Astronomers Discover the Most Massive Neutron Star… Or is it the Least Massive Black Hole?

Posted by Guy Pirro   01/24/2024 03:20PM

Astronomers Discover the Most Massive Neutron Star… Or is it the Least Massive Black Hole?

An artist’s impression of the binary system assuming that the massive companion star is a black hole. The brightest background star is its orbital companion, the radio pulsar PSR J0514-4002E. The two objects  are separated by 8 million km and orbit each other every 7 days. The Green Bank Telescope helped the South African Radio Astronomy Observatory’s MeerKAT uncover this mysterious object at the boundary between black holes and neutron stars. (Image Credit: Daniëlle Futselaar,


Astronomers Discover the Most Massive Neutron Star… Or is it the Least Massive Black Hole?

An international team of astronomers have discovered a massive dark object in orbit around a rapidly spinning millisecond pulsar. Is it a neutron star? Is it a black hole? Or is it something in between?

Neutron stars, the ultra-dense remains of a supernova explosion, can only be so heavy. Once they’ve acquired too much mass, perhaps by consuming another star or maybe by colliding with another of their kind, they will collapse. What exactly they become once they collapse is the cause of much speculation, with various wild and wonderful flavors of exotic stars being proposed. The prevailing opinion, however, is that neutron stars collapse to become black holes, objects so gravitationally attractive that even light cannot escape them.

“We found a pulsar orbiting a massive and mysterious companion star,” shares Scott Ransom, a scientist with the National Radio Astronomy Observatory, “It is either the most massive neutron star known, or the least massive black hole ever observed. Either one is amazing.”

This unusual object was found as part of a binary system, a pulsar and an unknown compact object, in the globular cluster NGC 1851. Globular clusters are unique environments with hundreds of thousands of stars packed closely together, and likely places to produce strange cosmological pairs. 

Astronomers used the pulsar, PSR J0514-4002E, to measure the pair’s precise location and reveal the unusual mass of its partner—simultaneously bigger than any known neutron star and yet smaller than any known black hole. This could be the first discovery of a long anticipated pulsar-black hole binary; a stellar pairing that would allow new tests of Einstein’s general relativity. “We don’t know how big neutron stars can get before they collapse into black holes. This source will help us figure that out,” adds Ransom.



This research combined several years of archival data from the National Science Foundations Green Bank Telescope with new data from the South African Radio Astronomy Observatory’s MeerKAT. 

“This exciting discovery shows how important access to legacy data from our major instruments is,” shares Ransom. “After we found the pulsar in the fantastic new MeerKAT data, we were able to detect the pulsar in several GBT observations of the cluster from 2005 and 2006, and show that the orbit had subtly changed over time due to the massive companion star. The Green Bank Telescope has nearly two decades worth of other data that could be really valuable for the astronomy community, and the Observatory is working hard to make all of it available.”

While astronomers cannot conclusively say whether they have discovered the most massive neutron star known, the least massive black hole known, or even some new exotic star variant, what is certain is that they have uncovered a unique approach for probing the properties of matter under the most extreme conditions in the Universe. 


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