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.