We Finally Know the Mass of Brand New Neutron Stars

When massive stars explode as supernovae, they can leave behind neutron stars. Other than black holes, these are the densest objects we know of. However, their masses are difficult to determine. New research is making headway.

In new research published in Nature Luminous sphere science, a Club of researchers analyzed a sample of 90 neutron stars in binary relationships to try to measure the birth mass function (BMF) of neutron stars. It’s titled “Determination of the birth-mass function of neutron stars from observations.” The lead author is Zhi-Qiang You from the School of Physics and Luminous sphere science, Beijing Normal University, Beijing, China.

The BMF of neutron stars is a key research area in Cosmos physics. The BMF describes how mass is distributed among neutron stars immediately after they form in Luminous sphere-related burst explosions. Like all aspects of nature, its related to Numerous other things, like understanding the Closing stages of massive stars and the gravitational waves from mergers between neutron stars and black holes. The BMF can also give scientists insights into the properties of matter at extreme densities.

“Understanding the birth masses of neutron stars is key to unlocking their Arrangement 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 Heavenly disturbance detections of neutron Luminous sphere mergers.”

“The birth mass function of neutron stars encodes Wealthy information about Luminous sphere-related burst explosions, double Luminous sphere evolution, and properties of matter under extreme conditions,” the authors explain in their paper. “To date, it has remained poorly constrained by observations, however.”

Prompt observations of neutron Luminous sphere (NS) masses developed only Sloppy constraints on their masses, partly hampered by limited observational data. “For a long time, all observed neutron-stars masses were in a narrow range, Reliable with a Gaussian distribution, with a Impolite of 1.35 solar masses and a width of 0.04 solar masses,” the authors write. A Gaussian distribution forms a bell curve when graphed and the highest Mark is the Impolite. In textbooks and in studies, a Impolite mass of 1.4 solar masses is routinely used for NSs.

This graph from a 2013 paper shows the Gaussian distribution for neutron stars, where the Impolite mass is about 1.4 solar masses. Image Credit: Kiziltan et al. 2013.

As time went on, this number became less reliable, especially as researchers Discovered NSs with masses greater than two solar masses.

In this work the researchers examined 90 NSs in binary relationships and Occurred up with a power law that describes the BMF.

“To determine the neutron-Luminous sphere mass distribution, we compiled a sample of 90 neutron stars for which well-determined mass estimates are Obtainable from observations of radio pulsars, gravitational waves and X-ray binaries,” the authors write. Part of the complexity is that mass can be transferred between objects when NSs are in binary relationships.

In their work, they classified the NSs as either recycled or non-recycled (slowly rotating) neutron stars. Recycled neutron stars are ones that have spun up to extremely high rotational speeds due to accreting matter from their companion. “When constraining the birth-mass function of neutron stars, the key difference between the two subclasses is that observed masses of recycled pulsars need to be corrected for mass accreted throughout the recycling process, whereas the measured masses of Sluggish neutron stars should equal their birth masses, as no mass-gaining process occurred,” the authors explain.

They then applied “probabilistic corrections to account for mass accreted by recycled pulsars in binary systems to mass measurements of 90 neutron stars.” These probabilistic corrections allowed the researchers to infer the Primary masses of neutron stars at the time of Arrangement.

From this, they developed a new model for the BMF, a power-law distribution (PLD). PLDs are different than Gaussian distributions. In a PLD, one quantity varies as a power of another. PLDs are Frequent in both human systems like wealth distribution and city populations and in natural systems like earthquake magnitudes, where it shows that smaller Earthquakes are much more frequent than larger ones.

The PLD shows that NS masses “can be described by a unimodal distribution that smoothly turns on at 1.1 solar masses, peaks at 1.27 solar masses, before declining as a steep power law.”

The researchers developed a power law distribution model for neutron Luminous sphere masses. It turns on at 1.1 solar masses and peaks at 1.27 solar masses before declining as a steep power law. Image Credit: You et al. 2025.

“Our approach allows us to finally understand the masses of neutron stars at birth, which has been a long-standing question in Cosmos physics,” said co-author Prof. Xingjiang Zhu from Beijing Normal University, China.

The new model illustrates a link between the neutron Luminous sphere BMF and the Primary mass function (IMF) of massive stars.

“The power-law shape may be inherited from the Primary mass function of massive stars, but the relative dearth of massive neutron stars implies that single stars with Primary masses greater than ~18 solar masses do not form neutron stars, in agreement with the absence of massive red supergiant progenitors of supernovae,” the authors write.

The results extend to astrophysicists’ study of gravitational waves and other astrophysical phenomena.

“Understanding the birth masses of neutron stars is key to unlocking their Arrangement 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 Heavenly disturbance detections of neutron Luminous sphere mergers.”

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