DELTA AUR (Delta Aurigae). Given that it carries the fourth letter of the Greek alphabet, one would think that Delta Aurigae -- the most northerly star in the classic constellation of Auriga, the Charioteer -- would shine much brighter than fourth magnitude (3.72). Instead, it is beaten out by all five stars that make the prominent pentagon to its south: Alpha (Capella), Beta (Menkalinan), Gamma (Elnath, Beta Tauri) as expected, but also by Iota (Al Kab) and Theta. Even two of the three "Kids" (Epsilon and Eta) are brighter. Adding to the star's obscurity is its nature as yet another class K (K0) helium- fusing giant that has few if any outstanding characteristics. But like the most important thing in real estate, what it does have is location. The front-bowl-star Pointers of Ursa Major's Big Dipper direct the eye to Polaris and the North Celestial Pole. They can't, however, compare with the three star lineup of (from south to north) Theta, Beta, and Delta Aurigae. In near-perfect northerly alignment, the three stars are more accurate in pointing to the northerly pole than any other set in the sky. Moreover, in one of the most remarkable of coincidences, all three are within a few minutes of arc of the "solstitial colure," the great circle that connects the celestial poles with the Summer and Winter Solstices in Sagittarius and Gemini. (The "equinoctial colure" connects the poles and equinoxes.) You can use the stellar trio as a little "dotted line" to trace out the solstitial colure. To the south, the colure passes just to the east of Betelgeuse in Orion. On current star maps the trio lie just to the west of the colure. Precession (the 26,000-year wobble of the Earth's axis), however, is moving them slowly east relative to
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.)
Between the beginning of 2004 to near the middle of 2006, one after the other, they crossed it, the best alignment occurring in May of 2005. Over the years, they will lose near-perfect alignment to the pole, but until the far-distant future (when actual stellar motions become important), they will maintain a fine direction toward Polaris, which itself moves relative to the pole (in actuality, the stars staying in place, the coordinate grid actually doing the moving). To these three we can also add Pi Aurigae, which is only slightly off the line. Delta Aur itself, with a surface temperature of 4825 Kelvin, lies 140 light years away, and shines with the light of 47 Suns, from which we find a radius of 12 times solar. This quiet helium-burner has a metal content that is closely solar. From a projected equatorial rotation speed of 1.7 kilometers per second, the star -- if the rotation axis is perpendicular to the line of sight -- takes nearly a year to rotate. The theory of stellar structure yields a mass about double that of the Sun and an age of about 1.3 billion years. Delta Aur is listed as a quadruple, with three faint "companions," a 10th magnitude star three minutes of arc away and an 11th magnitude one two minutes off, which itself seems double. Alas, the motions over the years show all to be merely line of sight coincidences, not surprising given the star's proximity to the Milky Way. (Thanks to Jerry Diekmann.)
Written by Jim Kaler 2/1/08. Return to STARS.