Bizarre neutron star spins every 76 seconds inside a stellar graveyard

A neutron star has been spotted spinning at a cosmic snail's pace.
John Loeffler

Neutron stars are normally extremely fast-spinning stellar corpses left over from the intense violence of a supernova, but researchers have found one in a "stellar graveyard" where one should not be – and it spins at a relatively glacial rate of once every 76 seconds.

Researchers with the University of Sydney found the bizarre radio signal, designated PSR J0901-4046,  emitted by the neutron star thanks to the MeerKAT radio telescope in South Africa and weren't even expecting to see it. The region of the sky they were observing was thought to be free of pulsars, since none had been observed there before.

Now they might know why. Capturing eight-second-long samples of the sky, they caught sight of a single pulse from the star, which had to be confirmed with subsequent observation due to its unexpectedly long rotational period.

"Amazingly we only detect radio emission from this source for 0.5 percent of its rotation period," said research lead Dr. Manisha Caleb, formerly of the University of Manchester but now with the University of Sydney, in a statement.

"This means that it is very lucky that the radio beam intersected with the Earth," Caleb added. "It is therefore likely that there are many more of these very slowly spinning stars in the galaxy, which has important implications for understanding how neutron stars are born and age."

Its long rotational period and the nature of the radio signals used to detect these stars, the way pulsars—as these kinds of stars are called—are detected might have to be reconsidered going forward.

"The majority of pulsar surveys do not search for periods this long," Caleb said, "so we have no idea how many of these stars might exist."

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The research was published this week in the journal Nature.

How rare is a neutron star?

A neutron star is the remnant core of a very large star that is compressed during a supernova but not so much that it becomes a black hole. Because they are rotating when they were a large star, the law of angular momentum means that the force of that rotation must be preserved even when the star is condensed to extremely small dimensions, relatively speaking.

This results in a neutron star that can complete a hundred or more rotations every second, despite being the size of a large city. And because neutron stars release powerful radio energy from their surface, we can spot them out in the universe by seeing their rapid "pulsation".

The radio signals they emit though are strongly directed, almost like a beam, which means that you have to get fairly lucky to be in the "line of sight" of the beam in order to detect the pulses. As such, it's hard to know exactly how rare neutron stars actually are, though we've identified about 3,000 of them so far in the Milky Way galaxy. Compared to the estimated 400 billion stars in our galaxy though, 3,000 is barely a drop in the cosmic bucket.

The discovery of this possibly-new class of neutron star complicates that picture even further. "This is the beginning of a new class of neutron stars," Caleb said. "How or whether it relates to other classes is yet to be explored. There are likely many more out there. We need only look."

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