PI HER (Pi Herculis). The 88 formal constellations are accompanied in the sky by many informal ones, a variety of beloved asterisms, of which Ursa Major's "Big Dipper" is probably foremost. One of the best known belongs to Hercules, a northern quartet of stars forming the "Keystone," which looks very much like the central keystone in a stone arch that keeps it from falling down. The southwestern corner of the Keystone is anchored by its brightest star, Zeta Herculis, while the northeastern corner is formed by Pi Herculis. An easily visible third magnitude (3.16), this class K (K3) star shines at us from a distance of 370 light years, showing it to be quite luminous. Usually given as a "bright giant" (but sometimes as a supergiant), Pi Her radiates 1330 solar luminosities into space from its coolish 4110 Kelvin surface. Even at that distance, it is so large that its angular diameter is easily measurable. Unlike the Sun, however, large stars have ill-defined and fuzzier "surfaces" such that the size is dependent on the color at which you measure. In the visual spectrum, the star measures 0.055 seconds of arc across, but in the infrared it is larger, 0.061 seconds (the difference caused by the way that the opacity of the stellar gases change with color). The resulting actual radii are 55 and 66 times that of the Sun, the latter corresponding to 0.30 astronomical units, 80 percent the size of Mercury's orbit. Calculation of the size from temperature and luminosity yields a larger value of 72 solar radii, showing that some of the numbers are a bit in error, most likely the allowance for invisible infrared radiation. Weighing in at 4.5 solar masses, born some 140 million years ago, and most likely fusing helium in its deep core, Pi Her was once a blue class B5 dwarf. Though you would never know it with visual observation, Pi Her is a kind of "variable star," the star periodically changing its velocity relative to Earth (found from the spectrum) over a 613 day period. The cause might be an orbiting substellar companion of at least 27 Jupiter masses at a distance of 3 AU from Pi itself, "non radial" pulsations (in which parts of the stellar surface pulse outward while others move inward), or rotation (that must be less than 1320 days) that brings spots in and out of the field of view. Given the star's size, gentle pulsation is probably the best bet.
Written by Jim Kaler. Return to STARS.