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IZAR (Epsilon Boo).Arcturus climbsthe eastern sky in northern spring evenings, the kite-shaped figureofBootes, the Herdsman, to the left. Centered in the eastern edge of the figure lies the constellation'ssecond brightest star, Izar (short or long "I"). The name derivesfrom a short Arabic phrase meaning the "girdle" or "loin cloth,"and means much the same as "Mizar" thatindicates the "loin" ofUrsa Major, theGreater Bear. Bayer must have had something other than brightnessin mind -- likely position -- when he gave Bootes' stars theirGreek letter names, as second-magnitude(2.59) Izar received the "Epsilon" designation and the nextbrightest, third magnitudeMuphrid, got"Eta." Izar's claim to fame is that it is one of the finest doublestars in the sky. It consists of a third magnitude (2.70) class K(K0) orange brightgiant only threeseconds of arc from a fifth magnitude (5.12) white class A (A2)main sequence dwarf. Note, however, that the magnitudes are quiteuncertain, as the proximity of the pair makes measurementdifficult. Izar B may be as bright as magnitude 4.8, and thecombined pair as bright as magnitude 2.3. The color contrast,enhanced by the stars' proximity to each other, is so striking thatthe discoverer (F. G. W. Struve) called the pair "Pulcherima" for"the most beautiful." At a distance of around 200 light years, thedimmer A star is found to have a total luminosity 27 times that oftheSun, while the brighter giant, radiating400 solar luminosities, outshines it by a factor of 15. Theuncertain distance, however, may be as great as 250 light years. The A dwarf's 8700 Kelvin temperature show it to be about twice thesize of the Sun, while the cooler 4500 Kelvin giant is 33 times aslarge. In the 170 years since discovery, the stars have completedless than three percent of their orbit. Separated by a distance ofat least 185 Astronomical Units (the AU the distance between theEarth and the Sun), the period is well over 1000 years long. Thepair wonderfully presents a chapter in the story of stellarevolution. The A star is about double the solar mass, the K starcloser to quadruple. Class K giants by their natures are evolved,fusing helium to carbon in their cores instead of hydrogen tohelium (as does the class A dwarf). More massive stars use theirfuel and evolve first. We know how rapidly stars age. The pairwas born some 300 million years ago as the white A star we see nowand a hotter, bluer, and brighter mid class B star. Ten or 20million years ago, the brighter star's central hydrogen fuel supplyran 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 thesmaller star, and it will become a lesser orange giant. By thattime, the bigger star will have ejected almost all of its outerenvelope leaving not much more than the old core, which will appearas a dim dense "whitedwarf" about the size of Earth that will nearly be lost in theorange glow of the giant-to-be (which will someday become a lesserwhite dwarf itself).
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