Omicron Sagittarii

Lore & Background

This object is positioned 0.86 degrees north of the ecliptic, so ο Sagittarii can be occulted by the Moon and very rarely by planets. The last occultation by a planet took place on 24 December 1937, when it was occulted by Mercury. It was almost eclipsed by the sun, which occupies a mean, rounded, half of one degree of the sky, on 5 January. Thus the star can be viewed the whole night, crossing the sky, in early July.

ο Sagittarii, Latinized as Omicron Sagittarii, is the star's Bayer designation. It is occasionally called by the name Manubrium, usually in an astrological context. This star is part of the Arabic asterism al-Qilāda 'the necklace'; the IAU Working Group on Star Names adopted the name Alqiladah for this star on 13 August 2026.

In Chinese astronomy, 'Establishment' refers to an asterism consisting of ο Sagittarii, ξ² Sagittarii, π Sagittarii, 43 Sagittarii, ρ¹ Sagittarii and υ Sagittarii. Consequently, the Chinese name for ο Sagittarii itself is derived from that asterism.

Reader's Guide

Omicron Sagittarii is notable as a red clump giant, an aging star on the horizontal branch undergoing core helium fusion. Its classification as G9IIIb and its properties—2.39 billion years old, 1.80 solar masses, expanded to 12 solar radii, radiating 67 times the Sun's luminosity at 4,474 K—place it in a late stage of stellar evolution. The star has a faint, magnitude 13.8 companion, designated component B, positioned away along a position angle of 252° as of 2010. Its position 0.86 degrees north of the ecliptic makes it subject to lunar and rare planetary occultations, the last planetary occultation being by Mercury on 24 December 1937. The star's historical names include Manubrium in astrological contexts and Alqiladah, adopted by the IAU Working Group on Star Names in 2026, reflecting its inclusion in the Arabic asterism al-Qilāda. In Chinese astronomy, it is part of the Establishment asterism. Its radial velocity of +26 km/s indicates it is moving away from the Sun, having passed its closest approach at 26.33 pc roughly a million years ago.

Stellar Evolution & Physical Character

Omicron Sagittarii is an aging giant star classified as G9IIIb, placing it firmly on the horizontal branch of stellar evolution. At roughly 2.39 billion years old, it has moved well past the main-sequence phase and is now fusing helium in its core — a hallmark of red clump giants. This evolutionary stage has dramatically reshaped its physical profile: the star has swollen to about 12 times the Sun's radius while packing 1.80 times the Sun's mass into that expanded envelope. Its effective temperature sits at 4,744 K, giving it the warm yellow hue visible to the unaided eye. Despite its cooler surface, the inflated photosphere radiates 67 times the luminosity of our Sun, making it a genuinely bright object in the night sky. A faint companion, designated component B, was catalogued in 2010 at magnitude 13.8, sitting 38.4 arcseconds away along a position angle of 252 degrees, though its physical association with the primary remains uncertain.

Names Across Cultures

The star carries a rich tapestry of names reflecting diverse astronomical traditions. Its Bayer designation, Omicron Sagittarii, is the standard Western identifier, but it also goes by the Latinized name Manubrium, a term more commonly encountered in astrological literature. In Arabic stellar tradition, it belongs to the asterism al-Qilāda, meaning "the necklace," a grouping that evokes a string of jewels strung across the Sagittarius sky. The IAU Working Group on Star Names formally adopted Alqiladah as the star's proper name on 13 August 2026, cementing that Arabic heritage in official nomenclature. Chinese astronomers placed it within the asterism Jiàn, meaning "Establishment," a six-star figure that also includes ξ², π, 43, ρ¹, and υ Sagittarii. Within that group, Omicron Sagittarii holds the second position, earning the designation Jiàn èr — literally "the Second Star of Establishment." These parallel naming systems illustrate how different civilizations mapped and gave meaning to the same point of light.

Observing the Star

At an apparent visual magnitude of +3.77, Omicron Sagittarii is comfortably visible to the naked eye under reasonably dark skies, presenting as a steady yellow point among the more crowded stars of Sagittarius. Its position just 0.86 degrees north of the ecliptic makes it a candidate for lunar occultations and, far more rarely, planetary ones. The most recent planetary occultation occurred on 24 December 1937, when Mercury passed in front of the star. On 5 January of an unspecified year, the star was nearly eclipsed by the Sun, whose mean angular diameter spans roughly half a degree. For observers in the Northern Hemisphere, the optimal window to track Omicron Sagittarii across the entire night sky arrives in early July, when it transits near midnight and remains above the horizon from dusk to dawn. Its distance of approximately 142 light years, derived from parallax measurements, places it in the nearby galactic neighborhood, though it is currently receding from the Sun at a radial velocity of +26 km/s.

Motion Through Space & Proximity

Omicron Sagittarii is not stationary relative to the Solar System. Its radial velocity of +26 km/s indicates it is steadily drifting away from us, adding a small but measurable increase to its distance each year. However, its trajectory has not always been one of recession. Roughly a million years ago, the star passed to within about 86 light years of the Sun — a remarkably close approach for an object that now sits at 142 light years. That ancient flyby would have made it a noticeably brighter and more prominent member of the local stellar field, though no human observer was present to record it. The star's motion through the galactic neighborhood, combined with its 2.39-billion-year age, means it has traversed a significant arc of the Milky Way's disk over its lifetime. Its faint companion, component B, detected in 2010 at 38.4 arcseconds separation, adds a subtle layer to the system's structure, though the wide angular gap raises questions about whether the two stars are gravitationally bound or merely aligned along our line of sight.