ARCTURUS (Alpha Bootis). Among the very brightest of stars,
shining with a soft orange light, Arcturus lights northern spring
skies. It is one of three luminaries that partition the northern
sky into very rough thirds, the others being summer's Vega and winter's Capella. Of the three, Arcturus, the Alpha
star of the constellation Bootes, the
Herdsman, is slightly the brighter, making it the brightest star of
the northern hemisphere and the fourth brightest star of the entire
sky, following only Sirius, Canopus, and Alpha Centauri. Arcturus, the "Bear Watcher,"
follows Ursa Major, the Great Bear, around
the pole, "arktos" being the Greek name for "bear," from which our
word "arctic" is derived by reference with the constellation of the
Greater Bear. Arcturus is located at a distance of 37 light years,
and became famous when its light was used to open the 1933 world's
fair in Chicago, as that light had left the star at about the time
of the previous Chicago fair in 1893. It is a classic orange class
K (K1) giant star with a precisely defined surface temperature of
4290 degrees Kelvin. To the eye, it shines 113 times more brightly
than our Sun. Its lower temperature,
however, causes it to radiate considerable energy in the infrared.
When this infrared radiation is taken into account, Arcturus
actually shines almost twice as brightly, releasing 215 times more
radiation than our Sun, from which we find a diameter 26 times
solar, about a quarter the size of Mercury's orbit. Arcturus is
close and large enough so that its angular diameter of 0.0210
seconds of arc can easily be measured, leading to a very similar
direct determination of 25 times the solar dimension and providing
nice confirmation of stellar parameters. Arcturus has a velocity
relative to the Sun that is higher than other bright stars.
Compared with the set of surrounding stars, which orbit the Galaxy
on more or less circular orbits, it falls behind by about 100
kilometers per second (as do several others of the "Arcturus
Group"). The lagging movement has long suggested that the star
comes from an older population of the Galaxy. Consistently, it is
somewhat deficient in metals, having only about 20 percent as much
iron relative to hydrogen as found in the Sun. A more intriguing
suggestion is that the star actually comes to us from a small
galaxy that merged with ours some 5 to 8 billion years ago. As a
giant, weighing in at around 1.5 times the mass of the Sun, it has
ceased the fusion of hydrogen in its core. Though it is somewhat
brighter than we would expect for a stable helium fusing star,
helium fusion to carbon has probably already begun. Such stars are
not expected to have magnetic activity like the Sun, but very weak
X-ray emission suggests that Arcturus indeed is magnetically active
and has a hard-to-observe "buried corona."