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A yellow star in Hercules, which stands out more prominently than its third magnitude might suggest because it forms part of the Keystone formation. This asterism is formed by a quadrangle of stars suggesting the trunk of Hercules, of which Eta Herculis forms the northwestern corner. This is the closest bright star in the sky to the Hercules Cluster: taking an imaginary line from Eta Herculis southward to Zeta Herculis (the southwestern corner of the Keystone), the globular cluster designated M13 lies approximately a third of the way along the line.
Eta Herculis is a yellow giant star in the later stages of its life, having expanded to approximately ten times the Sun's diameter. It has some forty times the Sun's luminosity, but nonetheless its outer layers are actually somewhat cooler than those of the Sun. Lying some 111 light years from the Solar System, this star was at one time suspected to be a binary, but its 'companion' is now considered to merely occupy the same line of sight from Earth, making Eta Herculis an optical double.
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