J0651: Shedding energy due to gravitational waves (Image: NASA)

You might need three satellites plus some not-yet-invented equipment to detect gravitational waves directly, but it turns out you can see the effects of these ripples in spacetime with an optical telescope and a stopwatch.

A pair of white dwarf stars, known as J0651, which are only a third as far from each other as the Earth and moon, are creeping closer together because they’re shedding energy in the form of elusive gravitational waves.

Though gravitational wave emitters are fairly common, the unique thing about J0651 is the high power of the waves it produces combined with the fact that it emits visible light.


“An amateur astronomer with a 2-metre telescope could measure this effect,” says Warren Brown, an astronomer at the Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, and a member of the team that indirectly measured J0651’s gravitational waves.

Testing Einstein

According to Einstein’s general relativity – the leading theory of gravity – accelerating objects, like stars orbiting one another, should create gravitational waves. The heavier they are, and the faster they’re moving, the more powerful those waves. Crucially, this loss of energy causes the two objects to orbit closer together, an effect that can be detected.

Previously detected sources of gravitational waves include pairs of pulsing stars called pulsars – but these only emit radio waves and orbit each other slowly, on the order of once per hour, meaning the change in orbit due to gravitational waves is small.

“With pulsars, it’s only because you can measure them with insane accuracy that you can see these small shifts,” says Gijs Nelemans, an astrophysicist at Radboud University in Nijmegen, the Netherlands, who was not involved in the work.

By contrast, J0651’s two stars emit visible light and orbit each other once every 12.75 minutes, or 113 times a day. When Brown and colleagues compared the current orbit of J0651 to when it was first discovered a year ago, they found it was a quarter of a millisecond faster. That may not sound like much, but it amounts to a shift of six seconds in the timing of a specific eclipse compared with what you would expect if a system wasn’t sending out ripples in spacetime – a large enough gap to be detected by a common stopwatch.

The researchers also calculated that of all the known steady sources of gravitational waves, J0651 is the second most powerful. Only the binary system HM Cancri is more powerful, but it is less useful to study, says Brown, because the interaction between its two stars is made “messy” by the transfer of matter between them. This makes it harder to figure out what factors are contributing to the loss of energy. J0651 is rare because there is no exchange of material, despite the two stars’ relative proximity to one another.

Cosmic blink

“We’ve seen this with pulsars, but it’s very nice to see this here,” says Nelemans. “Optical has such a long tradition that as a community we know very well how to use optical instrumentation.”

The final confirmation of gravitational waves’ existence will come from detecting them directly. One promising planned experiment is eLISA, a set of three spacecrafts orbiting the sun that will detect slight movements in onboard, shielded objects that are buffeted only by the gravitational waves emitted by systems like J0651.

Indeed, the discovery of this system, due to be published in The Astrophysical Journal Letters, will be a great target for eLISA, which should start taking data in about 2025, says Brown’s colleague J. J. Hermes from the University of Texas, Austin.

“We need to know where to look to be able to say we see something above the noise,” he says. “It’s like looking for a needle in a hay stack, and now we’ve got GPS on the needle.”

Nelemans points out that the team was very lucky to find the system since it is relatively close to the end of its life: within 2 million years the stars will merge – a mere blink in cosmic timescales.

Reference: arxiv.org/abs/1208.5051