Astronomers spot a pair of stars with shortest orbit detected to date

Astronomers have discovered a pair of stars with such a short orbit they appear to circle each other in just 51 minutes.

The system seems to be one of a rare class of binaries known as a ‘cataclysmic variable,’ in which a star similar to our sun orbits tightly around a white dwarf — a hot, dense core of a burned-out star. 

A cataclysmic variable occurs when the two stars draw close, over billions of years, causing the white dwarf to start accreting, or eating material away from its partner star. 

This process can give off enormous, variable flashes of light that, centuries ago, astronomers assumed to be a result of some unknown cataclysm. 

The ‘cataclysmic’ system, which resides about 3,000 light years from Earth in the Hercules constellation, has the shortest orbit detected to date of its type.

It was discovered by astronomers at MIT and has been named ZTF J1813+4251.

Unlike other such systems observed in the past, experts caught this cataclysmic variable as the stars eclipsed each other multiple times, allowing the team to precisely measure the properties of each star.

They then ran simulations of what the system is likely doing today and how it should evolve over the next hundreds of millions of years. 

This led the researchers to theorise that the stars are currently in transition, and that the sun-like star has been circling and ‘donating’ much of its hydrogen atmosphere to the voracious white dwarf. 

As time goes on, the astronomers say the sun-like star will eventually be stripped down to a mostly dense, helium-rich core. 

In another 70 million years, the stars will then migrate even closer together, with an ultrashort orbit reaching just 18 minutes, before they begin to expand and drift apart.

Decades ago, researchers at MIT and elsewhere predicted that such cataclysmic variables should transition to ultrashort orbits — but is the first time such a transitioning system has been directly observed.

‘This is a rare case where we caught one of these systems in the act of switching from hydrogen to helium accretion,’ said Kevin Burdge, from MIT’s Department of Physics. 

‘People predicted these objects should transition to ultrashort orbits, and it was debated for a long time whether they could get short enough to emit detectable gravitational waves. This discovery puts that to rest.’​

Burdge and colleagues discovered the new system within a vast catalogue of stars, observed by the Zwicky Transient Facility (ZTF), a survey that uses a camera attached to a telescope at the Palomar Observatory in California to take high-resolution pictures of wide swaths of the sky.

The survey has taken more than 1,000 images of each of the more than 1 billion stars in the sky, recording each star’s changing brightness over days, months, and years.

Burdge combed through the catalogue, looking for signals of systems with ultrashort orbits, the dynamics of which can be so extreme that they should give off dramatic bursts of light and emit gravitational waves.

These appeared to flash repeatedly, with a period of less than an hour — a frequency that typically signals a system of at least two closely orbiting objects, with one crossing the other and briefly blocking its light.

The discovery was made by the Zwicky Transient Facility (ZTF), which operates at Caltech’s Palomar Observatory, with the help of the W. M. Keck Observatory in Hawaii (pictured)

Burdge used an algorithm to weed through over 1 billion stars, each of which was recorded in more than 1,000 images, an ultimately zeroed in on ZTF J1813+4251.

‘This thing popped up, where I saw an eclipse happening every 51 minutes, and I said, ok, this is definitely a binary,’ Burdge said.

He and his colleagues further focused on the system using the W.M. Keck Observatory in Hawaii and the Gran Telescopio Canarias in Spain. 

They found that the first object was likely a white dwarf, at 1/100th the size of the sun and about half its mass. 

The second was a sun-like star near the end of its life, at a tenth the size and mass of the sun (about the size of Jupiter).

However, something didn’t quite add up.

‘This one star looked like the sun, but the sun can’t fit into an orbit shorter than eight hours — what’s up here?’ Burdge said.

He realised that ZTF J1813+4251 was likely a cataclysmic variable — a discovery that means it is the shortest orbit cataclysmic variable detected to date.

‘This is a special system,’ Burdge added. ‘We got doubly lucky to find a system that answers a big open question, and is one of the most beautifully behaved cataclysmic variables known.’

The discovery has been published in the journal Nature.