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DELTA CRB (Delta Coronae Borealis). At first glance, Delta CoronaeBorealis, a modest just-over-the-line fifth magnitude (4.63) starinCorona Borealis, the Northern Crown,looks like any other common evolving single (no companion known)giant. Two exceptions bring it to notice, however. First, thisclass G (G3.5) star is unusually warm, dim, and small for a giant,with a temperature of 5150 Kelvin, just 630 degrees cooler than ourSun. At a distance of 165 light years, itshines 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 thestar's rather rare state of evolution. When stars run out of theircore hydrogen fuel (after which they fire up a shell of hydrogenfusing to helium around the contracting dead helium core), theyfirst cool with roughly constant luminosity before they finallyincrease their sizes and luminosities to becomegiants in the true sense of theword. The passage at (sort of) constant luminosity is swift, therealm of cooling temperature through which they pass called the"Hertzsprung Gap" (after the astronomer Ejnar Hertzsprung). DeltaCrB has just finished the "crossing," and is consistently alsoclassified 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 isprimed to begin its serious expansion. The star is better known,however, for its sunlike magnetic activity. It varies slightly bya few hundredths of a magnitude over a period of 59 days, whichseems to be caused by starspots that swing in and out of view asthe star rotates. (The variability is consistent with an observed,projected 5 km/s rotation speed and an axial tilt of about 45degrees to the line of sight.) Moreover, Delta CrB is a potentsource of X-rays, implying an active, magnetically heated outercorona and chromosphere, the latter the thin layer between thestar's cool surface and outer coronal gases. The corona seems tobe heated to temperatures of close to 7 million Kelvin, much hotterthan the Sun's 2 million Kelvin coronal temperature. Constantflaring may raise the temperature to 10 million. Even a long termcycle similar to the 11-year sunspot cycle is suspected. Manydwarf stars (those like the Sun) are found to have spot andactivity cycles (caused by a rotation/convection dynamo), but themore ponderously rotating giants are not usually so inclined,making Delta Coronae Borealis a real oddball among the set.
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