BETA PHE (Beta Phoenicis). Fairly bright third magnitude (3.31) Beta Phoenicis, as expected the second-brightest star in the modern constellation of Phoenix (the mythical Firebird, whose luminary is second magnitude Ankaa), has a problem: no accurate distance. Fundamental stellar distances are determined through their parallaxes, the minute angular shifts in position caused by the Earth going about the Sun. The first parallax, and thus the first measured stellar distance, that of the nearby double star 61 Cygni, goes back to 1838, the shift a mere 2/3 of a second of arc (giving a distance of just under 10 light years). Astronomers have been refining the techniques ever since. The parallax business, at least for "local" stars, was extraordinarily advanced in 1997 by the publication of the space- based observations by the Hipparcos satellite, which had the capability of measuring accurate distances into the hundreds of light years. It's most unusual therefore to find a relatively bright, and by modern standards fairly nearby, star with no precise distance. The estimated error on the Hipparcos result for Beta Phe is more than 100 times the value itself. The observation was probably confused by the fact that the star is a close orbiting double with nearly identical class G (G8) giant components of magnitude 4.0 and 4.1 (after scaling to the star's brightness as seen as a single unit). Double giants, epitomized by Capella, are rather unusual, as both stars must evolve at the roughly the same time from dwarfs of closely- similar masses. The two go around each other on rather highly elliptical orbits with a period of 168 years. The angular orbital size is well known at 0.338 seconds of arc, but to translate that into a physical separation requires the distance.
Beta Phe Slightly fainter Beta Phoenicis B is plotted as if it goes about Beta Phe A (at the cross). Actually, the two go about a common center of mass that lies almost exactly mid-way between them. On a 168-year, highly elliptical orbit (e=0.72) that is tilted by 34 degrees to the plane of the sky, "B" was last closest to "A" in mid-2003. The scale is in seconds of arc: note how close the stars are to each other. As seen here, Beta Phe B orbits in the counterclockwise direction: look at how much more accurate the later observations are. (From W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)
A questionable parallax from a ground-based observatory gives 165 light years, while adopting a typical absolute visual brightness for a G8 giant gives 200 light years. Assuming a rough average of 180 light years then gives an orbital size of 52 Astronomical Units, the two looping between 96 and 16 AU apart (the pair closest in 2003). Kepler's laws then yield masses of about 2.5 solar for each of them. Taking a different approach, from the typical absolute visual brightness and an adopted temperature of 4950 Kelvin (from one measurement and a typical G8-giant value), the stars shine at roughly 100 solar luminosities apiece, which implies approximate masses of 2.7 Suns if they are ordinary giants fusing helium into carbon and oxygen, or 3 Suns if they are just becoming giants with dead helium cores. Given the huge uncertainties, the agreement with the double-star masses is remarkably good, suggesting that the above distance is not too far off, though that is not much to say about such a bright and nearby star.
Written by Jim Kaler 11/11/11. Return to STARS.