In the dense core of the globular cluster NGC 6397, where stars jostle like commuters at rush hour, astronomers have spotted a peculiar pair. One star is a white dwarf with a surface temperature of 16,000 K, the scorched remnant of a star that has been cooling for billions of years, and circling it is an unseen companion. They complete an orbit around their common center of mass in 0.54 days, faster than Earth spins on its axis.
Spectroscopic analysis reveals that the visible white dwarf is remarkably light — only 0.23 solar masses, and composed mostly of helium. Its invisible partner is heftier: at least 0.78 solar masses. Most likely, it is another white dwarf, but if the orbit is steeply inclined to our line of sight, it could even be a neutron star — an object where a teaspoon of material weighs as much as a mountain.
Residual radial velocity hints suggest a possible third body in the system. If confirmed, NF1 would become a triple system — even more exotic than previously thought.
The pair's orbital period is 0.54 days. If you stood on the surface of the visible white dwarf, the unseen companion would sweep overhead every 13 hours.
Similar systems of two dead stars have inspired stories of "stellar beacons," where gravitational waves from their merger serve as signals for distant civilizations.