Nunki, Sigma Sagittarii (σ Sgr), looks like a single bright point on the handle of Sagittarius’s Teapot. Its visual magnitude is about 2.1. That familiar point has two stories behind it: modern observations have uncovered a close stellar pair, while scholars have untangled a mistaken identification behind its ancient-sounding name.
The blue-white light belongs to hot B-type stars. Sigma Sagittarii has long served as a standard for the B2.5 V spectral class: B describes the hot stellar spectrum, 2.5 refines that classification, and V is the main-sequence luminosity class. Main-sequence stars generate their central energy by fusing hydrogen. The 2026 spectral analysis estimates an effective temperature around 18,500 kelvin, more than three times the Sun’s surface temperature. That heat gives the pair its blue-white appearance.
A 2025 study by Idel Waisberg, Ygal Klein and Boaz Katz resolved Nunki with the Very Large Telescope Interferometer’s GRAVITY instrument. The companion supplied nearly 88 percent as much K-band infrared light as the brighter component. The authors estimated masses of about 6.5 and 6.3 Suns and an age around 30 million years. Their measured angular separation was only about 8.6 milliarcseconds — 0.0086 of an arcsecond — corresponding to a projected separation of roughly 0.60 astronomical unit at that observation. This was a snapshot of the apparent spacing, not a complete measurement of the orbit. An astronomical unit is the Earth–Sun distance.
In March 2026, Waisberg and Katz combined observations from several epochs to solve that orbit. They found a period of 134.779 days, a relative orbital semimajor axis of about 1.26 astronomical units, and eccentricity about 0.49: the spacing varies substantially around an elongated orbit. At roughly 69 parsecs, or 225 light-years, the pair remains unresolved in an ordinary backyard view. The authors also explain how rapid rotation broadens the two stars’ spectral lines, helping a similar pair hide in what looks like a single stellar spectrum.
The close orbit matters for their eventual evolution. As a component expands, the system may exchange material and possibly merge into a more massive star. The 2026 authors therefore describe Nunki as a candidate for the nearest future core-collapse supernova progenitor. That is a proposed evolutionary outcome, not a prediction of an imminent explosion or a guaranteed fate.
The name has an equally surprising double identity. The IAU Working Group on Star Names’ etymology researchers trace Nunki to the Sumerian term for a celestial object associated with Eridu, an ancient Mesopotamian city. But they conclude that its modern attachment to Sigma Sagittarii arose from a nineteenth-century misreading of cuneiform material. Modern scholarship places the ancient designation in a different southern sky region, associated with the old Argo constellation, rather than Sagittarius. The name has ancient roots; its assignment to this particular star is a later historical transfer.
For finding your way, start with the whole Teapot — spout, body, lid and handle — then match Nunki to its plotted place on the handle. The Teapot is an asterism within Sagittarius, so its recognizable outline is useful even when the larger Archer figure is less obvious. Kaus Australis provides another conspicuous point in the pattern. Confirm the arrangement of several stars before beginning a hop; brightness alone can lead you to the wrong starting point.
The chart’s bright point is the pair’s combined light, not two visually separable stars. A nearby planet is a temporary visitor: use the current week’s position for the final hop. Visibility still depends on daylight and the horizon; being a bright guide does not make Nunki observable at every season or hour.
Sources: Waisberg, Klein & Katz (2025), Research Notes of the AAS 9, 71, doi:10.3847/2515-5172/adc739; Waisberg & Katz (2026), arXiv:2603.17011; The IACOB project XII, doi:10.1051/0004-6361/202449298; IAU-WGSN Etymology Group, All Skies Encyclopaedia, Nunki (ase.exopla.net/index.php/Nunki).
How to find it: Identify Sagittarius’s Teapot, then locate Nunki on its handle using the finder’s labeled stars. Match the whole pattern before following the current chart to a nearby target.
