The Chameleon Star: Where Did Theta Eridani's Brightness Go? ⚡ экспресс
Theta Eridani (V=2.9) was listed in ancient catalogs as one of the brightest stars, with a brightness discrepancy of ΔV~2.7 — the largest among the ~1000 stars of the Almagest. To unravel this mystery, a comprehensive analysis was carried out using data from: VLTI interferometry (PIONIER/GRAVITY), spectroscopy (ESPaDOns/FEROS), and TESS photometry. The inner pair Aa+Ab is a tight elliptical system (a=0.083 AU, e=0.105) with masses M_Aa≈2.3 M⊙ and M_Ab≈2.2 M⊙ and radii R_Aa≈4.3 R⊙, R_Ab≈4.0 R⊙, filling ~80% of their Roche lobes; the light curve is dominated by ellipsoidal variations. The primary component is in a rare phase immediately after hydrogen exhaustion in the core. The set of parameters points to a thousand-year transitional stage with a common envelope, triggered by eccentric Roche lobe overflow at a previous eccentricity of e≈0.6. This confirms the reality of historical brightening, rejecting the hypothesis of errors by ancient observers.
Thousands of years ago, the star Theta Eridani shone brightly in the sky, but today its light has almost vanished. For a long time, it seemed like ancient astronomers had made a mistake, but the real reason is a cosmic dance between two stars. Modern instruments—spectroscopes, which split light into colors, and photometers, which measure brightness—revealed: Theta Eridani is not a single star but a tight pair. Each star, twice as heavy as the Sun, orbits so close that they’d fit inside Mercury’s orbit. When one exhausted its hydrogen and bloated, it swirled its companion in a gaseous shroud. Like dancers tangled in a shared scarf, they became wrapped in scorching gas. Friction inside this “scarf” made the system flare up dozens of times brighter for a few centuries. Then the shroud dispersed, and the star dimmed again. Now scientists realize that ancient records of sudden flares might not be mistakes, but traces of episodes like this. And perhaps this pair will end their dance by merging into a single star.
🎯 At its peak brightness, Theta Eridani rivaled Vega, one of the main stars of the summer sky.