Izar, a stunning double star situated in Boötes

Izar (Epsilon (ε) Boötis) is the next star ‘up’ the eastern side of the Kite asterism, which begins in the south with the bright spring star Arcturus (Alpha (α) Boötis).

The Kite forms the main body of Boötes, the Herdsman. Izar is a beautiful double star consisting of a mag. +2.4 K0 II-III orange primary, separated from a mag. +5.1 A2 V white secondary by 2.8 arcseconds. The difference in colours between both stars is what makes the pair so striking through the eyepiece, something which was noted by their discoverer FGW Struve, who named them Pulcherrima, meaning ‘loveliest’ in Latin.

The Izar system lies 203 lightyears away. At that distance, the apparent separation indicates that the two stars are 185 AU apart. They are believed to be in an orbit that takes 1,000 years to complete.

The primary is a late-stage star which has exhausted its supply of core hydrogen.

It has a mass 4.6 times greater than our Sun, but its evolutionary state means it has swelled to be 33 times larger. Its secondary companion is more like our Sun: still on the main sequence of stellar evolution and estimated to be 2.2 times more massive and 2.7 times larger. But where our Sun rotates at the leisurely speed of 2km/s, Izar B rotates at 123km/s.

Eventually, like our Sun, Izar B will exhaust its core hydrogen and swell into a red giant. By the time this has occurred, the primary will have shed its outer layers to form a planetary nebula, and the core will have become a white dwarf.

The name Izar is from an Arabic phrase for ‘girdle’ or ‘loin cloth’, a meaning that is also attributed to the similar-sounding Mizar (Zeta (ζ) Ursae Majoris). It is highest in the sky, due south at 01:00 BST (00:00 UT) on 1 May, 00:00 BST (23:00 UT) on the 15th and 23:00 BST (22:00 UT) on the 31st.