Sigma Scorpii

Sigma Scorpii, also known by its Bayer designation σ Scorpii, is a multiple star system in Scorpius. It sits near the brighter red supergiant Antares and, with a combined apparent magnitude of +2.88, ranks among the constellation's more luminous members. Parallax data from the Hipparcos mission places the system roughly 696 light-years (214 parsecs) away, with a refined estimate by North et al. (2007) providing a slightly different value. The system comprises a spectroscopic binary—its two components are designated Sigma Scorpii Aa1 (the star formally named Alniyat, which is also the traditional name for the whole system) and Aa2—plus a third star, Sigma Scorpii Ab, located 0.4 arcseconds from the binary pair, and a fourth, Sigma Scorpii B, at about 20 arcseconds.

The traditional name Al Niyat (or Alniyat) comes from the Arabic *al-niyāţ*, meaning "the arteries," and was shared with Tau Scorpii. Both stars flank Antares, the scorpion's heart, with Sigma Scorpii lying just north of it. In 2016, the International Astronomical Union’s Working Group on Star Names (WGSN) decided to assign proper names to individual stars rather than entire systems, and on February 1, 2017, it approved the name Alniyat specifically for Sigma Scorpii Aa1. In Chinese astronomy, Sigma Scorpii, Antares, and Tau Scorpii form an asterism called Heart; Sigma Scorpii itself is known as "the First Star of Heart." The Boorong people of northwestern Victoria, Australia, saw this star and Tau Scorpii as wives of Djuit (Antares). In Hawaiian tradition, the star is called Au-haele and forms a line of three stars with Hōkū-‘ula (Antares) and Paikauhale (Tau Scorpii).

The brightest member, Sigma Scorpii Aa, is a double-lined spectroscopic binary—its two stars cannot be resolved by telescope, but their orbit is deduced from Doppler shifts in their combined spectrum. They complete an orbit every 33.01 days with an eccentricity of 0.32. The primary, Sigma Scorpii Aa1, is an evolved giant classified as B1 III, with about 18 solar masses and 12 solar radii. It is a Beta Cephei variable, causing its apparent magnitude to vary between +2.86 and +2.94 with multiple periods; during each pulsation cycle, its temperature changes. The secondary, Sigma Scorpii Aa2, is a main-sequence star of class B1 V.