Alpha Bootis - Arcturus

Arcturus, or α Bootis, lights northern spring skies with a soft orange light. At magnitude -0.05, it the brightest star in the northern hemisphere, and is the fourth brightest star in the night sky (following only Sirius, Canopus, and Alpha Centauri). Arcturus, whose name means "Bear Watcher," follows the Great Bear, Ursa Major, around the pole. The Greek word for bear is "arktos", from which our word "arctic" is also derived, in reference to the northern polar constellations of the Greater and Lesser Bears.

Properties

At 37 light-years away, Arcturus is the closest giant star to Earth. Its light was used to open the 1933 Chicago World's Fair, as that light had left the star during the previous Chicago World's Fair in 1893. Arcturus is a classic orange class K1 III giant star with a surface temperature of 4290 K. In visible light, its luminosity is 113 times the Sun's. Because of its lower temperature, however, it radiates considerable energy in the infrared, and when this is taken into account, Arcturus' total luminosity is 215 times the Sun's. From this, we find its diameter to be 26 times solar. Arcturus is large and close enough that its angular diameter can easily be measured, at 0.0210". This provides a direct determination of its diameter (25 times the Sun's), which nicely confirms the other parameters.

Evolution

As a giant, containing about 1.5 times the Sun's mass, Arcturus has ceased fusing 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 our Sun, but very weak X-ray emission suggests that Arcturus indeed is magnetically active and has a "buried corona" hard to observe.

Proper Motion

Arcturus has a relatively large proper motion, and its position in the sky changes significantly over the centuries. It has moved over 1/2 degree in the past thousand years. Compared with the surrounding stars, whose orbits around the galactic center are more-or-less circular, Arcturus falls behind by about 100 km/sec. So do several others of the "Arcturus Group". This lagging movement suggests that the star comes from an older galactic population. Consistently, it is somewhat deficient in metals, having only about 20% as much iron relative to hydrogen as does 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.