IZAR (Epsilon Boo). Arcturus climbs the eastern sky in northern spring evenings, the kite-shaped figure of Bootes, the Herdsman, to the left. Centered in the eastern edge of the figure lies the constellation's second brightest star, Izar (short or long "I"). The name derives from a short Arabic phrase meaning the "girdle" or "loin cloth," and means much the same as "Mizar" that indicates the "loin" of Ursa Major, the Greater Bear. Bayer must have had something other than brightness in mind -- likely position -- when he gave Bootes' stars their Greek letter names, as second-magnitude (2.59) Izar received the "Epsilon" designation and the next brightest, third magnitude Muphrid, got "Eta." Izar's claim to fame is that it is one of the finest double stars in the sky. It consists of a third magnitude (2.70) class K (K0) orange bright giant only three seconds of arc from a fifth magnitude (5.12) white class A (A2) main sequence dwarf. Note, however, that the magnitudes are quite uncertain, as the proximity of the pair makes measurement difficult. Izar B may be as bright as magnitude 4.8, and the combined pair as bright as magnitude 2.3. The color contrast, enhanced by the stars' proximity to each other, is so striking that the discoverer (F. G. W. Struve) called the pair "Pulcherima" for "the most beautiful." At a distance of around 200 light years, the dimmer A star is found to have a total luminosity 27 times that of the Sun, while the brighter giant, radiating 400 solar luminosities, outshines it by a factor of 15. The uncertain distance, however, may be as great as 250 light years. The A dwarf's 8700 Kelvin temperature show it to be about twice the size of the Sun, while the cooler 4500 Kelvin giant is 33 times as large. In the 170 years since discovery, the stars have completed less than three percent of their orbit. Separated by a distance of at least 185 Astronomical Units (the AU the distance between the Earth and the Sun), the period is well over 1000 years long. The pair wonderfully presents a chapter in the story of stellar evolution. The A star is about double the solar mass, the K star closer to quadruple. Class K giants by their natures are evolved, fusing helium to carbon in their cores instead of hydrogen to helium (as does the class A dwarf). More massive stars use their fuel and evolve first. We know how rapidly stars age. The pair was born some 300 million years ago as the white A star we see now and a hotter, bluer, and brighter mid class B star. Ten or 20 million years ago, the brighter star's central hydrogen fuel supply ran out and now it is a bright giant with little time left to it. In a little over a billion years, the same thing will happen to the smaller star, and it will become a lesser orange giant. By that time, the bigger star will have ejected almost all of its outer envelope leaving not much more than the old core, which will appear as a dim dense "white dwarf" about the size of Earth that will nearly be lost in the orange glow of the giant-to-be (which will someday become a lesser white dwarf itself).
Written by Jim Kaler. Return to STARS.