Sigma Scorpii system

Sigma Scorpii (σ Sco, σ Scorpii) is a star system. It has the traditional name Al Niyat, although this name is sometimes also applied to Tau Scorpii.

The brightest component of the system is a binary pair that completes an orbit every 33.01 days and has an orbital eccentricity of 0.32. The primary component of the spectroscopic binary, σ Scorpii A, is a bright evolved giant star with a stellar classification of B1 III. Its outer envelope has an effective temperature of 26,150 K. This is a variable star of the Beta Cephei type, causing the magnitude to vary with multiple periods of 0.2468429, 0.239671, and 8.2 days. During each pulsation cycle, the temperature of the star varies by 4,000 ± 2,000 K.

The other member of the core pair, σ Scorpii B, is a main sequence star with a classification of B1 V. Orbiting this binary at around 120 AU is σ Scorpii C, which has an orbital period of over a hundred years. Even farther out at more than 4500 AU, is σ Scorpii D, classified as a B9 dwarf.

Given its position, youth, and space velocity, the Sigma Scorpii system is a likely member of the Gould Belt, and in particular the Upper Scorpius subgroup of the Scorpius-Centaurus Association (Sco OB2). Recent isochronal age estimates for the Sigma Scorpii system yield ages of 8–10 million years through comparison of the HR diagram positions for the stars to modern evolutionary tracks. This agrees well with mean age for the Upper Scorpius group which is approximately 11 million years.