Our understanding of how the Universe works has been upended by a flash of light.
As the two stars merged, it was found that one of them was far heavier than the maximum star mass.
The magnetar seemed to have continued for a day before collapsing into a black hole.
Nuria Jordana-Mitjans, an astronomer at the University of Bath in the UK, told The Guardian that it's not normally thought of as possible. It's a mystery why this one lasted so long.
There are different ways a star can end up at the end of their lives. A star will travel for millions or billions of years, powered by a hot pressurized core.
At this point, the stars' atoms will run out and everything will explode. The star's core collapses under the pressure of gravity after it ejected its outer mass.
The mass of the object affects how we categorize them. White dwarfs, which have an upper mass limit of 1.4 solar mass, are the result of the collapse of stars that were up to 8 times the mass of the sun.
A sphere just 20 kilometers (12 miles) across has the cores of stars between 8 and 30 solar mass turning into neutron stars. The theory says that the biggest stars collapse into black holes.
There isn't a lot of black holes below 5 solar mass so what happens in that mass regime is a mystery.
Astronomers find neutron star mergers fascinating. When two neutron stars are in a system and have reached the point of decay, they inevitably become one object, combining the two.
The theoretical upper mass limit for neutron stars doesn't account for the combined mass of the stars. The products of these mergers are likely to be in the mass gap.
A burst of high-energy radiation known as a short-duration gamma-ray burst can be released when there is a collision. The scientists thought that these could only be emitted when a black hole is formed.
It has been difficult to understand how merging stars turn into black holes. Does the black hole form quickly or does the two stars produce a heavy neutron star that collapses into a black hole very quickly?
A light that traveled over 10 billion years to reach us was detected in June of last year. The burst itself, the kilonova explosion, and the longer-lived afterglow were all looked at by Jordana-Mitjans and her colleagues.
Something wasn't right when they looked at the radiation produced over the course of the event.
35 minutes after the burst, the optical emission vanished. The team found that the expansion was being accelerated by a continuous energy source.
It was consistent with a neutron star. That's not just any star. It looked like it was a magnetar, one with a magnetic field 1,000 times more powerful than an ordinary neutron star and a quadrillion times more powerful than Earth. It stayed around for over a day and a half.
"For the first time," Jordana-Mitjans said, "our observations highlight multiple signals from a surviving neutron star that lived for at least one day after the death of the original neutron star."
It's not clear what could have helped the magnetar. It's possible that the magnetic field gave it a little help, giving it an outward pull that prevented it from collapsing all the way.
We can no longer assume the presence of a black hole because the team's work shows that the stars are capable of launching short-term gamma-ray bursts.
It's important that they confirm that newborn neutron stars can power some short-term GRBs and the bright emissions that have been detected accompanying them.
When we're looking for signals in the sky, this discovery may offer a new way to locate stars.
The research is in a journal.
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