An international team of astrophysicists from China and Australia, led by former Australian Research Council (ARC) Centre of Excellence for Gravitational-wave Discovery (OzGrav) researcher Prof. Xingjiang Zhu (now a Professor at Beijing Normal University, China), 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 8 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, published in Nature Astronomy, analyses 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.  Zhu.

This finding is important for interpreting new observations of neutron star masses from gravitational wave observations.

The team used Bilby, a software package that OzGrav researchers developed to model neutron star mass distributions. The study’s Australian co-authors are members of the Australian Research Council (ARC) Centre of Excellence for Gravitational-wave Discovery (OzGrav).

To learn more about this discovery, watch Dr. Kirsten Banks interview Dr. Simon Stevenson below!

Paper published in Nature Astronomy

https://www.nature.com/articles/s41550-025-02487-w

Zhi-Qiang You, Xingjiang Zhu et al.

Preprint link: https://arxiv.org/abs/2412.05524

Media Contact

Email: ozgrav.comms@swin.edu.au

Available for interview 

Dr Simon Stevenson | ARC DECRA Fellow, Swinburne University of Technology

Dr Lilli Sun  | ARC DECRA Fellow, Australian National University (ANU)

Professor Eric Thrane  | Monash University