In the dense star cluster NGC 6791, which is older than the Sun and rich in heavy elements, the light from many stars blends into a common glow. Astronomers set out to disentangle this light into individual stars using data from the Gaia space telescope. They noticed that on the diagram where stars are plotted by color and brightness, the main-sequence band (where stars spend most of their lives) is wider than expected for single stars.
To explain this width, the scientists compared two hypotheses: stellar rotation and unseen companions. Rotation can slightly alter brightness, but a Bayesian analysis showed that it does not fit. Instead, the presence of a close companion around many stars, which adds its own light and shifts the star on the diagram, explains everything perfectly. Such pairs are called unresolved because the telescope sees them as a single point.
It turned out that the fraction of such pairs with nearly equal masses (when the secondary is heavier than half the primary's mass) is 54.3% with an uncertainty of 2.8%. This is noticeably higher than in other open clusters and in the Galactic field. Moreover, the mass ratio distribution is not uniform: pairs with similar masses are more common, as if low-mass companions were lost over time while tight pairs survived.
In addition, some stars lie above the line corresponding to a pair of two identical stars. Their light is too bright for a binary system. These are likely triple or even more complex systems, where three or more stars orbit together but merge into a single point for us.
Interestingly, the radial distribution of single and multiple stars within the cluster does not differ. This means that heavy binaries have neither sunk to the center nor been ejected outward, but are uniformly mixed. Such a picture is typical for a cluster that was once much more massive but lost its outer layers and remained as a dense core where the strongest pairs survived.
The cluster NGC 6791 is so old (about 8 billion years) that its stars resemble the Sun, yet the cluster itself has survived many passages through the Galactic disk that should have disrupted it.
In Isaac Asimov's novel 'The Gods Themselves', a civilization is described in a world with three stars, where complex orbits dictate all life — similar to the triple systems possibly found in NGC 6791.