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.
Written by Jim Kaler. Return to STARS.