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Study of stars

Study Space / May 4, 2017

Matters of the heart can be puzzling and mysterious - so too with unusual astronomical objects called heartbeat stars.

Heartbeat stars, discovered in large numbers by NASA's Kepler space telescope, are binary stars (systems of two stars orbiting each other) that got their name because if you were to map out their brightness over time, the result would look like an electrocardiogram, a graph of the electrical activity of the heart. Scientists are interested in them because they are binary systems in elongated elliptical orbits. This makes them natural laboratories for studying the gravitational effects of stars on each other.

In a heartbeat star system, the distance between the two stars varies drastically as they orbit each other. Heartbeat stars can get as close as a few stellar radii to each other, and as far as 10 times that distance during the course of one orbit.

At the point of their closest encounter, the stars' mutual gravitational pull causes them to become slightly ellipsoidal in shape, which is one of the reasons their light is so variable. This is the same type of "tidal force" that causes ocean tides on Earth. By studying heartbeat stars, astronomers can gain a better understanding of how this phenomenon works for different kinds of stars.

Tidal forces also cause heartbeat stars to vibrate or "ring" - in other words, the diameters of the stars rapidly fluctuate as they orbit each other. This effect is most noticeable at the point of closest approach.

"You can think about the stars as bells, and once every orbital revolution, when the stars reach their closest approach, it's as if they hit each other with a hammer, " said Avi Shporer, NASA Sagan postdoctoral fellow at NASA's Jet Propulsion Laboratory, Pasadena, California, and lead author of a recent study on heartbeat stars. "One or both stars vibrate throughout their orbits, and when they get nearer to each other, it's as though they are ringing very loudly."

Kepler, now in its K2 Mission, discovered large numbers of heartbeat stars just in the last several years. A 2011 study discussed a star called KOI-54 that shows an increase in brightness every 41.8 days. In 2012, a subsequent study characterized 17 additional objects in the Kepler data and dubbed them "heartbeat stars." To characterize these unique systems, further data and research were required.

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Elizabeth Landau
Jet Propulsion Laboratory, Pasadena, Calif.