Triple Stars in Far-Flung Relationships

Astronomers have wondered for nearly a century why some binary stars seem to orbit each other at impossibly large distances. New simulations might reveal the underlying cause: they're often triples in disguise.

Last month, while chasing the Moon's shadow in the South Pacific, I had a few nights to lead some stargazing sessions. Among the southern splendors on display was Alpha Centauri, which, I told my group, was the closest star in the night sky. Technically, it was a little white lie. The Alpha Centauri that gleams so brightly in the southern sky, actually a tight double star, lies 4.37 light-years from us. But this duo has faint, 11th-magnitude companion, Proxima Centauri, that's actually a bit closer, 4.21 light-years.

Proxima has been a puzzle ever since its discovery in 1915. It's very far from Alpha Centauri A and B, about 15,000 times farther than the Earth-Sun distance or roughly 0.25 light-year. Astronomers don't think it formed so far away — no single clump of condensing interstellar gas would likely have been so large. Perhaps Proxima was captured as a cluster of newly formed stars started to disperse — just as the Pleiades are doing today.

But Proxima is hardly a rare exception. In recent years observers have found that about 10% of such ultrawide binaries are in fact triple-star systems, typically anchored by a tightly bound pair of relative massive stars and a lower-mass outlier (often an M dwarf, like Proxima) quite far away.

Bo Reipurth (University of Hawaii) and Seppo Mikkola (University of Turku, Finland) think they know what's going on. They ran computer simulations that followed the evolution of 180,000 hypothetical triple stars. These triplets form close together in dense, localized "cores" within larger interstellar clouds in which a great many stars are coming together. Within 100 million years, 90% of these trios become gravitationally unstable (usually through close encounters with their neighbors) and get flung apart. But about 10% stay together — matching their abundance in the real universe.

The key to survival seems to be establishing a hierarchical pecking order: the triplets interact in a way that brings two members closer together while pushing the third ever farther away. The pair whirling in close proximity can appear to be a single star telescopically unless examined carefully. As Reipurth and Mikkola conclude in the December 5th issue of Nature, their simulations imply that the widest binaries should actually have three stars, not just two.

However, some ultrawide binaries really are just twosomes. In these cases, Reipurth suggests, initially close pairs have merged into a single star. That can happen if there's enough gas remaining in the cloud core to create orbital drag and cause them to spiral into each other. Or perhaps there's some other cause that awaits discovery.

Observers can test Reipurth and Mikkola's outcome by seeing if the distant outcasts in each ultrawide system do in fact have the lowest masses, or by confirming that all three stars in a hierarchical triple have the same age. Another test would be to determine whether all this chaotic rearrangement can still allow planets to form. Just two months ago, a team of European observers announced that there's an Earth-mass planet orbiting Alpha Centauri B.

I'm headed to Costa Rica in March to get another southern-sky stargazing fix, and I hope to get my hands on a scope large enough (at least 8 inches) to resolve Proxima Centauri. But, successful in that quest or not, I've got newfound respect for this well-known stellar survivor.