How far can a pair of stars be separated and still maintain a stable orbit around each other?

There are two issues here: First, how far apart can two stars form and remain bound? And second, once formed, how far apart can they survive as a pair? There are a number of ideas being debated for how very wide binaries can form. The Gaia satellite will, with its precision measurements, identify many thousands of ultrawide binaries that we can study, and their properties will help astronomers to determine the most likely formation mechanism behind them.

The second issue is better understood. If the two stars in a very wide binary were the only stars in the universe, the pairing could survive forever. But the universe is a busy place, and our Milky Way alone contains more than 100 billion stars moving around its center. A very wide binary has a very weak gravitational bond, so if another star passes near the binary, the pair can break apart. Eighty years ago, Armenian astronomer Victor Ambartsumian calculated that a wide binary rarely breaks apart as the result of a single close encounter with another star, but rather through numerous distant passages that each gently tug on the binary until it imperceptibly passes from being bound to being unbound.

For a very long time, the two stars will still travel together through space until eventually they part ways. An ultrawide binary with a separation of 0.5 parsec (1.6 light-years) is statistically likely to break up within just 100 million years, while a slightly less extreme binary with separation around 0.1 pc (0.3 light-years) may survive for more than 1 billion years.

In summary, there is no known fixed upper limit for binary separations, but the wider a binary is, the more difficult it will be to find.