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As it is obvious from the question title is there any restriction or cases that we can see the binary stars as not in pairs

also is there any binary stars doesn't linked together by gravity ?

I know the definition of binary stars that it is a pair of stars that revolve around their common center of mass but my question is about special cases if it is exist

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  • $\begingroup$ According to Wikipedia, pairs of stars that appear close together in the sky but are not gravitationally bound are called "optical doubles". The two stars might be many light-years from each other but happen, by coincidence, to be on nearly the same line of sight as seen from Earth. $\endgroup$ – Keith Thompson Nov 25 '14 at 20:01
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To answer the main part of your question: Yes, there do exist such systems. They're called visual binaries. We generally need a telescope to tell them apart. Most binary systems look like a single star when viewed with only the naked eye; many cannot be resolved without the aid of a telescope. But visual binaries can.

also is there any binary stars doesn't linked together by gravity

No, there aren't. You were right when you said that the definition of a binary system is basically two stars orbiting a common center of mass. If the stars don't orbit each other, they aren't a binary system.

By the way, check out Proxima Centauri, the nearest star to the Sun. It's one of the three stars in the triple-star Alpha Centauri system. The reason I mention it is that even though it's seemed for years like it was gravitationally bound to the system, that idea is now under debate.


Let me address something that Mitch pointed out. The classification of star systems as "visual binaries" is based solely on our ability to observe them. If we had better telescope, this class could change. An analogy might be our naming certain wavelengths of light "visible". They only depend on our perceptions, not some objective characteristic that all observers in the universe could agree on.

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    $\begingroup$ Looks like I misinterpreted the first part! It may be notable that this is a classification based on our ability to determine that a system is binary. There are other classifications such as spectroscopic binaries, eclipsing binaries, astrometric binaries, photometric binaries, and potentially more. These of course define is an objective classification, as it's related to our methods of observing the subject to determine that it is a binary. $\endgroup$ – Mitch Goshorn Nov 24 '14 at 22:32
  • $\begingroup$ @MitchGoshorn I'm not entirely sure I interpreted the first part correctly! But that's a good point that our classification is based on our limitations, and not the physical properties of the system. $\endgroup$ – HDE 226868 Nov 24 '14 at 22:34
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As you've said, the definition of binary star systems would mean that the cases you are describing would not be considered binary star systems.

That said, there are other arrangements which exceed binary, but are often thought of as including binary relationships. This is due to star systems which reach beyond binary often resulting in hierarchical arrangements. For instance, many triple star systems involve what is largely a close binary star system with a third star that orbits the pair at a much greater distance. For an extreme example of this, consider Castor, which is a sextuple star system containing three binary star pairs.

These systems wouldn't technically be said to be binary star systems, but rather to contain binary pairs, but given the question I can't think of a better way of having a binary star system that isn't a binary star system.

Note that the hierarchical nature of super-binary systems is a matter of system evolution. Systems which would not be considered hierarchical are believed to be unstable, which would eventually result in the ejection of a star from the system.

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  • $\begingroup$ I can make my question more clear $\endgroup$ – Mohamed IBrahim Nov 24 '14 at 21:17
  • $\begingroup$ double stars or binaries, that are connected by gravity, if we look at them by telescope, can we see any pair of them as distinct pair of stars? $\endgroup$ – Mohamed IBrahim Nov 24 '14 at 21:17
  • $\begingroup$ This is just as @HDE stated in his answer. In many cases we can't, however there is a classification of stars we can, which are called visual binaries. $\endgroup$ – Mitch Goshorn Nov 24 '14 at 22:34
  • $\begingroup$ Alcor and Mizar in the big dipper is likely the best known visual binary:en.wikipedia.org/wiki/Mizar_and_Alcor As stated in link, it's recently been shown the two stars are gravitationally bound, but that's not the case for all historically known visual binaries. $\endgroup$ – Wayfaring Stranger Nov 25 '14 at 16:00

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