Wide binary stars may have started out as triplets

The old adage, ‘two’s company three’s a crowd’ could also be true for wide binary star systems. 

Astronomers investigating wide binary systems believe that a third star may have been present when they formed, explaining a long held mystery about how the relationship between the two stars form at such great distances. 

Most stars in the universe are part of a binary star system, in which two stars are linked and orbit around a common centre of mass. Wide binary star systems, stars that are separated by long distances, are also common in the Milky Way. For example, our nearest stellar neighbour, ‘Proxima Centauri’, is partnered to our next nearest neighbour  ‘α Centauri’ by a distance of 0.1 parsecs; about 15,000 times the Earth–Sun separation or about 500 times the size of our Solar System. 

However how such long-distance relationships between these stars form has long baffled astronomers because the physics suggests that binary-star orbits should be small enough to fit within the protostellar ‘cloud cores’ in which stars form. In the case of wide binary star systems, the connecting distances between the stars far exceeds the size of their cloud cores.

Bo Reipurth and Seppo Mikkola of the University of Hawaii, Manoa, conducted a series of 180,000 simulations that looked at the formation of binary star systems which started out as triplet stars. In a paper published in Nature this week, they found that around 10% of these triplets go onto form wide binary systems, which is in keeping with observations of these systems out in space. 

Reipurth and Mikkola believe that early on in the stellar cloud core, two of the stars form a tight pair, at the expense of the third which is cast out on an increasingly distant orbit. These hierarchical triplets are therefore what astronomers see as wide binaries systems, the tight pair looks like a single star due to the limited imaging resolution of our telescopes.

“This has been a long-time mystery about these very wide binaries,” Bo Reipurth, told the site SPACE.com.

Astronomers are already working on testing the triplet hypothesis on ultra-wide star systems. So far, one high resolution imaging study has found that as many as 30% of such wide binary systems were indeed triplets.