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THETA CRB (Theta Coronae Borealis). An unassuming star anchors thenorthwestern end of the semicircle that makesCorona Borealis, the Northern Crown. While well-placedfor observation, it is both under-studied, and -- as a result --not very well understood. Theta CrB is at first appearances justanother hot class B (B6) dwarf with an estimated temperature of14,000 Kelvin (an actual value has never been measured). Becauseof its rather good distance of 310 light years, this star, shiningwith a luminosity of 380Suns (whichincludes a lot of optically invisible ultraviolet radiation),appears as only fourth magnitude (4.14) in our sky. From thetemperature and luminosity (after subtrating the light from acompanion), we find a radius 3.3 times that of the Sun and a heftymass of 4.2 solar. What gives the star its notoriety is itsrotation, which is close to a record. The equatorial spin speed ismeasured at 393 kilometers per second, 200 times that of the Sun! (Since we do not know the tilt of the rotation axis, the speed islikely to be even higher.) Given the radius, the star takes nomore than 10 hours to make a full turn, as opposed to 25 DAYS forthe Sun. Rapid class B rotators somehow generate disks of gasaround their equators, which then radiate, making them "B-emission"("Be") stars. Many are they, topped byGamma Cassiopeiae andZeta Tauri. They can be eruptive aswell, as Gamma Cas andDschubba(Delta Scorpii) will attest. In 1970, Theta CrB did just theopposite, and dimmed by some 0.7 magnitudes (to about 50 percentnormal brightness), perhaps as a result of some kind of dustejection. At the same time it went into a series of brightnessoscillations. It is now very peaceful, as it has been for wellover a decade. For a time its dimming behavior was thought to bethe result of an eclipse by a companion, but that notion has beenput to rest. Theta DOES have a companion however, though typicalfor this star, neither the companion nor the orbit have beenstudied. Now about a second of arc apart, the companion shines atmagnitude 6.6. Given its calculated absolute brightness, it mustbe a class A (A2) dwarf. The primary star thus has a magnitude of4.26 (taken into account in the properties given above). With anestimated mass of 2.5 solar, the companion is separated from theprimary by at least 86 Astronomical Units and must take at least300 years to orbit. With its madly spinning disk, Theta-A must bequite the sight from Theta-B. Born 85 million years ago, Theta-Ahas about 75 million years left before its core hydrogen fuel runsout and it begins to die. Theta-B will follow half a billion yearslater.
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