THETA PER (Theta Persei). Perseus is so filled with bright, hot, blue, distant, massive stars -- many grouped in associations with one another -- that it comes almost as a surprise to find a cool nearby one not all that much different from the Sun. Not usually part of the connect-the-dots classical outline of the constellation, fourth magnitude (4.12) Theta Persei (with no proper name) lies not quite seven degrees west of the figure's luminary, Mirfak (Alpha Per), which is centered on a prominent sprawling cluster. With a temperature of 6250 Kelvin, Theta Per is a class F (F8) hydrogen- fusing dwarf just 470 degrees warmer than the Sun. The star is so close, just 36.3 light years away, that its distance is known with remarkable accuracy, well within a tenth of a light year. That (with virtually no correction for radiation outside the visual spectrum) leads to a luminosity just 2.25 times that of the Sun, and a radius 1.3 times as large. The star is SO close that it has been possible to make direct measure of angular diameter, which with distance also gives a radius of 1.3 solar, showing that all our data are quite good, including the resulting mass of 1.25 times that of the Sun. Even the metal content is close to the solar value. However, now we deviate some, as Theta Per is notably younger, just 730 or so million years old, a mere 15 percent of its projected 5-billion-year dwarf lifetime. In contrast the Sun is already five billion years old and has gone through half its original core hydrogen. A projected rotation speed of between 6 and 9 kilometers per second gives a rotation period for Theta Per of under 11 days. There seems, however, to be little in the way of magnetic activity. Theta's main distinction (other than its solar comparison) is that it has a dim 10th (10.0) magnitude red dwarf companion now around 21 seconds of arc away. Theta Per B has been classed as M1, while its absolute magnitude (how bright it would appear at the standard distance of 32.6 light years) makes it out to be M2, the total luminosity (factoring in infrared radiation) only six percent solar, which leads to a mass of 0.6 solar (a bit high for the class). Remarkably, even though the separation is large, there has been enough movement to allow the construction of an orbit, the red dwarf taking 2720 years to go around its brighter companion at an average separation of 248 Astronomical Units. A modest eccentricity makes the distance change between 280 and 216 AU. They were last closest in the year 1613 and will be farthest apart around 2970, a long wait indeed.
Theta Per A dim, 10th magnitude red dwarf companion, Theta Persei B, orbits much brighter Theta-A, the scale in seconds of arc. An ellipse fitted to the short segment of data reveals a period of almost 3000 years and an average separation of just under 250 Astronomical Units. A tilt to the orbital plane of 75 degrees relative to the plane of the sky distorts the curve from what would be seen face-on and makes the orbit appear much more flattened than it really is. The arrow in the lower right corner shows the direction of motion. The stars were closest early in the seventeenth century and are now actually getting farther apart even though, because of the orbital tilt, it does not look like it. In spite of the small section of the path observed, the orbit appears to be remarkably accurate. From the Sixth Catalog of Orbits of Visual Binary Stars, W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.
Application of Kepler's Laws gives a total mass of 2.06 times that of the Sun, just 11 percent greater than that of the mass-sum of the two found from luminosity and temperature, which is remarkable agreement given the relatively short time-base for observation. We can bring the two mass-sums into agreement by lowering the mean separation to 239 AU. Off in the distance, over a minute of arc away, is another dim (11th magnitude) "companion," Theta Per C, but its motion clearly shows it to be a line-of-sight coincidence. Thanks to Bill Hartkopf for hepful comments.
Written by Jim Kaler 10/22/10. Return to STARS.