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KAUS BOREALIS

(Lambda Sagittarii). Occupying a key position within the constellation Sagittarius, Kaus Borealis makes the northern tip of the Archer's bow, the hybrid name meaning just that, the "northern (Borealis from Latin) bow (kaus, from Arabic). Below it lie the other stars of the bow, Kaus Media and Kaus Australis, and at the tip of the arrow to the right, Alnasl, "the Point." Kaus Borealis also serves as the prominent end of the handle of the famed asterism "The Little Milk Dipper," and moreover as the top of the equally famed "Teapot," which is formed from the Little Milk Dipper and the Bow. Though Bayer (who worked around 1600) is usually thought to have labelled the stars of a constellation rather in order of brightness, he certainly missed with Sagittarius. The Alpha star is only fourth magnitude, while the brightest (Kaus Australis) is called Epsilon (the fifth letter of the Greek alphabet), and Kaus Borealis, at the fainter end of second magnitude (2.81) and ranked fifth, was given the Lambda (eleventh!) designation. Though cool (4700 Kelvin) by solar standards, Kaus Borealis is on the warm side of class K, and is an orange giant star rather similar to Gemini's Pollux. Relatively nearby, its distance of 77 light years shows it to have a luminosity (accounting for some infrared radiation) 52 times that of the Sun. The radius derived from temperature and luminosity and from the star's measured angular diameter (0.0044 seconds of arc) agree nicely at 11 times solar, not all that much for a star called a giant. With a mass about 2.3 times solar, Kaus Borealis is a prime example of what astronomers call a "clump star," one that though dying is currently quite stable and fusing helium into carbon and oxygen in its deep core (the name "clump" coming from a congregation of such stars at about this temperature and luminosity). It is also a modest X-ray source, showing it to have magnetic activity rather like that found on the Sun, something of a surprise. More to home, Kaus Borealis lies in a rich part of the Milky Way, the handle of the Little Milk Dipper pointing to the famed "Lagoon Nebula," a naked eye interstellar cloud that marks a region of fierce star formation and is a lovely sight in binoculars. Just to the west of Kaus Borealis and above the Lagoon lies the winter solstice, the southern-most point of the Sun's path, the star the closest bright marker.