Eridanus constellation
© Idel Waisberg & Boaz Katz, arXiv (2026). DOI: https://doi.org/10.48550/arXiv.2606.30748Photo of the southern part of the Eridanus constellation as seen from Jacutinga, MG, Brazil on 2026-03-22 UTC 23:06 shows Theta (ฮธ) Eridani.
For more than a century, astronomers have puzzled over a strange discrepancy in the historical record. Theta Eridani, a modest star in the constellation Eridanus, is the third-brightest star in its constellation, with a magnitude of 2.9. But ancient astronomers described it very differently. A new study, posted to the arXiv preprint server on June 29, has proposed a plausible explanation for this mystery.

The 'forgotten' bright star

Ancient astronomers, including Hipparchus, Ptolemy and al-Sufi, all described Theta Eridani as one of the brightest stars in the night sky. Hipparchus, writing around 129 B.C., called it "the brightest and preceding and southernmost of all in the River." Ptolemy's Almagest, compiled in 137 A.D., ranked it among the 13 brightest stars in the entire sky. And nearly 800 years later, in 964 A.D., the Persian astronomer al-Sufi still listed it as having magnitude 1.

Then, in 1603 A.D., one of the members of the Dutch expeditions, Frederick de Houtman, published a star catalog of the southern sky in which the star was assigned a magnitude of 3, consistent with its modern, ordinary brightness. Whether this sudden drop in brightness was real or a mistake in the historical record has been debated for centuries.

Interestingly, Theta Eridani, long thought to be a single star, is actually a system of three stars. Modern studies have shown that it consists of a close binary pair of stars, plus a third, more distant companion.

Uranometria published in 1603
© arXiv (2026). DOI: 10.48550/arxiv.2606.30748
Eridanus constellation in Johann Bayer's Uranometria published in 1603. The extension of the constellation to Achernar= ๐›ผ Eridani was based on the stellar globes containing the southern sky produced just a few years prior based on the Eerste Schipvaart expedition. ๐œƒ Eridani is already depicted as much fainter than ๐›ผ .

Ancient puzzle

In this new study, two researchers โ€” Idel Waisberg and Boaz Katz โ€” argue that the historical reports of Theta Eridani's brightness were likely accurate, not mistaken. They first rule out the mundane explanations proposed over the past century: confusion with the much brighter star Alpha Eridani, a typographical error in the ancient texts and an overcorrection for atmospheric extinction at low observing altitudes.

"... we find no convincing reason to refute the fact that Theta Eridani was visually brighter by about a factor of ten between about 2,000 and 1,000 years ago," they write in the paper.

Having found none of these explanations convincing, they turn to the star system itself, combining interferometry, spectroscopy and satellite photometry to uncover a possible mechanism behind its dramatic dimming. They mapped the inner binary stars and found that they orbit very close together โ€” 0.083 AU, roughly a fifth of Mercury's distance from the sun โ€” and that their orbit is nearly circular, with slight elongation. Both stars have similar masses, about 2.3 and 2.2 times the sun's mass.

Living dangerously

Both stars are expanded to about 80% of their "Roche lobe" size. The Roche lobe is the maximum size a star can reach in a binary before its own gravity loses the tug-of-war with its companion and starts spilling material onto it. Being at 80% of that limit means these stars are living dangerously close to that threshold.

In particular, the larger of the two stars has been caught at a special moment. It has just finished burning hydrogen in its core and is at a transitional stage before entering the next evolutionary phase.

All these properties line up well to explain the historical brightening reported by ancient astronomers. The researchers propose that as this star aged and expanded, it likely spilled material onto its companion during a previous, more elongated orbit. This process would have released a burst of energy that powered a long-term glow, coinciding with the roughly 1,000-year period during which ancient observers reported the star as unusually bright. Over time, this process gradually calmed the orbit into the quieter, near-circular configuration seen today.

"This strengthens the case that the apparent brightening was real and not due to an error by three different ancient observers, as has been commonly claimed in the past," they conclude.

Researchers note that a key piece of the puzzle is still missing โ€” what determines how long such a glow could last and how bright it could become. Finding and studying more binary systems going through a similar process in modern photometric surveys could help them better understand how close binary stars evolve.