My first Magazine Sky & Telescope - 04.2019

dim, 11th-magnitude star lying some 2° from Alpha Centauri had the same proper motion as the binary star across the sky. Shortly after, Dutch astronomer Joan Voûte calculated this star’s parallax and distance, showing that it was moving in the same direction and at the same speed as Alpha Centauri and was equally close to Earth. This was the ﬁ rst indication that the three stars formed a triple system. This star, Alpha Centauri C, is actually the closest star to the Sun. It’s a red dwarf with 0.12 solar mass, describing a 550,000-year orbit around AB at a distance of 13,000 a.u., or one-ﬁ fth of a light-year. Currently it’s on the Earth-side of its orbit, placing it closer to us than AB and earning it its com- mon name: Proxima Centauri. Historically, astronomers have questioned if Proxima is really orbiting Alpha Centauri or is just a passerby. In 2017, Pierre Kervella (Paris Observatory, France) and others pro- vided the ﬁ rst solid evidence of the trio’s gravitational bond by measuring Proxima’s speed relative to AB. The team used the planet-hunting instrument HARPS, installed on the 3.6-meter telescope at La Silla Observatory in Chile. HARPS depends on the Doppler effect to gauge the radial velocity of stars — their speeds towards or away from Earth — to reveal the gravitational tug that orbiting planets exert on them. But what HARPS ultimately does is pre- cisely measure stellar movements recorded in a star’s light. Kervella used these measurements to calculate Proxima’s velocity relative to Alpha Centauri rather than its movement relative to Earth. “This is much more difﬁ cult to measure because the spectrum of Proxima, which is used to mea- sure the Doppler effect, is full of molecular lines all mixed together, which makes it impossible to measure an absolute radial velocity,” Kervella says. Kervella and his colleagues worked around this problem by measuring the radial velocity of Proxima’s extended atmo- sphere, a layer of hot gas that surrounds the star. With this approach, they showed that Proxima’s velocity relative to AB is below the system’s escape velocity by a signiﬁ cant factor. But that doesn’t mean that Proxima has always been part of the Alpha Centauri system. It could have formed along with A and B from the same primeval cloud of gas and dust, or, maybe, it formed elsewhere and was later captured by the binary star. Double and triple stars are common in the universe. Perhaps half of all known Sun-like stars have companions, although astronomers aren’t quite sure why. Recent research suggests that all stars may be born in orbiting pairs from dense, egg-shaped cores found within cosmic gas clouds. Around 60% of them move farther apart until they eventu- ally split up, while the rest shrink their orbits instead to form tight binaries. Most researchers think that Alpha Centauri and Proxima formed together. “These kinds of hierarchical triple systems or multiple systems are really quite common,” Ayres says. One example, he points out, is the Capella system, made of four stars in two binaries (two Sun-like, two red dwarfs), separated by 10,000 a.u. “It’s just the way the stars form, the smaller stuff gets thrown in wider orbits.” But Proxima could have been captured by the binary after a close encounter, says Fabo Feng (Univer- sity of Hertfordshire, UK). Feng has created simulations that account for the gravitational inﬂ uence of the Milky Way and close encoun- ters with other stars, which show that the system’s current conﬁ gu- ration might be unstable for its age. That points to a capture scenario for Proxima. Kervella thinks that if Proxima was captured, it had to be very soon after the three stars formed in close quarters. Otherwise their relative speeds would have been too high to allow the capture. “It’s very difﬁ cult for an old system to cap- ture a passing star,” Kervella says. “This is why we think they might have formed at the same time from the same cluster.” Betelgeuse D = 887 q Sun D = 1 Alpha Centauri A D = 1.2 Alpha Centauri B D = 0.86 Proxima Centauri D = 0.15 u OUR STELLAR CAST In our three- part series we discuss Polaris, the Alpha Centauri system, and Betelgeuse. These stars span a range of sizes, shown to scale. Listed diameters (“D”) are approxi- mate and given as multiples of the Sun’s diameter. We’ve included the Sun here for visual reference. Polaris D = 45 Most researchers think that Alpha Centauri and Proxima formed together. But Proxima could have been captured by the binary after a close encounter. sk yandtele scope.com • A PR I L 2 019 37

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