A neutron star is the core left behind after a massive star explodes. Its matter is so compressed that a teaspoonful would weigh billions of tons. But the exact size and mass of such objects remain debated. A team of astronomers studied the system EXO 0748-676, where a neutron star pulls matter from a normal companion star. It occasionally flares up and then quiets down. During quiescent periods, its surface cools, and X-ray emission can reveal key parameters.
The researchers gathered 20 observations from 2008 to 2025—14 from Chandra and 6 from XMM-Newton. They ran the data through a hydrogen atmosphere model and applied a Markov Chain Monte Carlo method. They found the star's mass to be 1.77 solar masses and its radius 12.62 km. That's like squeezing one and a half Earths into a ball the size of a small city. These numbers favor a fairly stiff equation of state — meaning the matter inside doesn't compress easily.
They also compared two quiescent periods — before and after the 2024–2025 outburst. After the new flare, the star's crust began cooling again, and they could track the process. This is a rare chance to see a neutron star recover from violent accretion.
A neutron star spins so fast that its surface can move at tens of thousands of kilometers per second.
In Robert Forward's novel "Dragon's Egg," life is depicted on the surface of a neutron star, where time flows millions of times faster.