PI AUR (Pi Aurigae). From south to north, Theta, Beta (Menkalinan), and Delta Aurigae are in remarkable alignment almost exactly along the "solstitial colure," the great circle in the sky that connects the celestial poles with the summer and winter solstices. As such they act as "pointers" to the north celestial pole and Polaris. Precession (the 26,000-year wobble in the Earth's axis) carried them all across the colure to the west within a couple years of one another, the best alignment occurring around May of 2005. More remarkably, Auriga (the Charioteer) presents yet another colure star, Pi Aurigae, which lies just north of Menkalinan, making it four stars in a row. At fourth magnitude (4.26, almost fifth) the faintest of the set, it is also the most out of the perfect alignment, falling just barely slightly west of the others. As such, precession took it past the colure in October of 2000, a few years earlier than it did the others. Nevertheless, one would have a hard time coming up with anything quite like this curious quartet. The stars, of course, really have nothing to do with one another, the alignment entirely accidental from our perspective in space.
Auriga Map Dominated by Capella, Auriga lies within the Milky Way. Beta Tauri (Elnath), toward the bottom, is also Gamma Aur (see the Greek alphabet). The yellow curve that sweeps from upper right toward lower left is the Galactic Equator, where "180 degrees" marks the Galaxy's Anticenter. Theta, Beta (Menkalinan), Pi, and Delta Aurigae closely line up along the Solstitial Colure (the curved line immediately to the left of them), and thus point the way to the North Celestial Pole and Polaris. The map is based on coordinates for 1950, when the line that connects Theta, Beta, and Delta was to the west of the Colure. Precession has since moved the line to the other side. Theta, Beta, and Delta lay almost exactly on the Colure in mid-2005; Pi passed over a bit earlier, in late- 2000. (See Auriga in context on the larger map.)
A class M (M3) bright giant, Pi also stands out as the reddest and (with a temperature of 3490 Kelvin) the coolest of the quartet, as well as the most massive and the farthest along its evolutionary pathway. From a substantial distance of 840 light years, Pi shines to us (after allowing for a lot of infrared radiation) with a luminosity of just over 9000 Suns (more if there is a bit of dimming by interstellar dust, which could raise the luminosity by up to 20 percent). Temperature and luminosity then yield a radius of 261 times that of the Sun, very close to the value of 265 solar radii (23 percent larger than Earth's orbit) derived from direct measure of the angular diameter by sophisticated interferometry, showing that all parameters are in good shape. Theory then gives a mass of five times that of the Sun, and also reveals that Pi Aur is brightening as a giant for the second time with a dead carbon- oxygen core (as opposed to the first-time brightening with a dead helium core), the C and O having come from advanced helium fusion. Having begun life as a hot class B dwarf some 100 million years ago, and having begun its evolution as a giant only 13 million years ago, the star seems now to be entering a phase in which deep helium- and hydrogen-fusing shells alternately turn on and off, the former violently, leading to a "helium flash" (which is not thought to be visible at the surface). Such stars begin to vary in brightness. Now an irregular variable with a range of about a tenth of a magnitude (about 10 percent), it may someday appear more like Mira. "Visible" in the radio spectrum, Pi Aurigae is also losing mass. Its wind will eventually increase to the point where the star will lose its entire outer envelope, the revealed core to become a massive white dwarf just a bit lighter than Sirius B.
Written by Jim Kaler 2/22/08. Return to STARS.