In 2021, a star in the constellation Hercules flared up and faded almost instantly: its brightness dropped by half in a single day. Novae usually shine for weeks, but this one lasted just a day. Astronomers used to think such haste revealed a white dwarf (the compressed core of a dead star) at its limit: a little more and it would collapse into a neutron star or black hole. But the weight measured directly turned out to be more modest.
A team led by Ashish Vaswani pointed two X-ray telescopes, XMM-Newton and NuSTAR, at V1674 Her after the star had settled down. X-rays here work like a stethoscope: they pass through the outer layers and reveal what's happening near the white dwarf's surface. Hot gas falls onto it, accelerates to enormous speeds, and crashes, creating a shock wave that glows in X-rays. By examining how this "shock column" shines, one can calculate the dwarf's mass, just as the pitch of a plucked string reveals its tension.
It turned out the dwarf weighs about 1.1 solar masses — far from the limit. A second method, akin to analyzing the flickering of a candle flame (measuring rapid oscillations in the X-ray flux), gave the same number. So the old formulas that predicted mass from fading speed missed the mark here. Why was the star in such a hurry? Part of the answer lies in its powerful magnetic field. Measurements showed the field at the surface reaches tens of millions of gauss (for comparison, Earth's magnetic field is about half a gauss). Such a field acts like an invisible dam: it intercepts infalling matter far from the surface and channels it into narrow funnels, speeding up the entire process.
Thus, the fastest nova taught us a lesson: don't judge weight by gait. Even when a star seems to race at its limit, inside it might be a sturdy average Joe, and the pace is set by entirely different forces, like a magnetic leash that yanks matter faster than usual.
The fading speed of V1674 Her is so extreme that if our eyes could see it in real time, we'd notice the star dimming literally by the hour, as if someone were slowly turning down a dimmer switch.
In Stanisław Lem's novel "Solaris," the ocean could control the planet's orbit like a living magnet; here, the white dwarf's magnetic field conducts the fall of matter, preventing the star from exploding prematurely.