Nunki, the third-brightest star in Sagittarius, has served as a navigational marker for millennia. Until recently it was considered a single, fast-rotating hot star. New research based on data from the VLTI interferometer tells a different story: Nunki is a tight binary system, and it is now the closest known candidate to the Sun for a core collapse event — the final stage in the life of a massive star, often ending in a supernova.
The system lies at a distance of roughly 69 parsecs, or about 220 light-years. On a cosmic scale, that is practically next door. Most stars capable of ending their lives in core collapse sit thousands of light-years farther away.
Piecing the orbit together
The first hint of duplicity came from a VLTI/GRAVITY observation that resolved Nunki into two stars with a projected separation of just 0.60 AU. To pin down the full orbit, researchers Idel Waisberg and Boaz Katz combined this measurement with three archival observations from another instrument, PIONIER, plus one earlier MAPPIT observation.
Putting all the data points together yielded a semi-major axis of 1.26 AU (with an uncertainty of 0.05 AU) and an orbital period of 134.779 days. The two components turned out to be nearly equal in mass — 6.5 and 6.3 solar masses. The orbit's eccentricity came out to 0.492, meaning the stars swing between closer approaches and much wider separations over the course of each orbit.
One detail stands out: the orbital inclination is remarkably low, just 19.7 degrees relative to the line of sight. Combined with a high projected rotational velocity of about 160 km/s for the primary star and the absence of a decretion disk — a feature that usually accompanies rapidly rotating hot stars — this hints at a misalignment between the star's spin axis and the binary's orbital plane.
How the binary hid from spectroscopy
The most striking part of the study is not the binary itself, but how long it went undetected. The authors examined two archival spectra of Nunki and showed the mechanism at play: when two stars have nearly equal mass and their absorption lines are heavily broadened by fast rotation, the radial velocity separation between the components can be several times smaller than the width of the lines themselves.
In that case, the two spectra simply blend into one, and the system looks like an ordinary single star even under careful spectroscopic scrutiny. That explains why Nunki's binary nature was revealed only through direct interferometric measurements rather than classical spectroscopy.
Merger as the likely outcome
The eccentric orbit means that once the primary star expands to roughly 50 solar radii — something massive stars inevitably do as they burn through the hydrogen in their cores — it will overflow its Roche lobe precisely at the point of closest approach in the orbit. This process is known as eccentric Roche lobe overflow.
The most likely outcome of that overflow is a merger of the two stars into a single object exceeding 10 solar masses. From there, the star follows the familiar evolutionary path of a massive single star: exhausting its nuclear fuel, undergoing core collapse, and possibly ending in a supernova.
Nunki against Spica and Bellatrix
The authors compare Nunki to two other core collapse candidates, Spica and Bellatrix, both at a distance of about 77 parsecs. Since Nunki sits closer — at 69 parsecs — it takes over as the nearest known core collapse progenitor candidate to the Sun.
The team also checked Bellatrix with the same GRAVITY instrument and found no close companion with a K-band flux ratio above 1%. That settles an earlier question: Bellatrix is not a close, equal-mass binary, as had previously been suspected.
