DELTA CRB (Delta Coronae Borealis). At first glance, Delta Coronae
Borealis, a modest just-over-the-line fifth magnitude (4.63) star
in Corona Borealis, the Northern Crown,
looks like any other common evolving single (no companion known)
giant. Two exceptions bring it to notice, however. First, this
class G (G3.5) star is unusually warm, dim, and small for a giant,
with a temperature of 5150 Kelvin, just 630 degrees cooler than our
Sun. At a distance of 165 light years, it
shines with the light of but 36 Suns, its radius only 7.6 solar,
which is tiny by the usual giant standards. The reason lies in the
star's rather rare state of evolution. When stars run out of their
core hydrogen fuel (after which they fire up a shell of hydrogen
fusing to helium around the contracting dead helium core), they
first cool with roughly constant luminosity before they finally
increase their sizes and luminosities to become giants in the true sense of the
word. The passage at (sort of) constant luminosity is swift, the
realm of cooling temperature through which they pass called the
"Hertzsprung Gap" (after the astronomer Ejnar Hertzsprung). Delta
CrB has just finished the "crossing," and is consistently also
classified as a "giant-subgiant." Having started life as a cool-
end class B star with a mass of 2.3 times that of the Sun, it is
primed to begin its serious expansion. The star is better known,
however, for its sunlike magnetic activity. It varies slightly by
a few hundredths of a magnitude over a period of 59 days, which
seems to be caused by starspots that swing in and out of view as
the star rotates. (The variability is consistent with an observed,
projected 5 km/s rotation speed and an axial tilt of about 45
degrees to the line of sight.) Moreover, Delta CrB is a potent
source of X-rays, implying an active, magnetically heated outer
corona and chromosphere, the latter the thin layer between the
star's cool surface and outer coronal gases. The corona seems to
be heated to temperatures of close to 7 million Kelvin, much hotter
than the Sun's 2 million Kelvin coronal temperature. Constant
flaring may raise the temperature to 10 million. Even a long term
cycle similar to the 11-year sunspot cycle is suspected. Many
dwarf stars (those like the Sun) are found to have spot and
activity cycles (caused by a rotation/convection dynamo), but the
more ponderously rotating giants are not usually so inclined,
making Delta Coronae Borealis a real oddball among the set.