The universe is filled with almost incomprehensibly bizarre phenomena, but astronomers may be a step closer to understanding the life cycle of stars. Astronomers observing a distant star system have identified what may be the most massive neutron star ever discovered. This could help shed light on the murky division between black holes and neutron stars.
Neutron stars like the newly discovered J0740+6620 are the remains of dead stars. While stars burn for millions or billions of years, they all eventually run out of fuel. Some stars, those between eight and 29 solar masses end up as neutron stars. Smaller stars like the sun become white dwarfs, and larger ones collapse into black holes.
A neutron star is extremely dense, with a mass greater than the sun in a sphere measured in tens of kilometers. The rest of the star’s mass is blown away in a supernova explosion, leaving just the dense, iron-rich core. It has so much mass that it collapses inward until all the protons and electrons merge into neutrons. Some neutron stars like J0740+6620 rotate and emit flashes of radiation from their poles — we call these pulsars. This pulsing is the key to characterizing J0740+6620.
J0740+6620 is not alone in its solar system. It’s in a binary arrangement with a less massive white dwarf. Luckily, the pulsar’s pole is pointed at Earth, sweeping us with radio frequency signals that we can measure from 4,600 light-years away. Using the Green Bank Telescope in West Virginia, the researchers monitored the signal from J0740+6620, which rotates on millisecond scales. When the white dwarf passes in front of the pulsar, its gravity causes tiny disruptions in the regularity of the pulses known as a Shapiro time delay. The team measured these delays, which amount to a difference of about ten-millionths of a second.
Your average neutron star compared with New York City. Credit: NASA/Goddard Space Flight Center
The Shapiro time delay gave the team an important piece of information: the mass of the white dwarf. If you know the mass of one object in a binary system, it’s comparatively simple to determine the other’s mass. Based on that, the team determined that J0740+6620 has a mass of 2.14 solar masses, which is tantalizingly close to the theoretical upper limit of 2.3 solar masses for a neutron star (based on gravity wave analysis).
Some other studies have pointed to neutron stars that might be 2.4 or 2.5 solar masses, but they weren’t measured as accurately as this one. We don’t know exactly how massive neutron stars can get, but we’ve never spotted a black hole less than five solar masses. What happens in between is still a mystery, but studying J0740+6620 could shed light on how stars live and die.