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Kaus Borealis

Lambda Sagittarii, 22 Sagittarii

Sagittarius the Archer carries a bow made up from an arc of three prominent stars; Kaus Borealis, Kaus Media and Kaus Australis. The 'Kaus' of these names come from the Arabic for 'bow', with an additional Latin term indicating their position within the bow shape ('Borealis' means 'northern'). The entire bow extends southwards from the Ecliptic, with its nocked arrow pointing westwards towards the heart of the Milky Way, and Kaus Borealis marks the top, or northerm extent, of the Archer's bow. This star also belongs to the asterism known as the Milk Dipper; in this context, it forms the Dipper's handle.

All three of the bow stars are brighter than magnitude +3, and therefore clear and prominent, but Kaus Borealis is the faintest of the three at magnitude +2.81. Nonetheless, it is in fact the closest of the trio to the Solar System, at a distance of about 77 light years. (the middle bow star, Kaus Media, is more than 300 light years distant).

Physically, Kaus Borealis belongs to the class known as subgiants, and is substantially larger and more massive than the Sun without approaching the mass of the true giants. Its surface is rather cooler than that of the Sun (about 4,700K by comparison with the Sun's 5,700K) causing it to shine with the distinctive orange light of a K-type star.


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