Amateur observers helped reveal a rare eclipse of a close stellar pair known as b Persei by a third companion star.

b Persei
An animated model shows the orbits of the close stellar pair in the b Persei system.
Jason Sanborn

A team of professional and amateur astronomers has succeeded in observing a distant companion as it eclipsed the bright variable star b Persei (not to be confused with Beta Persei) in a rare event.

The b Persei system was already known as a binary, with two stars that orbit each other every 1.5 days. But a suspected third companion popped up during a survey of radio-emitting stars. The Navy Precision Optical Interferometer (NPOI) began to trace out the companion’s orbit, and what began as routine observations turned into a rare eclipse prediction: the distant companion was set to cross in front of the other two stars in February 2013, and then again in January 2015.

Astronomers Robert Zavala (United States Naval Observatory), Jason Sanborn (Lowell Observatory), and Donald Collins (Warren Wilson College) put together an observing plan to catch the eclipse as it happened. And to do that, the participation of amateur astronomers, especially the American Association of Variable Star Observers (AAVSO), would be essential.

The long orbital period of the third star — 702 days — makes eclipses rare in this system. (The mutual orbit of the closer pair faces Earth, so it doesn’t produce eclipses.) But the eclipse events themselves last a long time, since the third star takes three days to pass in front of the other two stars. So monitoring the whole event requires a worldwide network of observers. The predicted brightness changes are complicated by the fact that, even as the third star eclipses the closer binary, the binary itself will continue orbiting and changing brightness over the 3-day window.

Collins, an AAVSO member himself, detected a probable eclipse of b Persei in 2013. But though the few tantalizing data points revealed that an eclipse probably occurred, better time coverage during the next eclipse two years later would confirm the event and reveal the sizes and relative brightnesses of the three stars.

The stellar sizes, especially of the close pair, reveal the evolutionary state of the binary system — larger sizes suggest a later stage of evolution, in which the stars begin sharing material before eventually merging into a single star. Understanding the complex dynamics of multiple star systems could help us predict their long-term stability, a question of particular interest when considering habitability on exoplanets within multiple systems.

Collins’ 2013 observing campaign led to a secure prediction of the next eclipse in January 2015. Partnering with the AAVSO, his team put out a call for observing support and six amateurs in Europe and the United States responded. The amateurs recorded the brightness of the system over the critical three-day window, watching it fade and re-brighten multiple times as both the eclipse and the binary stars’ orbits changed the system’s brightness.

Watch the animation below to witness how the AAVSO’s observations captured the system’s orbital motions.

b Persei eclipse
The simulation above traces out the 2015 eclipse of b Persei, while the black line traces the simulation's progress as compared to the observed light curve. For more information, see the b Persei 2015 eclipse page.
Donald Collins

This work is still in its early stages — the team is working on combining information obtained from the light curve (both in and out of eclipse), interferometry, and spectroscopy to measure the stars’ sizes and evolutionary age.

Authors of this piece are Robert T. Zavala, astronomer at the U.S. Naval Observatory, Flagstaff Station, Donald F. Collins, Emeritus Professor of Physics at Warren Wilson College, and Jason J. Sanborn at Lowell Observatory and Northern Arizona University.


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