SEGINUS (Gamma Bootis). Seginus, almost defining third magnitude (3.03), and the fourth brightest star in Bootes, the Herdsman, for the Gamma star is not far off the mark. Oddly, however, it is not beaten out by the Beta star (Nekkar) nor even by the Delta star, but by Epsilon (Izar) and of all things Eta (Muphrid). Usually, Arabic star names have been badly corrupted by translation into Latin. Here we have the reverse, a star name that actually originally meant "Bootes" in Greek and that was mangled in translation by the Arabs (and then RE-mangled when put back to Latin). It no longer sounds anything like the original. At first, Seginus seems to be yet another boring white class A (A7) star (only boring of course because there are so many of them!). It has a number of things to recommend it, however. First, unlike the class A stars of the Big Dipper, it is a giant. Class A giants are different from the cool giants, not as large. They are stars that are just now beginning their destinies with the "real" giants (those cool ones of classes K and M that really ARE large), and are just now starting to swell. From Seginus's distance of 85 light years and its surface temperature of 7600 Kelvin, we calculate a total luminosity 34 times that of the Sun and a radius only about 3.5 times solar (hardly "giant" status). Not only is Seginus also still a rapid rotator (giants slow down as they expand), spinning at least 139 kilometers per second, but it is also variable, of a class known as a " Delta Scuti" star. These are all dwarfs and giants of classes A and cooler F that chatter away in brightness by a few percent as a result of their internal constructions. Seginus varies by around 5 percent, pulsating over a period of about 7 hours. It is also been found to be a "non- radial" pulsator over a much shorter period, some parts of the star moving outward while others move inward. Seginus has a visible 12th magnitude "companion" about half a minute of arc away from it, but the little star fools us in being only accidentally in the line of sight. "Speckle" observations, however (in which the astronomer compiles myriad short exposures to overcome twinkling), reveal that Seginus really does have a companion only 0.07 seconds of arc distant from it (which corresponds to 1.8 astronomical units, a bit farther than Mars is from the Sun). Nothing whatever is known about the little star. With a mass of around 2.5 times that of the Sun, Seginus has recently shut down core hydrogen fusion (or at least it will very soon), and is roughly what the brighter Dipper stars will become when they begin to die. Oddly, it is surrounded by a small cloud of dust visible only through its infrared radiation. Why it should exist is a mystery.
Written by Jim Kaler. Return to STARS.