The Most Massive Neutron Star Ever Discovered

Ferrebeekeeper is baffled and alarmed by neutron stars (here is a post about them from back in the day).  A factoid from that post summarizes what makes these super-dense stellar remnants so disconcerting: a 1.27 square centimeter cube of neutron star material has approximately the same mass as all of Earth’s 7.7 billion human inhabitants (although the tiny cube of pure neutrons presumably lacks the same lively personality).  It is almost impossible to conceive of such a material…which is why we are reporting today’s space news! Astronomers at the Greenbank Radio telescope in West Virginia (pictured above) have discovered the largest known neutron star 4600 light years from Earth.  The star is known by the unlovely name J0740+6620 and it has 2.14 times the mass of the sun packed into a sphere with a diameter of 25 kilometers (to contextualize in instantly familiar terms, 25 km is the distance from Hell’s Kitchen to JFK airport).  This particular star is a rotating neutron star—a pulsar–which emits two radio beams from its poles as it rotates at hundreds of revolutions per second.  lies at the upper theoretical limit of how large a neutron star can be without collapsing into a black hole.

The star was discovered by luck as astronomers researched gravitational waves (which are vast invisible ripples in space time).  Because the neutron star has a white dwarf companion, astronomers were able to precisely calculate the star’s mass with some fancy math.   The mass of the white dwarf distorts spacetime around the neutron star to a degree which causes the pulsar’s radio beacons to be delayed by tenths of millionths of a second.  Astronomers measured these delays (the phenomenon is known as “Shapiro Delay”) and calculated the mass of the neutron star accordingly.

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