Two thousand years ago, a brilliant beacon shone in the constellation Eridanus, recorded in the registers of ancient astronomers. Hipparchus in 129 BCE noted it as a first-magnitude star, Ptolemy confirmed its brightness in the Almagest, and the Persian scholar Al-Sufi still saw it among the brightest in 964. Yet by the early 17th century, Acamar, Theta Eridani, had turned into an ordinary speck, visible only on a clear night. A drop by a factor of tens over centuries: that's impossible by normal stellar evolution for an intermediate-mass star. The mystery demanded modern tools.

The solution emerged when astronomers combined high-resolution spectroscopy with data from VLTI interferometers and the space-based photometry of the TESS telescope. It turned out that Acamar is a triple system, where the two central suns whirl with a period of just over four days. Their masses are 2.3 and 2.2 solar masses, radii about 4.3 and 4.0 solar radii; each occupies roughly 80% of its Roche lobe — the critical boundary beyond which material begins to spill onto its neighbor. Both components have recently left the core hydrogen burning phase, where helium is now accumulating; the primary star has already swollen to twice its original size, becoming a subgiant. But the key to the past came from orbital analysis: today it's nearly circular, yet according to the law of conservation of angular momentum, it couldn't have always been that way.

Reconstruction suggests: about two thousand years ago, the orbit was elongated, with an eccentricity of around 0.6. During close approaches, the subgiant star filled its Roche lobe, triggering a violent mass transfer. The material falling onto the companion didn't settle immediately: it enveloped both stars in a common envelope, like a fiery cocoon. Friction in this envelope relentlessly devoured the gravitational energy of the orbit, converting it into light. It was a veritable stellar fever: temperature and pressure skyrocketed, and the system blazed like 1,500 Suns. The orbital energy of the pair, described by the simple expression \(E_{\rm orb} = -\frac{G M_1 M_2}{2a}\), dwindled, shrinking the orbit while radiating the excess. Thus Acamar became a celestial floodlight for a thousand years, until the envelope dissipated, leaving behind a tighter, nearly circular dance and barely perceptible traces of recent nucleosynthesis in the surface layers. If planets had existed around Acamar back then, their climates would have endured apocalyptic warming, then an eternal winter after the envelope cleared.

This scenario fills a gap in understanding the evolution of close binaries. It turns out that the common envelope phase during eccentric Roche-lobe overflow can stretch over centuries and millennia — meaning modern sky surveys like TESS and the upcoming PLATO can spot similar "protracted transients" around other stars. Perhaps many stars that seem ordinary today have gone through outbursts comparable to Acamar's; we just didn't know where to look. Advances in numerical simulations of such processes and searches for anomalous luminosity in stars with known masses will open a new chapter in binary physics. Moreover, understanding energy dissipation in common envelopes is crucial for solving the puzzles of Type Ia supernovae and gamma-ray bursts — events where nature itself conducts experiments at the limits of the possible. And Acamar's metallicity, close to solar, hints that such dramas unfold in every corner of the Galaxy.