70 days. That's exactly how long the plateau, the steady glow after the blast, lasted for supernova SN 2018gj. Usually, these plateaus stretch a hundred days or more. This short plateau was the first crack in the star's mask.

Images from the Hubble Space Telescope pinpointed the progenitor: a red supergiant. Enormous. Cold. On its last breath. Spectroscopy measured its temperature—3470 K. Photometry pegged its luminosity at 100,000 Suns. The helium core, where hydrogen had long since fused, had grown to 4.6 solar masses. But oddities piled up. The hydrogen envelope: three and a half solar masses. For a typical II-P, it's twice as thick. And the age of the surroundings: more than 40 million years. A single star of that mass lives 30 million, so this one had lingered.

Numerical simulations with the POSYDON code sifted through thousands of scenarios. Bayesian analysis ruled out single-star evolution — probability near zero. But for a binary path: over 99%. Two stars, 8–9 solar masses each, orbited with a period of several days. They merged. Helium cores combined, envelopes were ejected. Thus was born a red supergiant with an anomalously thin hydrogen coat. A simple formula — log₁₀($M_{He}$/M_☉) = 0.659 log₁₀(L/L_☉) – 2.630—links luminosity to core mass and fits the data perfectly.

Now 70 days isn't just an anomaly. It's a marker. By measuring plateau duration, environment age, and envelope thickness, telescopes like Euclid, Roman, and LSST can fish out hidden mergers among thousands of supernovae. Each discovery redraws the map of chemical evolution in galaxies. And answers an old question: why astronomers couldn't find the progenitors of the most massive supernovae. The mask is off. But how many more secrets are hidden in stellar cores?