Two incredibly fast-orbiting stars seem to be the wrong temperature

Round and round the two stars go. Whether they’ll stop, nobody knows. Two white dwarf stars are twirling around one another at an extraordinarily fast rate, and we don’t know whether they will eventually smash together or just keep orbiting.

Kevin Burdge at the California Institute of Technology and his colleagues sifted through about 20 million data sets from the Zwicky Transient Facility, a sky survey that watches for objects with rapid changes in their brightness, to look for signals from pairs of white dwarf stars. They found one binary, called ZTF J153932.16 + 502738.8, that was particularly interesting.

Further observations revealed that there was a deep dip in the light from the system every 6.91 minutes. In a binary star system, this type of dip is caused when the cooler star passes between the hotter one and our telescopes, blocking out its light in an eclipse. This means that the two stars take less than 7 minutes to circle around one another.

“There have been other ultra-short orbital period binaries found before, but the signal was so low that we were never really sure that they were what we thought,” says Burdge. “This one is eclipsing, so it’s the first unambiguous detection of a system under 10-minute period.”

The two-star system could fit within the diameter of Saturn. The two stars are strange: the less massive one is colder than we’d expect, and the more massive one is far too hot at more than 48,000°C. Burdge and his colleagues plan to use the Hubble Space Telescope to investigate why.

It isn’t clear what the future holds for these two objects: they might smash together and merge in about 130,000 years, or one might start stealing matter from the other, slowing down their orbit and pushing them farther apart.

But we may not have to wait that long to get a hint of what will happen. “I think the game is, instead of waiting on this one for 130,000 years, you find a lot of them and chances are you’ll eventually find one that’s right on the edge of merging,” says Burge. “If you don’t find one, maybe that’s how you find out that pairs like this don’t actually merge.” This sort of binary should be easy to spot with the Laser Interferometer Space Antenna (LISA), a space-based gravitational wave detector expected to launch in 2034, he says.