NUSAKAN (Beta Coronae Borealis). The name of the second brightest star in Corona Borealis, the Northern Crown, has nothing whatever to do with the Crown itself, but refers to two "lines" of stars that outline a huge Arabic constellation called "The Pasture," which incorporates much of Hercules and Ophiuchus. Nevertheless, Nusakan beautifully fits into the semi-circlet that makes this small, exquisite constellation that lies to the east of Bootes. Just over the line into fourth magnitude (3.68), Nusakan is one of the more unusual of the brighter naked-eye stars. It is first a "spectroscopic" binary (one detectable through Doppler shifts in the spectrum, caused by orbital motion), but one in which the two components have actually been distinguished. Appearing a mere 0.3 seconds of arc apart, they are separated in space by about 10 astronomical units (about the distance between Saturn and the Sun). The two orbit with a period of 10.5 years, the brighter star four times more luminous than the other. From the combined magnitude and distance of 114 light years, the brighter is found to be 26 times more luminous than the Sun, the fainter about 7 times more. Of much more importance is Nusakan's stellar class. It is usually classified as a hot F (F0) dwarf (one fusing hydrogen into helium in its core), but one with a difference, F0p, where the "p" stands for "peculiar." Nusakan is a classic "chemically peculiar" star in which some chemical elements are wildly altered relative to others. Oxygen is extremely deficient, whereas elements such as strontium, chromium, and europium (one of the "rare earths") are powerfully enriched. All of these chemically peculiar dwarfs -- of which there are several varieties -- are of warm classes F, A, and B. The stars are not making the enriched elements. Like most of the "peculiar" stars, Nusakan is a slow rotator, spinning only once every 18.5 days, not much less than the period of solar rotation. Moreover, the atmospheres of stars in these classes are not in a state of convection. In the quiet unstirred atmospheres, the atoms of some chemical elements fall downward, while others are lifted up by radiation. Nusakan and others of the "p" variety (including bright Cor Caroli) also have powerful spotty magnetic fields in which various elements are concentrated. As the stars rotate and the spots go in and out of the field of view, the magnetic fields and the apparent chemical compositions vary with time. Nusakan's magnetic fields are very strong, well over 10,000 times that of the Earth (over double strength of sunspot magnetic fields). The peculiar spectra make the stars difficult to classify. Nusakan's temperature is actually 8300 Kelvin, more appropriate to that of a warm class A star. From its luminosity and temperature we find a radius 2.5 times that of the Sun and a mass about twice solar, the companion's mass perhaps 1.3 solar or so.
Written by Jim Kaler. Return to STARS.