​Gravitational Waves Can Play Stars Like an Instrument

The effect, to quote astronomer Barry McKernan, is like “a piano on steroids.”

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Sep 23 2014, 7:40pm

Images: USGOV/NASA.

Astronomers have proposed a novel speculative method for detecting gravitational waves—watching them "play" stars like the keys of an instrument. This phenomenon was originally predicted by Albert Einstein in his 1916 general theory of relativity, but has been overlooked in the decades since. Now, astronomers based out of the American Museum of Natural History have revived Einstein's idea, publishing their findings yesterday in The Monthly Notices of The Royal Astronomical Society.

"Basically, Einstein's theory allows stars to absorb energy from gravitational waves that hit them," co-author Barry McKernan told me. "This means two things: one, the star can brighten, i.e. look brighter than it normally should. Two, stars like our Sun can eclipse sources of gravitational waves."

Finding new ways to hunt for gravitational waves is essential, because these ripples in spacetime are notoriously difficult to pin down (as evidenced by yesterday's report about BICEP2). They are produced by extreme cosmic events, like supernovae, the merging of black holes, or even the Big Bang itself. But though they are forged by seismic disturbances, gravitational waves interact weakly with matter, which makes detecting and interpreting their behavior an ongoing challenge for astronomers.

But according to McKernan, the answer may be to simply follow the trail of brightened stars. You will be tracking a gravitational wave of a certain frequency pass through stars that oscillate at a matching frequency, which produces a burst of energy and luminosity.

If you're close to the merger, it's like a piano on steroids!

"A stringed instrument might be a good analogy," explained McKernan. "Each string has its own natural frequencies. If you place a violin near a piano, and strike the correct key on the piano, you can cause a string on the violin to resonate. The merging binary is like a piano striking the keys in sequence from low to high frequency—each time the frequency matches one of the violin strings, it will vibrate in sympathy."

"If you're close to the merger, it's like a piano on steroids!" he added, using arguably the best metaphor ever. "If the star is close to the source of the gravitational waves, the energy transfer can be quite spectacular. Giant stars could have their outer layers blown off them and maybe onto the gravitational wave source."

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Merging black holes produce strong gravitational waves. Image: NASA.

So, a source of gravitational waves—merging black holes, for example—would initially brighten surrounding large stars capable of harvesting the higher frequencies. But as the source dies off, smaller stars will begin to light up, reflecting the ebbing of the gravitational tide. That's some next-level cosmic harmony, folks, worthy of a Cosmos episode.

"The notion that you could drive [star] populations off the main sequence of the [Hertzsprung-Russell] diagram in order of decreasing mass is pretty cool," said McKernan.

But there's much more work ahead before this theoretical spacetime symphony will mature into a proven method for identifying gravitational waves. Having resurrected the theory from Einstein's opus, the AMNH team will now have to back it up with more observational data. The researchers plan to start by studying stars that are most likely to catch a passing wave.

"Some stars are better than others for this effect," McKernan told me. "Probably the best targets for this sort of behavior are small fully convective stars, which can't trap the energy for very long. So we'd look for a whole population of low mass stars in a galactic nucleus looking 'brighter than normal'."

McKernan also suspects that giant stars hit by waves will shed their outer layers, perhaps feeding the gravitational wave source, and pulsating stars might be "pumped off their period-luminosity relation, in the direction of greater luminosity."

It goes to show that there are still gems to be found in Einstein's general theory of relativity, nearly a full century after he first published it.