ArcturusArcturus, Alpha Boötis, is the fourth brightest star in the sky at magnitude -0.05.
Declination: 19°10'57" Right Ascension: 14h15m39.7s
It is a K1.5 IIIpe red giant star -- the letters "p" and "e" stand for "peculiar" and "emission", which indicates that the spectrum of light given off by the star is unusual and full of emission lines. This is not too unusual for red giants, but Arcturus has a particularly bad case of the phenomenon. It is 110 times more luminous than the Sun, but this underestimates its strength as much of the "light" it gives off is in the infrared; total power output is about 180 times than of the Sun.
According to the Hipparcos satellite, Arcturus is 36.7 light years (11.3 parsecs) from Earth, relatively close as these things go. The precise observations by this orbiting observatory have also added two recent facts to our knowledge of the star. First, it is now known to be slightly variable, by about 0.04 magnitudes over 8.3 days. It is believed that the surface of the star oscillates slightly, a common feature of red giant stars. In the case of Arcturus, this was an interesting discovery as it is known that the redder (more towards or within the M spectral class) a giant gets, the more variable it will be. Extreme cases like Mira undergo large swings over hundreds of days; Arcturus is not very red and is a borderline case between variability and stability with its short period and tiny range.
Hipparcos also suggested that Arcturus is a binary star, with the companion about twenty times dimmer than the primary and orbiting close enough to be at the very limits of our current ability to make it out. The most recent studies of the issue are generally coming down on the side of it being a single star, however.
The name of the star derives from Ancient Greek Arktouros and means "Bear Guard". This is a reference to it being the brightest star in the constellation Boötes, the Hunter, which is next to the Big and Little Bears, Ursa Major and Ursa Minor.