Astronomers using NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) realized that an object conventionally thought to be a black hole was actually a dead star emitting energy comparable to that of 10 million suns.

“This compact little stellar remnant is a real powerhouse. We’ve never seen anything quite like it,” said Fiona Harrison, NuSTAR’s principal investigator and professor of physics at Caltech. “We all thought an object with that much energy had to be a black hole.”

According to the scientists, the object is a pulsar– the super-dense rotating remnants of a star– that projects a class of extremely bright X-rays known as ultraluminous X-ray sources (ULXs). In fact, this pulsar is between one and two time the mass of the sun, but shines 100 times brighter than theory suggests objects of its size should be able to.

“We’ve never seen a pulsar even close to being this bright,” said Dom Walton, a postdoctoral scholar at Caltech who analyzed NuSTAR data. “Honestly, we don’t know how this happens, and theorists will be chewing on it for a long time.”

By focusing NuSTAR at a precise angle, the scientists noticed that the object was emitting radiation from its poles rather than at a constant rate. However the data was confusing, because the ULXs were much brighter than the Eddington limit, a physics tenet that sets limits on the brightness that an object of a given mass can achieve.

“This is the most extreme violation of that limit that we’ve ever seen,” said Walton. “We have known that things can go above that by a small amount, but this blows that limit away.

Until now, the general consensus among astronomers that black holes were the only sources of ULXs in space. Black holes have a much greater density and gravitational pull than pulsars; as a black hole consumes matter, it generates the heat energy to emit ULXs.

“Everybody had assumed all of these sources were black holes. Now I think people have to go back to the drawing board and decide whether that’s really true,” said Harrison. “This could just be a very unique, strange object, or it could be that they’re not that uncommon. We just don’t know. We need more observations to see if other ULXs are pulsing.”