Astronomers finally used the 'fingerprints' of oxygen and neon to figure out exactly how heavy and big a neutron star is.
arXiv · March 17, 2026 · 2603.14146
The Takeaway
Neutron stars are so dense that a single teaspoon of their material would weigh a billion tons, making them notoriously difficult to measure. By detecting specific elements on the star's surface and seeing how the star's intense gravity warps their light, researchers have moved beyond estimates to create the first purely spectroscopic measurement of these city-sized dead stars.
From the abstract
We present evidence for atomic absorption lines in the high-resolution 4-30 A X-ray spectrum of the neutron star RX J0822-4300 in the supernova remnant Puppis A. Comparison with model atmosphere calculations shows that features in the observed spectrum can be uniquely associated with redshifted and pressure-broadened transitions in highly ionized oxygen and neon. We also spectroscopically confirm the previously estimated strength of the surface magnetic dipole field; we detect both the linear an