NUSAKAN (Beta Coronae Borealis). The name of the second brightest
star in Corona Borealis, the Northern
Crown, has nothing whatever to do with the Crown itself, but refers
to two "lines" of stars that outline a huge Arabic constellation
called "The Pasture," which incorporates much of Hercules and Ophiuchus.
Nevertheless, Nusakan beautifully fits into the semi-circlet that
makes this small, exquisite constellation that lies to the east of
Bootes. Just over the line into fourth
magnitude (3.68), Nusakan is one of the more unusual of the
brighter naked-eye stars. It is first a "spectroscopic" binary
(one detectable through Doppler shifts in the spectrum, caused by
orbital motion), but one in which the two components have actually
been distinguished. Appearing a mere 0.3 seconds of arc apart,
they are separated in space by about 10 astronomical units (about
the distance between Saturn and the Sun). The two orbit with a
period of 10.5 years, the brighter star four times more luminous
than the other. From the combined magnitude and distance of 114
light years, the brighter is found to be 26 times more luminous
than the Sun, the fainter about 7 times
more. Of much more importance is Nusakan's stellar class. It is
usually classified as a hot F (F0) dwarf (one fusing hydrogen into
helium in its core), but one with a difference, F0p, where the "p"
stands for "peculiar." Nusakan is a classic "chemically peculiar"
star in which some chemical elements are wildly altered relative to
others. Oxygen is extremely deficient, whereas elements such as
strontium, chromium, and europium (one of the "rare earths") are
powerfully enriched. All of these chemically peculiar dwarfs -- of
which there are several varieties -- are of warm classes F, A, and
B. The stars are not making the enriched elements. Like most of
the "peculiar" stars, Nusakan is a slow rotator, spinning only once
every 18.5 days, not much less than the period of solar rotation.
Moreover, the atmospheres of stars in these classes are not in a
state of convection. In the quiet unstirred atmospheres, the atoms
of some chemical elements fall downward, while others are lifted up
by radiation. Nusakan and others of the "p" variety (including
bright Cor Caroli) also have powerful
spotty magnetic fields in which various elements are concentrated.
As the stars rotate and the spots go in and out of the field of
view, the magnetic fields and the apparent chemical compositions
vary with time. Nusakan's magnetic fields are very strong, well
over 10,000 times that of the Earth (over double strength of
sunspot magnetic fields). The peculiar spectra make the stars
difficult to classify. Nusakan's temperature is actually 8300
Kelvin, more appropriate to that of a warm class A star. From its
luminosity and temperature we find a radius 2.5 times that of the
Sun and a mass about twice solar, the companion's mass perhaps 1.3
solar or so.