For over a century, historians of astronomy have faced an awkward choice: either three of the most careful observers of the ancient world all made the same mistake, or a star in the southern sky has quietly changed. A new analysis argues for the second option – and offers a physical mechanism to explain it.
The star is Theta Eridani, today an unremarkable magnitude 2.9 point of light about 167 light years away, third-brightest in the constellation Eridanus. Yet Hipparchus, writing around 129 B.C., called it the brightest star in the celestial River. Ptolemy's Almagest of 137 A.D. placed it among the 13 brightest objects in the entire sky, and the Persian astronomer al-Sufi still recorded it at magnitude 1 in 964 A.D. Only in 1603, when the Dutch navigator Frederick de Houtman catalogued the southern sky, does it appear at magnitude 3 – essentially where we find it now.
In a paper posted to the arXiv preprint server on 29 June, Idel Waisberg and Boaz Katz of the Weizmann Institute of Science work through the mundane escapes first. Confusion with the far brighter Alpha Eridani, a copying error in the ancient manuscripts, an overcorrection for atmospheric dimming near the horizon – none, they argue, survives scrutiny. The gap between the historical and modern magnitudes, roughly 2.7, is the largest of any of the thousand or so stars in the Almagest, corresponding to a star about ten times brighter than today's.
A pair of stars living close to the edge
The explanation they propose lies in the star's true structure. What ancient observers took for a single star is a triple system: a distant companion plus an inner binary so tight that the two components orbit at 0.083 astronomical units – about a fifth of Mercury's distance from the Sun. The two stars carry similar masses, roughly 2.3 and 2.2 times the Sun's, and each has swollen to about 80 percent of its Roche lobe, the limit beyond which a star begins spilling material onto its partner.
The larger of the pair, the researchers find, has just exhausted the hydrogen in its core and sits at a transition between evolutionary stages. Their scenario: as that star aged and expanded on a more elongated orbit, it shed material onto its companion, releasing energy that sustained a glow for something like a thousand years – matching the window in which ancient astronomers described it as brilliant. The transfer then gradually circularised the orbit into the calm configuration seen today, and the star faded.
The authors are careful about what remains open, chiefly what sets how bright such a glow gets and how long it lasts. Finding comparable binaries in modern photometric surveys would test the idea – and would turn a footnote in ancient star catalogues into a data point on how close binary stars evolve.