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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.


Andromeda (Credits: NASA/JPL-Caltech/GSFC)

While everyone else was making popcorn garlands and giving sentimental presents and eating marshmallow candies shaped like Santa, astronomers were busy too…busy scanning the Andromeda galaxy with a super powerful x-ray telescope array in space! (Yeah, that’s right, astronomers are no joke, boy!)  According to NASA’s mission overview, “The NuSTAR instrument consists of two co-aligned grazing incidence telescopes with specially coated optics and newly developed detectors that extend sensitivity to higher energies as compared to previous missions such as Chandra and XMM. After launching into orbit on a small rocket, the NuSTAR telescope extends to achieve a 10-meter focal length. The observatory will provide a combination of sensitivity, spatial, and spectral resolution factors of 10 to 100 improved over previous missions that have operated at these X-ray energies.”

The astronomers operating this device (devices?) chose to look at Andromeda (AKA M31) the Milky Way’s big sister galaxy which is located relatively close by in galactic terms…a mere 2.5 million light years away.  They wished to observe X-ray binary systems–disturbing star systems where a supermassive star collapsed either into a black hole or a neutron star.  The huge mass left over from such a collapse plays havoc with the remaining living star.  Frequently great plumes of matter are stripped away from the living star into the gravity well of the white dwarf or the black hole.  As star stuff falls into the massive stellar fragment it produces large amounts of exotic radiation like x-rays.

Astronomers hope that by determining which of these systems harbor black holes versus neutron stars, they can find out more about such systems, which are theorized to have played a critical role in heating the interstellar gas nebulae which gave birth to the galaxies.


NuSTAR X-Ray Observatory (NASA)

Ye Olde Ferrebeekeeper Archives

July 2020