Astronomers baffled by bizarre ‘zombie star’ that shouldn’t exist

A collapsed star around 13,000 light years away is so unusual that the researchers who have discovered it say it shouldn’t exist.

It was first detected in January 2024 by the ASKAP radio telescope in Western Australia and is likely to be a kind of pulsar that has never been seen before.

When supermassive stars reach the end of their lives and explode in a supernova, the remnants form a super-dense object called a neutron star. Pulsars are neutron stars that spin rapidly, emitting radio waves from their magnetic poles as they rotate. Most pulsars spin at speeds of more than one revolution per second and we receive a pulse at the same frequency, each time a radio beam points towards us.

But in recent years, astronomers have begun to find compact objects that emit pulses of radio waves at a much slower rate. This has baffled scientists, who had thought that radio wave flashes should cease when the rotation slows to more than a minute for each spin.

These slow-spinning objects are known as long-period radio transients. Last year, a team led by Manisha Caleb at the University of Sydney, Australia, announced the discovery of a transient with a period of 54 minutes.

Now, Caleb and her colleagues say a new object they found a year ago, named ASKAP J1839-0756, is rotating at a new record slow pace of 6.45 hours per rotation.

It is also the first transient that has ever been discovered with an interpulse: a weaker pulse halfway between the main pulses, coming from the opposite magnetic pole.

At first, the team thought that ASKAP J1839-0756 might be a white dwarf, a smaller star like our sun that has died. “But we’ve never seen an isolated white dwarf emitting radio pulses and our calculation suggests that it is too big to be an isolated white dwarf based on the properties of the pulse,” says Joshua Lee, a team member at the University of Sydney.

Next, the team thought it might be a magnetar, a neutron star with an immense magnetic field – as much as 10 trillion times more powerful than the strongest MRI machines on Earth.

A magnetar with a similar rotation period of 6.67 hours has been found before, but, so far, it has only emitted X-rays, not radio waves.

Caleb says that if the star is an isolated magnetar, it would be the first that emits in the radio wave frequency with a period that is this slow.

“This new object is completely rewriting what we thought we knew about radio emission mechanisms from neutron stars of the last 60 years,” says Caleb. “It is definitely one of the weirdest objects in recent times, because we didn’t think these things existed. But now we’re finding them. If it is a magnetar, it is certainly unique amongst the neutron star population.”

She says the idea that pulsars cease emitting radio waves when they spin too slowly needs to be reconsidered.

“We’re seeing objects in recent years which seem to cross this death line, but they’re still emitting in the radio [frequency],” says Caleb. “So they’re like zombie stars where you don’t expect them to be alive, but they’re still alive, and they’re pulsing away.”