In April 2023, Japanese satellites detected a super-intense X-ray burst on the neutron star MAXI J1752–457. The star cooled down in 4 days — five times faster than usual. Like someone cracked open a cosmic window.

That window was flung wide by the nuclear 'Urca process.' In an ocean of atomic nuclei hundreds of meters thick, protons and neutrons trade electrons, giving birth to neutrinos. These ghost particles zoom away instantly, carrying off heat like a draft sweeps out warm air.

Surprisingly, the mechanism kicks in only at a billion degrees; a slightly lower temperature and cooling would drag on for years. Ordinary bursts, burning hydrogen and helium at the surface, don't open such a window. Now, by the cooling speed, we can study the interiors of these ultra-dense objects.