoris as a multiple system. At about 3 o'clock position I saw two small points of light next to the primary. Tau is a a visual double consisting of 2 O-type stars separated by 0.151". Was I actually splitting the double or triple system with my Royce 11" mirror or were they foreground stars or even stars within the cluster not associated with Tau? I'll post this as well on the forum Doubles category. Thanks

#2 blb

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Posted 01 March 2014 - 10:38 AM

Tau-Canis Major is the bright central star of an open cluster. This cluster is known as NGC2362 and is sometimes called the Mexican Jumping Bean Cluster.

I am confused by the fact that you only saw three stars when looking at Tau. In my little AT66 refractor (66mm, f/6) I can count 23 members in the cluster and it is reported that you can see 40 stars with a 6" scope. Hey, I can see close to 30 stars with my 10" dob from home in a white zone where the sky is orangeish in color, and I live at 36 degrees north latitude. I wonder if in fact you were looking at Tau-CMA?

#3 gustave

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Posted 01 March 2014 - 11:19 AM

I was referring to the immediate view of Tau. In fact I was really enthralled by the number of apparent member stars in the NGC 2362 which I would render a guess came close or perhaps exceeded the 60 figure as reported on various astronomy sites. Again they may have been un-associtated with the cluster but they did seem to be within the circular pattern of 2362.

#4 Tony Flanders

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Posted 01 March 2014 - 05:42 PM

I have recently moved to Florida at latitude 27 and last night had my first dob sweep of the sky in the mid - 20's dec and came across NGC 2362. What a gem. This morning I "think" I confirmed that I was viewing the central star Tau Canis Majoris as a multiple system. At about 3 o'clock position I saw two small points of light next to the primary. Tau is a a visual double consisting of 2 O-type stars separated by 0.151". Was I actually splitting the double or triple system with my Royce 11" mirror?


Not a chance -- on multiple fronts.

First of all, if you saw two fainter stars near the primary rather than just one, you obviously weren't splitting a binary. If there were a third star equally close, you'd know about it.

Second, splitting an 0.15" double visually is almost out of the question. It would require a huge scope, perfect seeing, and superb eyesight. Barnard and Burnham have done such things, but few normal mortals can.

Finally, think about the magnification needed. Most people need an apparent separation around 4' to split an even double, though some can do as tight as 2' and a tiny handful (perhaps) can split 1' doubles.

Even 1' would require a magnification of 60/0.15 = 360X. A more conventional 4' would require 1440X. What magnification were you using?

#5 gustave

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Posted 01 March 2014 - 07:17 PM

mag-145x (brandon 24 plus Televue 2.5 barlow), 288x straight Nagler 4.8
The 0.15 came off a website that was obviously a typo. That led me to pose the questions here on the forum. You have provided me with a very good, detailed explanation and for that I am grateful.

#6 stevecoe

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Posted 02 March 2014 - 02:34 AM

Tony, et al;

Here is a drawing with a 13" f/5.6 Newtonian at 150X. The triple in the middle (we call it Tau and the Boys) is rather easy on a good night, I have split it with 6" of aperture.

Clear skies;
Steve Coe

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#7 Tony Flanders

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Posted 02 March 2014 - 07:51 AM

The 0.15 came off a website that was obviously a typo.


No, Tau CMa is in fact a very tight double with separation 0.15". That doesn't necessarily mean it was discovered by looking through the eyepiece of a telescope, though. There are several other ways: interferometry, radial shift, light curve for eclipsing binary.

I've always been a little suspicious of the term "double star" inside a dense cluster. Who's to say what's a multiple star versus a bunch of cluster components that happen to appear close to each other?

The Washington Double Star Catalog does in fact list a faint, more distant companion to Tau.

#8 RussL

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Posted 25 March 2014 - 11:42 AM

Don't forget to also look at the "Winter Albireo" just a little northerly (maybe NE?) of Tau. Very pretty orange/blue pair.

#9 tigerroach

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Posted 27 March 2014 - 10:38 AM

I just observed this cluster for the first time 3 weeks ago. It is really easy to overlook since it huddles around such a bright star. At moderate power it really jumps out.






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