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Visible from the Earth, which binary star system has greatest angular distance ?

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    $\begingroup$ Naked-eye visible, or will you allow for one or both components to be below naked eye visibility? $\endgroup$ – James K Mar 8 at 18:14
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    $\begingroup$ First guess Proxima Centauri from Alpha A-B pair angular separation is 2.18 deg. Proxima requires telescope to see it's visually 11th mag. A_B components visible to eye but require small telescope to split. $\endgroup$ – TazAstroSpacial Mar 9 at 6:56
  • $\begingroup$ Alcor-Mizar prime components of multi-star system both naked eye but separated by 12 arcminutes a lot less than the Alpha A-B Proxima separation. $\endgroup$ – TazAstroSpacial Mar 9 at 6:57
  • $\begingroup$ @JamesK, both components can be below naked eye visibility. $\endgroup$ – user3715778 Mar 9 at 14:47
  • $\begingroup$ @TazAstroSpacial Stupid answer: The Milky Way and its two satellite galaxies are technically a giant, multibillion star system, with an angular separation of 20° $\endgroup$ – fasterthanlight Mar 11 at 17:21
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Wide binary stars it turns out is an active research topic, with groups using statistical searches of catalogs (Tycho Gaia ...) to find in 3D space pairs of stars that are moving together taking into account possible orbital motion. Here people are looking at the physical separation not the angular separation. On the theory side it is suggested that nearly all stars were formed as part of a binary. So somewhere out there is sister of our Sun. One paper looked at nearby stars. They find several very wide binary pairs visible to the eye plus more fainter. In particular they mention Capella and 50 Peg which have an angular separation of 20 deg. This they class as an escaped binary : so no longer in orbit (thanks Connor for pointing this out).

There is however Fomalhaut; a faint (mag 12.7) 3rd member of the system was discovered in 2013. It is 5.7 deg away from the primary. The other companion is 2deg away in another direction. The whole system spans 8 deg. The Sky and Telescope article claims this as the widest (on sky) binary, beating Alpha AB Proxima system.

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  • $\begingroup$ Granted: updated answer with Fomalhaut $\endgroup$ – TazAstroSpacial Mar 11 at 4:16

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