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How heavy are neutron stars at birth? We now know the answer
An international team of astrophysicists from China and Australia has for the first time determined how massive neutron stars are when they are born.
"Understanding the birth masses of neutron stars is key to unlocking their formation history," said Dr. Simon Stevenson, an OzGrav researcher at Swinburne University and co-author of the study. "This work provides a crucial foundation for interpreting gravitational wave detections of neutron star mergers."
Neutron stars are the dense remnants of massive stars, more than eight times as massive as our sun, born at the end of their lives in a brilliant supernova explosion.
These incredibly dense objects have masses between one and two times the mass of our sun, compressed into a ball the size of a city, with a radius of just 10 km.
We can usually only weigh a neutron star (measure how massive it is) when it is in a binary star system with another object, such as a white dwarf or another neutron star. However, in these systems, the first-born neutron star typically gains extra mass from its companion, through a process called accretion, making it difficult to determine its original birth mass.
The research, in Nature Astronomy, analyzes a sample of 90 neutron stars in binary star systems with accurate mass measurements to measure the distribution of neutron star masses at birth, accounting for the mass gained since birth for each neutron star in a probabilistic manner.
The team found that neutron stars are typically born with a mass of around 1.3 times the mass of the sun, with heavier neutron stars being born more rarely.
"Our approach allows us to finally understand the masses of neutron stars at birth, which has been a long-standing question in astrophysics," said Prof. Xingjiang Zhu, former Australian Research Council (ARC) Center of Excellence for Gravitational-wave Discovery (OzGrav) researcher (now a Professor at Beijing Normal University, China).
This finding is important for interpreting new observations of neutron star masses from gravitational wave observations.
More information: Zhi-Qiang You et al, Determination of the birth-mass function of neutron stars from observations, Nature Astronomy (2025).
Journal information: Nature Astronomy
Provided by ARC Centre of Excellence for Gravitational Wave Discovery (OzGRav)
