Space Oddities: Heartbeat Stars

heartbeat stars
An artist's concept of two "heartbeat" stars in mutual orbit, with a chart of their brightness variations overlaid. The chart looks something like an electrocardiogram. NASA

Astronomers use an unusual kind of binary star called a "heartbeat" star to study the gravitational effects stars have on each other. These binaries got the "heartbeat" name because of the way they vary in their brightness. Binary stars themselves are simply systems with two stars orbiting each other (or to be technical, they orbit a common center of gravity).

Astronomers measure the luminosity (brightness) of a star over time to create a chart (called a "light curve"). Such measurements tell a lot about a star's characteristics. In the case of heartbeat stars, these look like an electrocardiogram. (That's the chart a doctor uses to measure the electrical activity of a patient's heart.)

It's All in the Orbit

What's so different about these binaries? Their orbits, unlike some binary orbits, are very elongated and elliptical (egg-shaped). As they orbit one another, their distances can be very small or very large. In some systems, the stars get very close to each other. Astronomers suggest that the shortest distance could be only a few times the actual width of a star. That would be analogous to the distance between the Sun and Mercury. At other times, when they're farthest apart, they could be ten times or more that distance.

Those changing distances also force changes in the shapes of the stars. At the closest, their mutual gravitational makes each star ellipsoidal (egg shaped). Then, as they pull apart, their shapes relax back to being more spherical. The mutual gravitational pull (called a tidal force) also makes the stars vibrate a bit in size. Their diameters get slightly smaller and larger very quickly. It's almost like they're fluttering, particularly as they get the closest to each other.

Astronomer Avi Shporer, who works at NASA's Jet Propulsion Laboratory, studied these stars, and in particular their "vibrating" tendency. "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," he said." 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."

Gravitational Changes Affect Brightness

The gravitational changes affect the brightness of the stars. At some points in their orbits, they're brighter due to the change of gravitational pull than at other times. This variability can be traced directly to the variation in gravity each star puts on the other. As these brightness changes are charted, the graphs show the typical "electrocardiogram" type of changes. That's why they're called "heartbeat" stars.

How Were These Found?

The Kepler Mission, which was sent to space to look for exoplanets, has also found many variable stars. It also discovered many of these heartbeat stars. After a number of them were found, astronomers turned to ground-based telescopes to follow up with more detailed observations. Some results show that the typical heartbeat star is hotter and bigger than the Sun. There may be others at different temperatures and sizes, and further observations should uncover them if they exist.

Still Some Mystery to These Stars

In some ways, the fact that heartbeat stars exist is still something of a mystery. That’s because gravitational influences usually cause orbits of objects to become more circular over time. That hasn’t happened with the stars studied so far. So, is there something else involved?

It’s possible that these systems each could have a third star involved. Its gravitational pull would also contribute to the elliptical orbits that showed up in the Kepler and ground-based studies. No third stars have been seen yet, which means that they could be much smaller or dimmer. If so, the observers will have to search harder for them. Follow-up studies should help determine if third-party contributions to heartbeat stars’ orbits are a reality. If so, what role do they play in the variations in brightness of the more-luminous members of their systems?

These are questions that future observations will help answer. Kepler 2 is still at work uncovering these stars, and there are plenty of ground-based observatories to do the important follow-up observations. There could be more interesting news about heartbeat stars as the studies progress.