What is the largest distance between stars in a binary star system?

Off hand, I don't know the answer to this. It would depend on where the star is located.

If it is in a dense star cluster where the mean stellar separations are a light year or less, then this distance would have to be, perhaps, 10 billion miles ( 100 AU) or less otherwise the stars would be so loosely bound together that a passing star could disrupt the binary system. In the vicinity of the Sun, I think the largest separations between stars in identifiable binary systems runs to something like 100 billion miles or 1000 Astronomical Units; some 25 times the distance to Pluto. In terms of observing binary stars with separations more than a few billion miles, they move so little in their orbits in a few decades that they can be easily overlooked. I believe many of the most common binary stars have orbits less than 50 years. There are about 70,000 binary stars known with several such as Castor with periods of 420 years, and 61 Cygni with a period of 720 years. The ones with the longest periods are also the closest so that their motion can be detectable. Here is a short list of some of the widest binaries I have come across:

Name....................Separation.........Period........Distance
+43 1326               44      156 AU      3000           11.5
Delta Geminorum         6.8    111 AU      1200           53
+50 1725               19       86 AU      1000           14.8
61 Cygni               25       85 AU       700           11.0
Eta Cass               11.9"    70 AU       478 yrs       19 LY
Castor                  5.8"    81 AU       349           45
40 Eridani BC           6.8     33 AU       247           15.8
Gamma Virginis          3.7     42 AU       171           36
Xi Bootes               4.9     32 AU       149           21  
.................................................................

A recent search for wide binaries in the Gleise Catalog of nearby stars by Arcadio Poveda at the Institute of Astronomy in Mexico revealed that of the 118 nearby binary stars, 33 had separations greater than 500 AU, and 14 had separations greater than 2000 AU.


Copyright 1997 Dr. Sten Odenwald
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