New observations solve longstanding mystery of tipped stars

Results: MIT researchers and colleagues have solved a longstanding mystery about a pair of stars called DI Herculis whose peculiar rotation (a shift in their orbit that was four times slower than expected) had remained a mystery for three decades. The real enation is simple: The stars are rotating tipped over on their sides, relative to their orbits around each other. This produces tidal effects that counteract the expected rate for the orbits to shift orientation over time (called precession), finally explaining the mysterious anomaly.

Why it matters: The discrepancy in the rate of precession had been seen as a possible refutation of Einstein’s theory of relativity, so finding a conventional explanation means that relativity has withstood another possible challenge. This discovery could also help to shed light on how binary stars (about half of all known stars) are formed and how their rotation and orbits evolve over time.

How they did it: Postdoctoral researcher Simon Albrecht and assistant professor of physics Joshua Winn and others used a high-resolution spectrograph called Sophie on a 1.93-meter telescope at the Observatoire de Haute-Provence in France to make highly detailed observations that revealed the unexpected tilt — one of more than 70 degrees from vertical, the other more than 80 degrees — of the stars’ rotation axes.

Next steps: The team hopes to study other unusual binary stars to try to determine how unusual this tipped-over configuration is.

Funding: The research was partly supported by grants from NASA’s Origins program, the Optical Infrared Coordination network, and a Rubicon fellowship from the Netherlands Organisation for Scientific Research.

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