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Astronomers using the NASA X-ray telescope aboard the International Space Station (ISS) have weighed a rapidly spinning, dead star that represents the heart of the millisecond pulsar closest to Earth.
Like all neutron stars, pulsars are born when massive stars die, but what really sets millisecond pulsars apart is that they spin hundreds of times per second. As they do so, beams of radiation and matter shoot out from the poles of these dead stars and sweep across the universe, making pulsars look like powerful “cosmic lighthouses.”
Located about 510 light-years from Earth in the constellation Pictor, PSR J0437-4715 (PSR J0437) is the closest example of a millisecond pulsar to our solar system, and the brightest such object in the night sky. PSR J0437 rotates 174 times per second, meaning it sends X-rays and radio waves back to Earth every 5.75 milliseconds. These pulses are so regular that, like other pulsars, this fast cosmic lighthouse could actually be used to keep time.
Now, scientists know that the neutron star-forming PSR J0437 is 14 miles (22.5 kilometers) across and has a mass equivalent to 1.4 times that of the sun. The team also discovered that the neutron star’s hot magnetic poles are misaligned and not perfectly opposite each other.
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To collect new measurements of PSR J0437, the team turned to NASA’s Neutron Star Interior Composition Explorer (NICER), which is attached to the ISS. They processed this X-ray data using a modeling method called “pulse profile modeling” and then created simulations of PSR J0437 using the Dutch national supercomputer Snellius.
“Previously, we were hoping to be able to calculate the radius accurately. And it would be great if we could show that the hot magnetic poles are not directly opposite each other on the star’s surface,” team leader Devarshi Choudhury of the University of Amsterdam said in a statement. “And we did both!”
The most extreme stars
Stars with masses between eight and 25 times the mass of the sun, once they run out of fuel after billions of years of existence, are no longer able to carry out nuclear fusion in their cores. This not only cuts off most of the energy a star emits, but also stops radiation pressure from flowing outward.
Throughout a star’s life, this radiation pressure supports it against the inward pressure of its own gravity. This means that once its fuel is exhausted, the star can no longer protect itself from gravitational collapse. As the core collapses, this process sends shock waves into the outer layers of the star, triggering a supernova explosion that rips off most of the star’s mass. Regardless of the star’s starting mass, the resulting neutron star will be born with a much narrower mass range of one to two times the mass of the sun.
However, the collapse of this dying star’s core shrinks the proto-neutron star’s width to about 12 miles (20 kilometers). As a result, the matter that makes up a neutron star is so dense that a sugar cube brought to Earth would weigh 1 billion tons—about 2,500 times the weight of the Empire State Building.
There is another consequence of a star’s core rapidly contracting to give birth to a neutron star. Due to conservation of angular momentum, the radical decrease in radius causes the stellar remnant to spin faster. This is similar to an ice skater on Earth pulling in her arms to increase the speed of a pirouette.
Neutron stars that form pulsars can also receive an additional speed boost from a companion star. When the neutron star and companion star are close enough, the former can strip material from the latter. This stellar material carries angular momentum, further increasing the neutron star’s rotation speed.
PSR J0437 may have engaged in this stellar cannibalism in the past to achieve its rotation rate of 174 rotations per second, as evidenced by the fact that it has a helium-rich white dwarf companion star that is about a quarter the mass of the Sun and whose outer layers appear to have been removed.
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While most measurements of PSR J0437 have confirmed scientists’ understanding of how these objects form, this millisecond pulsar offered a surprise. The mass of PSR J0437 tells the team that the maximum mass of neutron stars may be lower than some theories currently predict.
“This fits very well with what the gravitational wave observations suggest,” said team member and neutron star expert at the University of Amsterdam, Anna Watts.
The team’s research has been published in a series of peer-reviewed papers on the arXiv data repository and in the Astrophysical Journal.