Gamma¹ Sagittarii

A complex stellar system centred around a variable supergiant, Gamma¹ Sagittarii lies some 1,400 light years from the Sun in the approximate direction of the Galactic nucleus. As seen in the sky, it falls amid a dense part of the Milky Way as it runs through Sagittarius, near that constellation's western border with Scorpius.

Gamma¹ Sagittarii shares its 'Gamma' designation with Gamma² Sagittarii, better known as Alnasl. This is a brighter star less than a degree to the south of Gamma¹ Sagittarii, but the two stars have no connection other than appearing close to one another in the sky. Alnasl is actually rather less luminous than Gamma¹, but it is also very considerably closer to Earth, and so shines as the brighter of the two in the sky. The central stars of Sagittarius form a distinctive asterism known as the Teapot, and Alnasl represents the tip of the Teapot's 'spout', a fact that can help to locate Gamma¹ Sagittarii in the sky. It is the slightly fainter star that lies directly nearby to the north of the 'spout' star Alnasl.

The primary star of the system is a bright yellow supergiant, styled in full as Gamma¹ Sagittarii Aa1, which is estimated to be more than twenty times the Sun's diameter, and more than eight hundred times as luminous. This supergiant is a variable star, and is given its own distinct variable identifier of W Sagittarii. It is a highly regular pulsating variable of the kind known as classical or type I Cepheids. The star's apparent magnitude varies from +5.14 to +4.29 (that is, the star effectively doubles in brightness) over a predictable period of 7 days, 14 hours and 17 minutes. Part of the process that causes this pulsation also causes a change in the star's spectral characteristics, so that it also varies between a bright yellow F-type and a cooler, deeper yellow G-type.

The supergiant Aa1 is actually one member of a spectroscopic binary, with a less massive main-sequence star (Gamma¹ Sagittarii Aa2) in a close orbit. This close pair is in turn orbited by a third star, white Gamma¹ Sagittarii Ab, so that Gamma¹ Sagittarii is a triple or ternary system. To confuse matters slightly, there are two further stars identified as Gamma¹ Sagittarii B and Gamma¹ Sagittarii C, but these are merely optical companions and not truly part of the Gamma¹ Sagittarii system.