EPS SER (Epsilon Serpentis). Serpens, the Serpent, brackets its bearer, Ophiuchus, the two forever locked in both mythology and in the sky. Together, they have come down to us as the caduceus, the symbol of medicine. Serpens comes in two parts, Serpens Caput (the Head, to the west of Ophiuchus) and Serpens Cauda (the Tail, to the east), the Greek letters naming the stars divided between the two. The luminary, bright third magnitude Unukalhai (Alpha Serpentis), lies in Serpens Caput. Just to the southeast of it is dimmer (typical of the relatively faint constellation) fourth magnitude (3.71) Epsilon Serpentis (of no proper name, that honor reserved for Unukalhai and Alya, Theta Ser). While seeming at first to be just another white, Vega-like class A (A2) dwarf, it's one with a difference, actually classed as "Am", showing it to be a "metallic-line star," one whose spectrum and thus outer atmosphere are rich in a wide variety of metals, while depleted in others. First, though, a run-down on the stellar characteristics. The distance of 70.5 light years is known to considerable accuracy, half a percent. Typical of Am stars, the temperature of 8400 Kelvin is lower than that expected from the class (more like that expected from an A5 star), the result of the anomalous chemical composition. After accounting for a bit of ultraviolet radiation, the luminosity then comes in at 12.4 times that of the Sun and the radius at 1.7 solar. With a mass of 1.85 times that of the Sun, the fairly young star is roughly a quarter of the way through its 1.4-billion-year hydrogen- fusing lifetime. A relatively slow (for class A stars) rotation velocity of at least 39 kilometers per second gives a rotation period of under 2.1 days. And therein lies the star's essence. Fast rotation stirs up a star's spectrum-forming atmosphere, sort of homogenizing it. In a quieter atmosphere among stars of this class, some "metal" (the astronomical shorthand for anything heavier than helium) atoms sink under the influence of gravity (and become depleted), while others rise under the force of outbound radiation. (Such an effect does not happen in the turbulent, convective solar atmosphere.) Epsilon Serpentis provides a fine example. Calcium is present at 15 percent that of normal (solar), scandium at 5 percent, just as expected. But then look at the overabundances. Iron is up by 80 percent, nickel by a factor of nearly 3. Then things soar. Zirconium is over solar by a factor of 8, there is 11 times the normal barium, and in the so-called "rare earth" department of elements, we find 15 times too much lanthanum and 20 times normal cerium. The metallic-line stars and their cousins, the magnetic "Ap" stars ("p" for "peculiar") like Cor Caroli, and a variety of other chemically odd stars are difficult to classify and to analyze, but what wonderful variety they give to the nighttime sky.
Written by Jim Kaler 7/17/09. Return to STARS.