TAU PER (Tau Persei). At mid-fourth magnitude (3.95), one hardly notices Tau Per among the bright and famed stars of Perseus, dominated as it is by the Alpha Persei cluster (featuring Mirfak) and Algol, which is among the most famed stars of the sky and epitomizes eclipsing doubles. Tau Per, however, has its own fascinating -- and mostly unsung -- story to tell. The spectrum clearly shows the signatures of two stars, a dominant class G (G4) giant coupled with a significantly fainter class A (A4) dwarf. Doppler shifts reveal that each orbit the other with a period of 4.419 years, implying reasonably large separation. Extremely short- exposure images (to avoid atmospheric turbulence --"twinkling") resolved the pair and allowed the separation and an orbit to be determined, the two only a few hundredths of a second of arc apart. The odds that the orbital plane would be aligned in the line of sight such that the stars eclipse each other seemed impossibly high. Algol after all orbits in a mere 2.9 days, with the stars separated by only 0.05 Astronomical Units (AU). But sure enough they do. In late 1984, astronomers discovered a small dip in brightness as the smaller A dwarf moved partially in back of the G giant for just two days. A campaign was then mounted to observe the January 1989 eclipse, which involved observatories around the world such that the star could be continuously monitored throughout its short eclipse cycle. The result was the establishment of light curves in three colors, the visual brightness showing a dip of about five percent. Analysis of the combination of direct,
The graph shows the two-day partial eclipse that took place on January 15, 1989, of Tau Per B (the A dwarf) by Tau Per A (the G giant) at visual (yellow) wavelengths (top), in blue light (middle), and in the ultraviolet (bottom). Luminosity is plotted against the angle the stars make with each other. Since the A dwarf radiates more strongly at shorter wavelengths, the eclipse is deeper in the blue and ultraviolet than it is in the visual wavelength band. The continuous U-shaped curves show that the eclipse is partial, that is, that the dwarf is not completely hidden behind the giant. From D. S. Hall and 20 other authors in the Astronomical Journal, vol. 101, p. 1821, 1991.
spectroscopic, and eclipse observations allowed full establishment of orbital and stellar properties, as well as a distance of 272 light years (more accurate than the parallax that gave 248 light years). While the average separation is 4.2 AU, a very high and unusual eccentricity takes the stars from 7.2 AU to 1.13 AU. Tau Per A (the G giant) has a mass of 2.4 Suns and a radius of 14 solar, while Tau Per B respectively comes in at 1.8 solar masses and 2.2 solar radii. The measured respective temperatures of 5160 and 8970 Kelvin give luminosities of 150 and 26 solar, but larger masses of 3.3 and 2.1 solar (no surprise given the various uncertainties). The dwarf is still fusing hydrogen in its core, while the giant -- which started life as class B star (hotter than the dwarf) -- has given it up and is most likely fusing helium into carbon. The secondary eclipse, wherein the A dwarf clips the edge of the G giant, is too subtle to be observed. While the dwarf spins relatively quickly, at 50 km/sec at the equator (for a rotation period of 1.8 days), the giant -- as expected -- spins ponderously with about the same speed as the Sun for a rotation period of at least 165 days. A nearby faint seeming companion, Tau Per C, itself double, is probably just a line-of sight coincidence. All in all, the star, unlike so many, presents us with an amazing amount of information.
Written by Jim Kaler. Return to STARS.