Pi Sco

PI SCO (Pi Scorpii). Few celestial sights are more prominent than Scorpius with its bright supergiant (Antares) and its three-star head, from north to south made of Graffias (Beta), Dschubba (Delta), and otherwise un- named Pi Scorpii. While the faintest of the trio, this bright class B spectroscopic double star (made of B1 and B2 hydrogen-fusing dwarfs) still shines at a nice third magnitude (2.89) even though some 520 light years away, placing it in the "Upper Scorpius" subregion of the Sco OB2 association of young stars. In spite of Pi Sco's brightness, it presents some mystery, the result of simple neglect, much of our knowledge coming from educated estimation that tries to reconcile all characteristics into some kind of consistent whole. Best known are the binary characteristics. From the back-and-forth radial velocities from the Doppler effect, the two stars are found to be in circular orbit with a periodof only 1.5700925 days, the B1 dwarf 58 percent more massive than the B1 dwarf. But that is about it. There are

"Velocity curves" for the spectroscopic binary Pi Scorpii (derived from spectral Doppler shifts) show the back-and-forth velocities of each of the two components. The more massive class B1 star has the smaller variation, the less massive B2 star the greater, in inverse proportion to the mass ratio. Since the orbital tilt is (without additional information) unknown, the observed velocities are lower limits to the true velocities, and give lower limits to the orbital sizes and stellar masses. The symmetrical "sine waves" traced out by the stellar velocities reveal circular orbits. (From an article by D. J. Stickland, C. Lloyd, R. H. Koch, and I. Pachoulakis in the The Observatory.)

no temperature measures. The best guesses are 25,000 and 21,000 Kelvin. The best fit to luminosity gives 10,000 and 3000 times that of the Sun, that to mass gives 11 and 9 solar masses. Though the parallax gives 460 light years, we have to raise the distance to the maximum allowed by parallax error, 520 light years, to make it all work and to match the observed visual magnitude. If all these parameters are correct, then the stars must orbit at a separation of just 0.071 Astronomical Units (AU, the distance between Earth and Sun), only 15 solar radii. Given that the stars have estimated radii of about 5 and 4 solar radii, their surfaces are remarkably close together. The closeness of Pi Sco to the ecliptic allows it to be occulted by the Moon, observations of which indeed reveal two stars 0.0003 seconds of arc apart, entirely consistent with the above estimates and with the estimated orbital tilt of about 40 degrees to the plane of the sky (found from the comparison between estimated masses and the lower limits derived from the velocities). The tilt combined with rotation speeds of 90 kilometers per second give true rotation periods of about 1.8 days, consistent with "synchronous rotation," the stars perpetually showing the same "faces" to each other. The pair illuminates the local interstellar dust to create a huge faint reflection nebula 80 light years across. Off in the distance, over 8000 AU away, lies a 12th magnitude companion, probably a class K dwarf (which has its own fainter companion about 90 AU away) that orbits with a period of more than 160,000 years. From the K star, the bright double B stars would be inseparable to the naked eye, and together shine with the light of 30 full moons. The more massive of the two stars of Pi Sco proper is above the cutoff at which stars explode as supernovae, while the lesser one might either explode or become a massive white dwarf. More likely, the stars will engage each other as they expand into helium-fusing giants and will cause each other to lose mass, thus altering normal evolution, one possibly even destroying the other.

Written by Jim Kaler. Return to STARS.