KAPPA-1 CET (Kappa-1 Ceti). From studies of the apparent motions of sunspots across the solar disk, we see that the Sun rotates "differentially." At the equator, it takes 25 days to make a turn, while at higher latitudes it takes closer to 30, the shearing gases coupled with convection helping to create solar magnetic fields and the solar activity that leads to terrestrial aurorae. Other solar type stars exhibit similar activity and should rotate differentially too, but how can you tell when a distant star is but a point and you cannot see the surface? Kappa-1 Ceti -- and an extraordinary orbiting satellite -- lead the way. Kappa-1, in far eastern Cetus off the Sea Monster's head, is an intriguing, heavily studied, fifth magnitude (4.83) class G (G5) solar-type hydrogen fusing dwarf that lies only 29.9 light years from Earth. (Kappa-2, half a degree to the northeast, is an unrelated G8 giant nearly 10 times more distant.) Kappa-1's measured temperature of 5690 Kelvin and luminosity of 85 percent that of the Sun lead to a radius of 0.96 solar, a mass of about 0.9 solar, and an age under 1 1/4 billion years, which allows the star to give us a glimpse of what our own Sun might have been like in its relative youth (the Sun now 4.5 billion years old). Though called "strong lined" (from its strong absorption spectrum), its metal content is only about 10 percent greater than that of the Sun. The fascination with Kappa-1 Ceti comes from its activity. The star has long been known to vary slightly over a period of about 9.2 to 9.3 days, as rotation swings magnetically active features (including starspots) in and out of sight. Spectral indicators of activity also reveal a spot cycle (perhaps similar to the 11-year solar sunspot cycle) of 5.6 years. Kappa-1 has also been know to pop "flares" of huge proportion that may be some kind of super-energetic analogue to those seen on the Sun and that are caused by the collapse of powerful magnetic fields. A star's magnetic field is dragged outward by its wind. Since the field is still anchored at the star, the effect slows the stellar rotation. Kappa-1's rapid rotation suggests a much lower lifetime of only 650 million years! It was among the first stars to be studied with Canada's MOST (Microvariability and Oscillations of STars) satellite, a suitcase-sized package capable of detecting a brightness change of a mere ten-thousandth of a percent, about 0.000001 magnitude. A month's-worth of observation showed a second (and dominant) periodicity of 8.9 days of only a
The brightess of Kappa-1 Ceti, given by F(t) on the vertical axis, varies by about four percent over an 8.9 day period as a prominent spot near the equator moves in and out of view as the star rotates. Superimposed is another variation of 9.3 days caused by a spot at a higher latitude. The solid line shows what would be expected without the second spot. The difference in period is caused by differential rotation. (S. L. Rucinski et al., Publications of the Astronomical Society of the Pacific, vol. 116, p. 1093, 2004.)
few percent in brightness that must be coming from another large spot (or spot group) that is closer to (if not on) the star's equator, and is thus rotating around faster, consistent with differential rotation, making Kappa-1 the first star other than the Sun for which the phenomenon has been seen and measured. Given the projected equatorial rotation speed of 4.64 km/s, the stellar axis is tilted by 60 degrees to the line of sight and the stellar radius must be 0.95 solar, almost exactly that found above from luminosity and temperature. More and more we see solar-type behavior among stars, which in turn will help us understand our own star, which is crucial to understanding -- and caring for -- our own Earth and selves. (Much of the information in this story was taken from S. L. Rucinski et al. in the Publications of the Astronomical Society of the Pacific, vol. 116, p. 1093, 2004. Thanks to Jaymie Matthews.)
Written by Jim Kaler. Return to STARS.