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What is the "Rule" on determining the "A" star (Primary Star)

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#1 dmdouglass

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Posted 16 February 2019 - 06:32 PM

What is the “Rule” on determining the “A” star (Primary Star) ?

 

I used to understand that the “primary” star of a double, was the “Largest Star”, which was also the “Brightest Star”. Not sure where I learned that. Having a hard time finding an authority on that.  But I did find a couple of references.

 

“Visual Observing of Double Stars by Charles E. Worley, which was/is a reprint from the magazine Sky and Telescope, apparently published in 1961. In the article (a booklet), was a drawing explaining Position angle and Separation.“The brighter component is always chosen as the origin of coordinates.”

 

Also in a text “Observing and Measuring Visual Double Stars” (R.W. Argyle, 2nd Ed,  2004 and 2012), has this on page 2, when talking about “position angle”… “With the brighter of the two stars being taken as the origin……”

 

That seems to tie in with my understanding, but still, not sure of where the “authority” on that is. Anyone have a clue ?

 

By why do I ask this? Myself, and a few friends, where checking out this beautiful Double Star System.

 

South-437 (S-437 aka WDS 03463+2411) is a Double Star System (5 stars) located smack in the middle of M-047 (Pleiades).
http://www.az-dahut....Target-Area.jpg

 

My actual observation of this Double Star system appears here:
http://www.az-dahut....DS-20190131.jpg

 

Now, granted, the AB pair is the primary pair, with “A” being Mag 8.13, and “B” being Mag 9.39
I’m not doing Speckle imaging quite yet, so AB shows as a single star in the obersation.
So at that point, all is well.

 

Now enters “C”, as in AB,C
Of the group of 5 stars in this “System”…. The biggest and brightest is definitely “C”.
Which makes me stop and ponder……

 

Anyone care to jump in here and clear up my head ??



#2 fred1871

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Posted 16 February 2019 - 09:16 PM

Worley and Argyle are correct. What you've come across is an anomaly in the data recorded in the WDS (and no doubt appears the same in Stelle Doppie). The close AB pair was discovered long after James South (the S in the designation) discovered a wide pair here and measured it. You'll notice that the magnitudes in the AB listing are 8.13 and 9.39, which means that if you see AB unresolved (as a single star) it will appear to be 0.30 magnitudes brighter than A by itself, therefore, 7.83.

 

However the next line in the WDS listing has, for AB,C 8.13 and 7.70. The first number is wrong, because it gives the magnitude of A, not AB combined.

 

James South, looking at the wide pairing back in 1823, saw two stars of very similar brightness. Either he, or someone much later, muddled which was the brighter of the two (as it appeared) stars.  There's not much difference between them, as seen. Perhaps it was SW Burnham, who discovered the AB pairing in the 1870s, and later produced the first standard catalogue of all Northern and Equatorial doubles, in which he made changes to various listings.... 

 

The likely possibility is that James South saw two stars of much the same brightness, and picked one of them as A, the other as B; and post-Burnham, with A becoming AB, the old 'B' label was changed to 'C' for the wider other star.

 

It's not a big deal, really; merely one of the occasional anomalies that creep into a long term cataloguing system that sometimes includes re-lettering of stars. Professional interest these days is mostly on the Burnham AB pairing, which is a gravitational pair, with a calculated orbit listed. The C component is likely a merely optical component, suggested by linear motion and rather different Proper Motion in Declination.

 

Added in Edit: 

The GAIA DR2 listing gives similar parallaxes for A and C (B is unresolved); these differ somewhat even after allowing for the formal error bars to make them as close as possible.... In other words, C is at a similar but not the same distance from us as the distance to AB. Also, the GAIA Proper Motion numbers reinforce the unlikelihood of C being other than optical: a big difference in PM for Declination, modest difference for RA.


Edited by fred1871, 16 February 2019 - 09:42 PM.

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#3 Cotts

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Posted 17 February 2019 - 11:28 AM

If my memory serves,  when a pair is perfectly equal the A component is chosen such that the PA is a minimum or under 180 degrees.....

 

DAve



#4 Luna-tic

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Posted 17 February 2019 - 12:30 PM

What throws me is a "double" that is actually a multiple system where one of the pair is a double in itself, such as Zeta Cancri. All three can be split, but the tightest of the three may be paired with the primary star or the secondary and then it may be 'A-C/B' or 'A-B/C' or 'A/B-C'



#5 Cotts

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Posted 17 February 2019 - 03:29 PM

What throws me is a "double" that is actually a multiple system where one of the pair is a double in itself, such as Zeta Cancri. All three can be split, but the tightest of the three may be paired with the primary star or the secondary and then it may be 'A-C/B' or 'A-B/C' or 'A/B-C'

There are two distinct types of triple stars.  

 

One type is the AB x C   where a tight pair is orbited by a distant single component.

 

The other is A x BC  where a single star is orbited by a tight pair.

 

"Orbited by" is a convenient shortcut to the fact that all three orbit their mutual centre of gravity.  In double star science, for measuring and examining orbits, only two stars at a time are considered. The "A" component is considered 'stationary' and the "B" component prescribing some sort of ellipse around it.  This ellipse is not the true orbit but a projection of the true orbit on the 'flat' plane of the celestial sphere.... 

 

So, for an AB x C triple there will be a calculated orbit for B around A and another orbit for C around the gravitational centre of the AB system.   Similar for the A x BC pair....

 

 

Confounding the above is that, sometimes, the AB pair is just the first one discovered with a much closer and/or fainter companion, C, being discovered decades later.   This can give us the combination of AC x B.  

 

The fun really begins when close companions to A, B or C are then discovered.   Then, for example, A will become the pair AaAb.   There are systems where there are 6 components: AaAb, BaBb and CaCb.   Castor, Alpha Geminorum is one of these.   Fun to visualize the system in your mind.....   

 

Dave




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