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A Celestial Heavyweight

Duncan Lorimer and Maura McLaughlin among spinning lights.

Written by Mikenna Pierotti
Photographed by Raymond Thompson Jr.

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About 4,600 light years from Earth, two stars are trapped in an endless cosmic dance. And one of these dancers, a pulsar known as J0740+6620, has a secret. It might be the most massive of its kind ever measured.

“It wasn’t an instantaneous thing,” Maura McLaughlin said of the discovery. 


As the Eberly distinguished professor of physics and astronomy, she is a member of an interdisciplinary, multi-university team who first detected this pair of stars using the Green Bank Telescope in Green Bank, W.Va. 


“This is one of the pulsars we’ve been timing for about five years or so. And we noticed this … signature in the data,” she said. 


McLaughlin and her team, including her husband, Duncan Lorimer, astronomy professor and Eberly College of Arts and Sciences associate dean for research, are part of a collaboration called the North American Nanohertz Observatory for Gravitational Waves, or NANOGrav. 

Their data are some of the most accurate ever recorded. 


“We can now measure the position of the center of our solar system to 100 meters, which is better than anybody — even NASA — can do,” she said. 


And that’s not even the real purpose of their work. 


“[We hope] to detect gravitational waves by using pulsars as natural clocks,” Lorimer said. “This result is a by-product of that work. It’s really interesting in its own right, but it was not the main reason we established NANOGrav.” 


Pulsars are a type of neutron star, remnants of giant stars that went supernova, exploding and collapsing down from 10 to 30 times the size of our sun to the size of a city. 


For a neutron star to be seen as pulsar, it has to have enough energy to emit electromegnatic radiation from its poles. As the neutron star rotates, this beam of electromagnetic radiation sweeps by our line of sight. Rather like a lighthouse, the pulsar is seen as a regularly spaced set of pulses. 


As the rotational periods of these stars are so predictable, subtle changes in the arrival times of their pulses — even the faint brush of a gravitational wave — can be detected and measured. 

But in order to do that, the team had to understand and model all the other effects in the system. First, they realized that J0740+6620 was in a binary orbit with a white dwarf. They then observed that the star’s pulses were delayed due to the gravitational well of its companion star when it passed behind the companion. From this delay, they could calculate the mass of the white dwarf companion and also the inclination of the pulsar’s orbit to Earth. 


“Once we have those two things, we can also get the mass of the pulsar,” McLaughlin said. 

Turned out, J0740+6620 was a record-breaker. 


“The mass that we came up with was about 2.1 times the mass of the sun. The previous highest mass was two times the mass of the sun,” she said. 


Although McLaughlin cautions that these measurements are not without a margin of error, she’s confident this star is unique. 


“It’s almost certainly the most massive neutron star ever measured.” 


Although this particular system hasn’t helped scientists find gravitational waves yet, its sheer size is affecting other areas of science. 


“Neutron stars are a very special type of matter. They are denser than anything we can make on earth,” McLaughlin said. “We just don’t know what happens to matter when you squish it to densities this high.” 


Particle physicists theorize that in the hearts of these superdense stars, whole new kinds of particles may be created — exotic particles with properties we haven’t even dreamed of. But in order to support a massive pulsar like J0740+6620, most of it would have to be neutrons, ruling out many of the current equations. 


“Neutrons are what provide the pressure to support the star. Those exotic particles don’t provide the pressure that the neutrons provide,” McLaughlin said. 


Theorists can now rule out some of their predictions, which were based on smaller masses. 

Another big question this measurement could help answer is, “Where do you draw the line between neutron star and black hole?” 


It’s closer to the neutron star/black hole boundary than any other star. 


Knowing all this will help researchers make even more accurate predictions about the universe, discover new particles and even calculate how many black holes might be hiding in our backyard, all without leaving the Earth. 


“It’s important because there’s a lot of interest in this in the broader physics community — how matter behaves at very high densities. These mass measurements are one of the only ways we can actually probe these theories. 


“I guess it’s pretty cool that we can weigh a star that is thousands of light years away so precisely using the radio waves it emits,” McLaughlin said. “And the constraint gives us details about the most exotic state of matter in the universe.” 


“This result highlights the power of this technique to measure even higher neutron star masses in the future,” Lorimer said. “This is really a stepping-stone to bigger-picture projects. There are surely more exciting results to come as we find and characterize more of these cosmic clocks.”