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.
Written by Jim Kaler. Return to STARS.