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 (K1), 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.