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Gamma1 Sagittarii

W Sagittarii (Aa1 component)

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.

Gamma1 Sagittarii shines against one of the densest parts of the Milky Way, in the approximate direction of the Galaxy's core. Imagery provided by Aladin sky atlas

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.


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