A Double Star Inside a Double Star express
In the constellation Pisces, there is a star that people have seen for thousands of years, but only now have they discerned its inner structure. Alpha Piscium is not one star, nor even two, but a whole family: two pairs of stars bound by gravity. Astronomers had suspected that a third star lurked in the second pair, but they couldn't see it: it was too crowded.
To peer into that tight space, scientists used several telescopes linked into a single giant antenna. They combined the starlight so that fine details became visible, as if looking through a very powerful binocular. This allowed them, for the first time, to directly see the second star in the pair: it turned out to be very close, at an angular separation 500 times smaller than the thickness of a human hair viewed at arm's length.
Thanks to this discovery, astronomers were able to precisely weigh both stars. One is 1.668 times heavier than the Sun, the other 1.646 times. Such precise mass measurements are rare for stars in triple systems. Now Alpha Piscium will serve as a benchmark for testing theories of how stars live and age.
The new observational technique the scientists applied allows measuring stellar positions with a precision comparable to noticing that an ant at the other end of a football field has shifted by the thickness of its antenna. This paves the way for studying other close pairs of stars that were previously invisible.
Original abstract
as written by the authors arXiv:2603.09489
We present the first on-sky demonstration of dual-field interferometry at the CHARA Array and the first direct resolution of the inner Ba--Bb subsystem in the bright hierarchical triple $$\alpha$$ Piscium. Using $$H$$-band fringe tracking on component A with MIRC-X to stabilize $$K$$-band science fringes on component B with MYSTIC, we detected a companion at a projected separation of 7 mas, confirming a long-suspected but previously unresolved short-period subsystem within the B component. The nearly equal $$H/K$$-band flux ratio indicates that Ba and Bb are near-twin F-type stars, consistent with the two narrow-lined components seen in optical spectra of B. By combining CHARA interferometry with archival VLTI/GRAVITY astrometry and radial velocities from archival and new spectroscopy (NARVAL and ARCES), we derive a well-constrained orbit with a period of $$P = 25$$ d, eccentricity $$e \simeq 0.6$$, and inclination $$i \simeq 65^\circ$$, yielding precise dynamical masses of $$1.668\pm0.033\,M_\odot$$ and $$1.646\pm0.029\,M_\odot$$. No additional companion is detected down to $$\Delta H \approx 5$$ at separations of 0.2--2 AU. We also obtained dual-field differential astrometry of the wide A--B pair with a precision of ~0.234 mas at a separation of $$1.85''$$, with an error budget dominated by internal delay-line actuators, fringe-tracking performance and chromatic dispersion. While the long-period outer orbit is not refined by these measurements, their agreement with the published astrometric orbit provides an on-sky validation of the CHARA dual-field mode. These results establish $$\alpha$$ Psc as a well-characterized hierarchical system suitable for future benchmark studies and demonstrate CHARA's new capability for off-axis interferometry and sub-mas astrometry on arcsecond-scale binaries.
Alpha Piscium is one of the few stars visible to the naked eye even from a city, and it turns out to be a system of four stars.