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Fainter stars looking blue-ish?

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#1 bobcat83


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Posted 14 September 2017 - 01:03 AM

I am working my way through the AL Double Stars observing program, and I have noticed through reviewing my observing logsheets, that I often note blue-ish color in the fainter of double stars or multiple stars.  Is my eye just seeing blue in contrast to the brighter yellow or white stars?  Or is there really "blue" color in a lot of companion stars in the 5 to 8th magnitude range?   I am intrigued by the amount of color I am seeing in the doubles on the AL list!

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#2 DLuders



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Posted 14 September 2017 - 03:15 AM

Which Double Stars (in particular) on the Astronomical League Double Star List https://www.astrolea...dblstar2017.pdf are you talking about? 

#3 beggarly


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Posted 14 September 2017 - 03:29 AM

Tough question!

The WDS has information on the spectral types of the components of some double stars.






The Saguaro Astronomy Club Double Star Database version 4.0 contains color information for some stars: http://www.saguaroas...t/downloads.htm


Stars arranged by spectral class: http://stars.astro.i.../sow/class.html




imho every amateur astronomer should at least once in their lifetime buy a copy of The RASC Observer's Handbook. It contains very usefull information. There is a table of 'Coloured Double Stars'. A supplement is available on line: https://www.rasc.ca/supplements.

There are many blue (spectral type B (and O)) components in double stars.







Edited by beggarly, 14 September 2017 - 07:04 AM.

#4 happylimpet



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Posted 14 September 2017 - 04:47 AM

I think this is a stellar evolution question - ie why is the fainter of many coloured doubles the bluer one, something Ive noticed too, and not just because I look at Albireo and gamma And far too often.


Im going to have a think about this!

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#5 Redbetter


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Posted 14 September 2017 - 05:13 PM

I take the question to be along the lines of, "is it really blue or is it whiter than the companion?" 


Color difference and perception is relative and varies not just with the color difference of the two stars, but also with magnification and overall brightness.  This is even apparent on the moon and planets naked eye and in the eyepiece. 


Often time the brighter star will be some form of red giant while the companion will be a bright white star still on the main sequence.  That color contrast will make the dimmer companion appear bluer than it would by its lonesome.  Albireo's B component often strikes me as almost purple at first glance, even though I know that hue is not really possible.


Even the Double-double shows this sort of contrast although reversed from the relationship seen with giants vs. main sequence.  Look for the dimmest component of the four stars.  It is not only dimmer by roughly a magnitude but it is an F0 spectral class vs. A3 for its partner.  If you look for it the color contrast is apparent with this component being more yellow than the others. 

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#6 3c_273


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Posted 16 September 2017 - 01:33 PM

I suspect that your seeing blue is a physiological effect of how the brain interprets input from your retinal rod cells.


I have noticed a similar effect when I'm well dark adapted, observing globular clusters. I saw many of the stars in them as being blue. Blue stragglers? No! It's just how my eye saw them!


#7 JimP


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Posted 17 September 2017 - 07:09 PM

Well I understand what you all are saying but, to me, if it looks blue, it's blue.
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#8 cildastun


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Posted 22 September 2017 - 04:54 PM

May be the Purkinje effect - certainly I get it with fainter stars if I compare how a star looks with my ED80 and then look with my 180 Mak.



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