PI PEG (Pi Pegasi), both PI-1 AND PI-2. Appearing right next to each other, separated by but 10 minutes of arc (1/6 degree), the two Pi's of Pegasus can hardly be taken separately. Though they may remind us of Mizar and Alcor, appearing to be something of a naked-eye double star, they are unrelated, yet present a pretty sight (use binoculars) that is closer to the stars of Lacerta, the Lizard, than to the prominent Square than makes the bulk of Pegasus. The numbers relate not to brightness rank, but to position, just-barely-sixth magnitude (5.59) Pi-1 to the west of brighter fourth magnitude (4.29) Pi-2. While just an "optical" line-of-sight double (not related by gravity), the two are still fairly close together, Pi-1 283 light years away, Pi-2 252 light years, making them just 31 light years apart. Given the errors in the measured parallaxes, they could actually be at the same distance, which would separate them by but 0.22 light years, or 47,000 Astronomical Units. They are, however, moving in quite different directions at different speeds, separating at some 25 kilometers per second, so there is no question about their being unrelated; they are just visiting each other. Rather oddly, Pi-1 and Pi-2 Peg are similar stars with similar ages, respectively class G (G6) and F (F5) dying giants, though in different stages of "gianthood." Pi-1, the fainter, has a temperature of 4790 Kelvin, a luminosity 51 times that of the Sun, a radius 10 times solar (not so much for a "giant"), and a mass double solar. It is a "clump star" some 600 million years old that is quietly fusing helium into hydrogen in its deep core ("clump" referring to the large numbers of similar stars that lie plotted on a graph of temperature vs. luminosity). Slowly rotating with a velocity of at least 6 kilometers per second, it may take as much as 89 days to make a full spin. Nearby lie four 10th through 12th magnitude stars that are also accidental line-ups and not real companions. Pi-2 is much more interesting. A warmer temperature of 6320 Kelvin and a luminosity of 92 solar (radius 8 solar) reveal a larger mass of 2.5 solar and tell of a relatively rare kind of star with a quiet helium core that is in transition to becoming a true helium-burner. Pi-2 is also a rare class F "shell star," one with a circumstellar disk of surrounding matter that has an unusually large rotation speed for a giant, 145 kilometers per second, which gives a rotation period less than 1.8 days. Rotations slow as stars expand (conservation of angular momentum, the same reason skaters speed up as they bring in their arms), but Pi-2 is not far enough along for much braking to have taken place. About 590 million years ago it was a rapidly spinning class B8 star, and is therefore probably the descendent of a disk- encased B-emission star (similar to, say, Zeta Tauri). Consistently, the star reveals a magnetically heated outer chromosphere (produced in part by rotation). If nothing else, the pair shows us how coincidences abound in astronomy and why measurements are needed to ferret out the relational truth. And though unrelated, the stars' brief visitation makes them prominent in each others' skies. If at their stated distances, Pi-1 is first magnitude (0.8) as seen from Pi-2, while Pi-2 glows at magnitude zero (-0.2) from Pi-1.
Written by Jim Kaler. Return to STARS.