Bob,
I would think the relative closeness (34 AU separation) makes it safe to call it a physical pair. I guess count alberio as a double you cannot detect changes in the position angle (PA). Zeta Herc (12 AU seperation) is one that the changes in PA can be readily observed. I have included a link to an article, "Orbits you might live through" , that talks about several. Also, for those of you who are new to double stars it is a good article to read, it has 5 parts. http://www.prairieastronomyclub.org/dblstar5.htm While working on the AL Double Star list I moved the field of view in RA and Dec to estimate the position angles. I have observed many double stars for years without paying any attention to PA. From what I have found most physical doubles that PA changes can be observed occur over decades. I doubt my estimates are more accurate than + or - 10 degrees. I guess we can say Alberio is one of those that PA changes are undetectable. While on the subject of events in the future: I talked with Charlie Green and the subject of future Astronomical Events worth living for came up. Anybody have any Astro Events you are looking forward to? Erik It would be interesting to know for sure as Albeio is a frequent and
favorite star staple and we get a lot of questions about it. My astronomy software lists it as a binary and to Erick's point most of what I was able to find lists it as a true binary system as well although the distance was more often reported as 380 ly vs. 400 ly. Not a significant distance unless you were heading there soon.
Robert Taylor
-----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of erikhansen@TheBlueZone.net Sent: Sunday, May 17, 2009 1:23 PM To: Utah Astronomy Subject: Re: [Utah-astronomy] Albireo: a double star?
My 2 cents.
I compiled a list of 25 famous double stars some years back, as I recall Deep Sky and Sky&Telescope were my primary sources. I may have gotten some info from Burnham.
For Alberio I have it listed as 400 LY from earth and a separation of 34.3 Astronomical Units. I have only Zeta Hercules (12 AU) and Alula Australis (20 AU) as closer, although I don't have AU separation for 7 on the list, I assume that means they are further apart than AU's can be applied. IE: They are only optical doubles. Those are Delta Herc, Sigma Cor Bor, Almach, Porrima, Alpha Sag, Al Rischa, and Delta Lyra.
That indicates to me that they are a gravitational pair. Does anyone know the AU threshold for gravitational double stars? For instance I have Epsilon Persei at 2000 AU's.
BTW: Rob our our fellow SLAS'ers behaving themselves in Hawaii?
Erik
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