Astronomers have discovered a binary star system with the shortest orbit it has ever seen.
The system is believed to be one of a rare class of binaries known as the “catastrophic variable”. These systems include a single star about the size of our Sun, orbiting a white dwarf – the hot, dense core of a burning star.
About half of the stars in the Milky Way are isolated, like our Sun. The rest appear in pairs or multiples, orbiting around each other. These orbits are often so narrow that some star systems can fit between the Earth and the Moon.
Cataclysmic binaries form when two stars come close to each other over billions of years. The most dense white dwarf begins to accumulate – consuming material from the other star. This process can produce massive, occasional flashes of light, which astronomers have assumed for centuries were caused by an unknown cataclysm.
The newly discovered system was named ZTF J1813 + 4251. The system is located about 3,000 light-years from Earth in the constellation Hercules. At 51 minutes, the orbit of the stellar pair is the shortest catastrophic variable orbit detected. The discovery was reported on temper nature.
When the stars eclipsed each other, astronomers were able to accurately measure the properties of each star.
Thanks to their calculations, astronomers were able to perform simulations of how the system evolved. They concluded that the stars would migrate much closer, in 70 million years orbiting each other every 18 minutes, before beginning to expand and drift apart.
Currently, the stars are in transition — the sun-like star has been stripped of much of its hydrogen envelope, eventually reducing its density to its helium-rich core.
“This is a rare case where we have fallen into one of these systems during the transition from hydrogen to accumulating helium,” says first author Dr. Kevin Bridge of the Massachusetts Institute of Technology. People expected these objects to travel to ultrashort orbits, and it was debated for a long time whether they could get short enough to emit detectable gravitational waves. This discovery relieves that. “
The new system was discovered within an extensive catalog of stars, observed by the Zwicky Transient Facility (ZTF) – a survey using a camera attached to a telescope at Palomar Observatory in California.
The survey took more than 1,000 images of each star from more than a billion stars in the sky, recording each star’s variable brightness over days, months and years.
Berridge combed the data for signs of extremely short orbits such as dramatic bursts of light and gravitational waves. “Gravity waves allow us to study the universe in a completely new way,” says Bridge.
“This thing showed up, where I saw an eclipse that happens every 51 minutes, and I said, ‘OK, that’s definitely a binary,'” Burridge recalls.
Using the WM Keck Observatory in Hawaii and Gran Telescopio Canarias in Spain, the team focused on the new duo. The “clean” images of stars passing over each other meant they were able to accurately measure their properties.
They found that the first object is likely to be a white dwarf, with a diameter of 1% of our Sun and half of its mass. The second object was a sun-like star towards the end of its life, the size and mass of a tenth of the sun (about the size of Jupiter).
But it shouldn’t be possible for a sun-like star to orbit this fast.
“This star is like the sun, but the sun can’t fit into an orbit shorter than eight hours – what’s up here?” asked Bridge? It was then that Page and his colleagues realized that this must be a catastrophic variable, confirming a decades-old theory.
When the sun-like star is stripped to its core, it remains heavy and dense enough to keep the dead star in a very short orbit.
“This is a proprietary system,” says Bridge. “We were doubly lucky to find a system that answers a big open question, one of the most beautifully behaved catastrophic variables known.”