Astrophysicists from China and Australia have found that the birth masses of neutron stars can be described by a unimodal distribution that smoothly turns on at 1.1 solar masses and peaks at 1.27 solar masses, before declining as a steep power law.
An artist’s impression of a neutron star. Image credit: Sci.News.
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.
Astronomers 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.
“Understanding the birth masses of neutron stars is key to unlocking their formation history,” said Dr. Simon Stevenson, an OzGrav researcher at Swinburne University.
“This work provides a crucial foundation for interpreting gravitational wave detections of neutron star mergers.”
Dr. Stevenson and his colleagues analyzed 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.
They found that neutron stars are typically born with a mass of around 1.3 solar masses, 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 Beijing Normal University’s Professor Xingjiang Zhu.
“This finding is important for interpreting new observations of neutron star masses from gravitational wave observations.”
The study appears in the journal Nature Astronomy.
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ZQ. You et al. Determination of the birth-mass function of neutron stars from observations. Nat Astron, published online February 26, 2025; doi: 10.1038/s41550-025-02487-w
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