PI-5 ORI (Pi-5 Orionis). There is more to Orion than the bright seven-star figure for which he is so famed. Centered above a line between Betelgeuse and Bellatrix (at the upper right corner) is Meissa (Lambda Ori), which marks his head, while up and to the left of Betelgeuse rise the faint stars of his club, which terminates between Gemini and Taurus. To the right of Bellatrix, the Hunter holds a skin made of a vertical string of stars, all of which are named with the Greek letter Pi and distinguished by calling them, from north to south, Pi-1 through Pi-6. Here we look at number 5, an unusual and under- studied blue-white fourth magnitude (3.72) tight class B double that lies around 1340 light years away (with considerable uncertainty). The spectrum quickly reveals a dominant class B3 giant that is being rocked back and forth by a hot companion that (again from the spectrum) appears to be a class B0 hydrogen-fusing dwarf. The two mutually orbit each other with a period of only 3.7004 days, showing them to be very close, a small fraction of an Astronomical Unit apart. As a result of their proximity (and a rotation of at least 90 kilometers per second), each of the stars is ellipsoidal. As they go around each other in nearly circular orbits, they present different-size faces to the Earth, and thus the binary varies by around 0.07 of a magnitude. With an orbital tilt of perhaps 70 degrees to the plane of the sky, the stars cannot quite eclipse each other. The situation is similar to that of Virgo's Spica. The binary nature of the star was first observed via the spectrum in 1903 and the first spectroscopic orbit determined in 1913 by O. J. Lee. The variability, and the deduction that Pi-5 is (like Spica) an "ellipsoidal variable," was found in 1917 via observations made by Joel Stebbins with the historic 12-inch refractor of the University of Illinois (home base of Skylights, STARS, and the Star of the Week).
Pi-5 Ori
From the discovery paper, Pi-5 Orionis is seen to vary smoothly twice over the orbital period of 3.70 days, showing it to be a tidally distorted ellipsoidal variable that continuously presents different apparent "sizes" to us. The discovery was made around 1917 with the first operating photoelectric photometer, attached to the historic 12-inch refractor at the University of Illinois Observatory. The accuracy, of the order of a hundredth of a magnitude, is remarkable. The photometer, which began the era of electronic astronomy, had also been used to make the first accurate light curve of an eclipsing binary, Algol. (From an article by Joel Stebbins in the Astrophysical Journal, vol. 51, p. 218, 1920.)
Stebbins had been measuring the variations of the eclipsing binary Lambda Tauri using Pi-5 as a reference standard, and found inconsistencies that could only come about if his reference was variable too! (a classic form of discovery). The brightness ratio of the Pi-5's two stars is unknown, so it is not possible to estimate individual luminosities and masses. A "combined" temperature is around 20,800 Kelvin. If all the light (dimmed by 0.2 magnitudes by interstellar dust) were coming from the B3 giant, the luminosity would be around 24,000 times that of the Sun, appropriate to that of a 12 solar mass star. If the combined mass were 12 solar, then the separation would be a mere tenth of an Astronomical Unit, a very untrustworthy number. The spectral data and tilt suggest only a few hundredths of an AU. Though observed for more than a century, Pi-5 remains an enigma.
Written by Jim Kaler 2/06/09. Return to STARS.