NEKKAR (Beta Bootis). Rising in northern spring evenings, Bootes (the Herdsman) spreads along the
horizon like a giant kite on its side, the southern end pinioned to
brilliant Arcturus, the northern ending
at Nekkar. An eponymous star, one whose very name says it all,
Nekkar comes simply and directly from an Arabic phrase meaning "the
Ox Driver." One might think that such a significant name would go
with significant brightness. Nekkar, however, right at the
juncture between third and fourth magnitude (3.50), ranks only
sixth in brightness in the constellation, and is beat out by both
Gamma (Seginus)
and Delta, as well as by lowly-lettered Epsilon (Izar) and Eta (
Muphrid), the latter two
respectively ranking 2 and 3. Rather than name by brightness,
Bayer rather obviously began with Arcturus, and then went to the
northern end of the constellation and worked southward. Nekkar is
a bit of a curiosity for other reasons as well. A standard class
G giant (one actually used as a spectral standard for class G8)
with a surface temperature of 4950 Kelvin, a bit cooler than the Sun, Nekkar shines with a total luminosity of
190 Suns from a distance of 220 light years (a bit more than double
the luminosity of Virgo's Vindemiatrix). From that and the temperature,
we deduce true giant status, a diameter 19 times solar. Nekkar is
observed to be an X-ray source, and though
a slow rotator (taking about 3/4 of a year for one leisurely spin),
it displays activity similar to that seen on the Sun. While
observing Nekkar in August of 1993, the Rosat X-ray satellite
detected the star to pop a large 10-minute X-ray flare many times
the strength of a typical bright solar flare (which produces an
intense patch of brightness on the solar surface), indicating a
collapse of a magnetic field loop (one generated by rotation).
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 (like Alphard) are usually thought
to be contaminated by companions that once fed them nuclear-
enriched matter and are now dead white dwarfs. Yet there is no
indication that such a companion exists. Nekkar's luminosity and
temperature suggest a mass a bit over 3 times that of the Sun and
an age of perhaps 350 million years. Once a blue class B dwarf,
Nekkar seems about ready to become a much larger and brighter red
giant, as its dead helium core prepares to fire up and fuse itself
to carbon and oxygen.