GIAUSAR (Lambda Draconis). The front bowl stars of the Big Dipper are famed for pointing at Polaris in Ursa Minor's Little Dipper. What, however, of the stars along the way? The path to Polaris is so familiar that we rarely stop to see the other sights that lie along it. About a third of the way from Dubhe (the Big Dipper's front bowl star) to Polaris (and a just a bit to the east) lies Giausar, the tail star of Western Draco the Dragon, to which Bayer assigned the Greek letter Lambda. The Arabic name of this mid-fourth magnitude (3.84) star is confusing at best. At times thought to refer to a "central one" much as does the Arabic name of Orion, the word actually refers to the "nodes" of the lunar orbit, the points at which the Moon crosses the ecliptic plane twice a lunar month -- which makes little sense, since Draco contains the ecliptic POLE, and is therefore quite distant from the ecliptic itself. Rather clearly, the name was applied in error. The star is about as neglected by research astronomers as it is by even dedicated skywatchers, rather too bad as it has -- as a class M (M0) red giant -- one of the rarer of naked-eye types. Over the past 20 years it has been mentioned less than 40 times. It is neglected by other stars too, as it has no known companions. Giausar is one of the sky's cooler and larger stars. With a temperature of 3525 Kelvin, it shines to us (if the estimate of invisible infrared radiation is correct) from a distance of 335 light years with a luminosity 1870 times that of the Sun, which leads to a radius of 0.55 astronomical units, half the size of Earth's orbit. Large enough to have had its angular diameter measured (at 0.0073 seconds of arc), direct measure of radius makes it somewhat smaller, a "mere" 0.37 astronomical units, about the size of Mercury's orbit. Even the star's general behavior is obscure. Classified as a "semi-regular variable," there is some indication that it changes brightness erratically by about a tenth of the magnitude. Giausar appears to be on the "asymptotic giant branch," in a portion of its evolution in which it is brightening as a giant star for the second time. With a dead carbon core, the star (of perhaps two solar masses) will most likely begin to pulsate more vigorously as it prepares to shed its outer layers and to turn itself into a white dwarf, as someday will the Sun.
Written by Jim Kaler. Return to STARS.