THETA CRB (Theta Coronae Borealis). An unassuming star anchors the
northwestern end of the semicircle that makes Corona Borealis, the Northern Crown. While well-placed
for observation, it is both under-studied, and -- as a result --
not very well understood. Theta CrB is at first appearances just
another hot class B (B6) dwarf with an estimated temperature of
14,000 Kelvin (an actual value has never been measured). Because
of its rather good distance of 310 light years, this star, shining
with a luminosity of 380 Suns (which
includes a lot of optically invisible ultraviolet radiation),
appears as only fourth magnitude (4.14) in our sky. From the
temperature and luminosity (after subtrating the light from a
companion), we find a radius 3.3 times that of the Sun and a hefty
mass of 4.2 solar. What gives the star its notoriety is its
rotation, which is close to a record. The equatorial spin speed is
measured at 393 kilometers per second, 200 times that of the Sun!
(Since we do not know the tilt of the rotation axis, the speed is
likely to be even higher.) Given the radius, the star takes no
more than 10 hours to make a full turn, as opposed to 25 DAYS for
the Sun. Rapid class B rotators somehow generate disks of gas
around their equators, which then radiate, making them "B-emission"
("Be") stars. Many are they, topped by Gamma Cassiopeiae and Zeta Tauri. They can be eruptive as
well, as Gamma Cas and Dschubba
(Delta Scorpii) will attest. In 1970, Theta CrB did just the
opposite, and dimmed by some 0.7 magnitudes (to about 50 percent
normal brightness), perhaps as a result of some kind of dust
ejection. At the same time it went into a series of brightness
oscillations. It is now very peaceful, as it has been for well
over a decade. For a time its dimming behavior was thought to be
the result of an eclipse by a companion, but that notion has been
put to rest. Theta DOES have a companion however, though typical
for this star, neither the companion nor the orbit have been
studied. Now about a second of arc apart, the companion shines at
magnitude 6.6. Given its calculated absolute brightness, it must
be a class A (A2) dwarf. The primary star thus has a magnitude of
4.26 (taken into account in the properties given above). With an
estimated mass of 2.5 solar, the companion is separated from the
primary by at least 86 Astronomical Units and must take at least
300 years to orbit. With its madly spinning disk, Theta-A must be
quite the sight from Theta-B. Born 85 million years ago, Theta-A
has about 75 million years left before its core hydrogen fuel runs
out and it begins to die. Theta-B will follow half a billion years
later.