DELTA SER (DELTA SERPENTIS). Serpens, divided in two by better-known and more- easily-recognizable Ophiuchus, is somewhat underlooked. Where the celestial snake bends on its western side (Serpens Caput), find Delta Serpentis, an "elegant double star" that unfairly carries no proper name. Other than "Alpha" being given to the luminary Unukalhai, the Greek letters much more follow position, Alpha through Epsilon in Serpens Caput (the Serpent's Head), the tail (Serpens Cauda) beginning with Zeta Serpentis. To the eye alone, Delta Ser is a bright 4th (3.80) magnitude star that lies 210 light years from Earth. Through the telescope, it breaks into a pair at fourth (4.1) and fifth (5.2) magnitude separated by 3.9 seconds of arc that looks rather like Porrima in Virgo. Not observationally well-discriminated from each other, they are as a pair called class F (F0) subgiants. The brighter has a surface temperature of 7550 K (a bit high and more like a giant's temperature), which probably serves for the fainter as well. Though both are white in color, contrast effects caused older observers to see the fainter as bluish-white. Delta Ser A (the brighter) has a luminosity 71 times that of the Sun, while Delta Ser B comes in at 26 solar. Temperature and luminosity together give respective radii 5.0 and 3.0 solar and respective masses 2.4 and 2.1 solar. They rotate at roughly with minimum equatorial speeds of 75 kilometers per second, giving them periods of at most three and two days. Roughly 800 million years old, both are on the verge of becoming evolved giant stars. The "A" component, being the more massive, is a bit farther along, having just given up its core hydrogen fusion, while "B" is still in hydrogen fusion's last stages. As the slightly more advanced and more massive, "A" has developed into a "Delta Scuti star" (like Caph in Cassiopeia) that subtly varies by a few percent over periods of 3.04 and 3.74 hours. The observation of slow orbital motion suggests a true separation of 375 Astronomical Units (more than 9 times Pluto's distance from the Sun) and an orbital period of 3200 years. Together, they yield a total mass for the system of 5.3 solar masses, which given the difficulty of the observation is in decent accord with that derived from temperature and luminosity, suggesting that the orbital parameters are reasonable. From Alpha Ser A, "B" would shine with the light of 330 full Moons, while from "B", "A" would appear nearly 3 times brighter yet.
Written by Jim Kaler. Return to STARS.