Epsilon Bootis - Izar, Pulcherrima

Izar, or ε Bootis, is the constellation's second brightest star, at magnitude 2.59. It lies centered in the eastern edge of the Herdsman's figure. The name Izar derives from an Arabic phrase meaning "girdle" or "loin cloth," and means much the same as "Mizar" that indicates the "loin" of Ursa Major, the Great Bear.

Components

Izar is one of the finest double stars in the sky. It consists of a magnitude 2.70 class K0 orange bright giant only 3" from a magnitude 5.12 white class A2 main sequence star. The pair presents a striking color contrast of golden yellow and greenish stars, enhanced by their proximity to each other. The discoverer (F. G. W. Struve) called the pair "Pulcherrima" for "the most beautiful."

In the 170 years since the system's discovery, the stars have completed less than 3% of their orbit. Separated by a distance of at least 185 AU, their orbital period is well over 1000 years.

Properties

At a distance of around 200 light years, the dimmer component has a total luminosity 27 times the Sun's, while the brighter giant radiates 400 solar luminosities. Their distance is uncertain, however, and may be as great as 250 light years.

The B component's 8700 K surface temperature implies a diameter about twice the Sun's, while the cooler, 4500 K giant is 33 times as large as the Sun. The fainter, class A2, B component is about double the solar mass, and the K0 giant A component closer to quadruple.

The pair was born some 300 million years ago. At that time, both stars were fusing hydrogen into helium in their cores; the more massive component of spectral class B, and the smaller component of class A, similar to its present state. More massive stars use their fuel faster, and 10 or 20 million years ago, the more massive star's hydrogen fuel supply ran out. Now it is is fusing helium into carbon and oxygen in its core.

In a little over a billion years, the same thing will happen to the smaller star, and it will become a smaller orange giant. By that time, the larger star will have ejected almost all of its outer envelope into space, leaving its core behind as a dim, dense white dwarf. It will be nearly lost in the orange glow of the giant-to-be, which will eventually become a white dwarf as well.

[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]