Hidden Companions to Intermediate-mass Stars. XXVI. Uncovering <i>Nunki</i> = Sigma Sagittarii as a 6.5 <i>M</i> _⊙ + 6.3 <i>M</i> _⊙, 0.60 au Binary*

Abstract Nunki = Sigma Sagittarii is a nearby B2.5V star and the 51st visually brightest star in the night sky. There is some contradiction in the literature on whether it is single or a close binary and also on the nature of the possible companion. Here we report on a VLTI/GRAVITY observation which revealed that σ Sgr does indeed have a close companion with a K band flux ratio of 87.72% at a projected separation ρ = 8.644 mas ↔ 0.60 au. We estimate isochrone masses M Aa ≃ 6.5 M ⊙ and M Ab ≃ 6.3 M ⊙ and a system age of 30 Myr. The stars will most likely merge in about 20 Myr, so that Nunki is not only the most massive system within 70 pc but also the closest core-collapse supernova progenitor at the moment, edging Spica by at least a few parsec.