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How large? How far? How long?

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#1 equuleus


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Posted 17 October 2013 - 12:29 AM

I would enjoy observing doubles if there were a database that told me the relative sizes of the stars and how far apart they were and how long it took for an orbit, etc. Is there information like this, in minimal technical terms, for several or more doubles (multiples)? The WDS and similar lists seem a bit overwhelming. My interest is not so much in what I can split or measure but just to marvel at the scale of what I'm seeing.




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Posted 17 October 2013 - 02:52 AM

A relative size of stars (means diameter of spurious disk) is given nowhere as far as I know and depends anyway on the size of the used aperture (you don't see the star through the scope but only the by the star produced diffraction pattern).
The book from Sissy Haas on doubles for small telescopes may be a good starting point for beginning to observe doubles.

#3 Asbytec


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Posted 17 October 2013 - 04:42 AM

If you mean relative size as in the actual diameter, say relative to the sun, I am not aware of any sites that post that data. You may be able to derive some idea from the spectral class and plotting it on the HR diagram - from dwarfs to super giants.

Orbital data is usually measured in arc seconds as seen from Earth. You could derive some distance from the orbital data if you knew the distance to the star. That is often given.

In short, I don't know of any sites that give a 'real' feel for the double in question, but some data is available to figure out, "the scale of what you're seeing."

#4 Cotts


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Posted 17 October 2013 - 09:43 AM

Sky Safari gives data such as below (Gamma Equulei, just for you...) for virtually every star in its database. For many of the more 'popular' doubles it gives orbit info in practical terms - how long the orbit is and how many A.U. the stars are apart, etc.

Another source of orbit info in practical terms is the the three-volume "Burnham's Celestial Handbook which should be in every amateur's library, even if a lot of the data is out of date.


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#5 Ed Wiley

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Posted 17 October 2013 - 10:02 AM

David has it right. Other programs also have info, but I am impressed with the depth in Sky Safari. And, you can use it at the scope.

Unfortunately, if you want to know more than what is available in the "popular" literature and software you must tackle the complexities of the resources at the Naval Observatory and the databases of the CDS available via VizierR.


#6 drollere


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Posted 05 November 2013 - 01:42 AM

you can start with this link, just to get a sense of the scale we are talking about with double stars ...


not sure where your interest lies. the size (radius) of a giant star can be observed in a few dozen cases, and measured in a few hundred cases of eclipsing binary stars; the size (mass) can be inferred from the spectral type. the smallest star is about 8% the mass of the sun and the largest is thought to be be somewhere around 150% the mass of the sun.

the typical binary orbit is about 40 astronomical units, and one revolution takes about 250 years; but the range in the orbit dimensions is huge, from two stars that rotate under a single, peanut shaped photosphere to orbits approaching half a light year wide and millions of years to complete.

the typical (40 AU) orbit is too small to resolve or measure visually once the system is more than about 100 parsecs away. most of the double stars observed in amateur telescopes are much farther apart, on the order of hundreds or thousands of AUs.

nearly all the systems in my "ShortWDS", linked at my site, include the system distance, minimum orbit radius, the log period in years, the mass ratio, the total number of components; for about half the entries, the parallaxes are from hipparcos. most of that info you can't get from the off the shelf software.

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