Nekkar, or β Bootis, lies at the northern end of Bootes, the Herdsman. At magnitude 3.50, it ranks sixth in brightness in the constellation. The name Nekkar comes from an Arabic phrase meaning "the Ox Driver."
Properties and Evolution
Nekkar is used as a spectral standard for class G8. With a surface temperature of 4950 K, a bit cooler than the Sun, Nekkar shines with a total luminosity of 190 Suns from a distance of 220 light years. From that and the temperature, we deduce a diameter 19 times solar.
Nekkar's luminosity and temperature suggest a mass 3 times the Sun's, and an age of perhaps 350 million years. Once a blue main-sequence star of class B dwarf, Nekkar seems about ready to become a much larger and brighter red giant, as its helium core prepares to fuse into carbon and oxygen.
X-Ray Flare
Nekkar is also an X-ray source, and though it rotates slowly, taking about 3/4 of a year for one turn, it displays activity similar to that seen on the Sun. While observing Nekkar in August 1993, the Rosat X-ray satellite detected a large 10-minute X-ray flare many times the strength of a typical bright solar flare, indicating a collapse of a magnetic field loop. Nekkar is the first known single cool giant star to display such activity.
But is it single? Nekkar is also classed as a marginal "barium star," one enriched in barium and other elements. Such stars are thought to be contaminated by companions that once fed them nuclear-enriched matter and are now dead white dwarfs. But there is no indication that such a companion exists in the β Boo system.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]