Epsilon Boötis
Epsilon Boötis is a binary star system in the northern constellation of Boötes. Its name is a Bayer designation that is Latinized from ε Boötis, and abbreviated Epsilon Boo or ε Boo. The primary component has the official name Izar, pronounced . The star system can be viewed with the unaided eye at night, but resolving the pair with a small telescope is challenging; an aperture of or greater is required. Based on parallax measurements, it is located at a distance of. The system is drifting closer to the Sun with a radial velocity of −16 km/s.
Nomenclature
ε Boötis is the star's Bayer designation.It bore the traditional names Izar, Mirak and Mizar, and was named Pulcherrima by Friedrich Georg Wilhelm von Struve. Izar, and Mizar are from the ʾizār and مئزر Mi'zar and المراق al-maraqq' ; Pulcherrima is Latin for 'loveliest'. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalogue and standardize proper names for stars. The WGSN approved the name Izar for this star on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.
In the catalogue of stars in the Calendarium of Al Achsasi Al Mouakket, this star was designated Mintek al Aoua, which was translated into Latin as Cingulum Latratoris, meaning 'belt of barker'.
In Chinese astronomy, 梗河 Gěng Hé, refers to an asterism consisting of Epsilon Boötis, Sigma Boötis and Rho Boötis. Consequently, the Chinese name for Epsilon Boötis itself is 梗河一 Gěng Hé yī.
Properties
Epsilon Boötis consists of a pair of stars with an angular separation of at a position angle of. The brighter component has an apparent visual magnitude of 2.45, making it readily visible to the naked eye at night. The fainter component is at magnitude 4.8, which by itself would also be visible to the naked eye. Parallax measurements from the Hipparcos astrometry satellite put the system at a distance of about from the Earth. This means the pair has a projected separation of 185 Astronomical Units, and they orbit each other with a period of at least 1,000 years.The brighter member has a stellar classification of K0 II-III, which means it is a fairly late-stage star well into its stellar evolution, having already exhausted its supply of hydrogen fuel at the core. Evolutionary models suggest it is on the red giant branch, where hydrogen is being fused in a shell around a helium core. With more than four times the mass of the Sun, it has expanded to about 38 times the Sun's radius and is emitting 650 times the luminosity of the Sun. This energy is being radiated from its outer envelope at an effective temperature of, giving it the orange hue of a K-type star.
The companion star has a classification of A2 V, so it is a main sequence star that is generating energy through the thermonuclear fusion of hydrogen at its core. This star is rotating rapidly, with a projected rotational velocity of. It has a surface temperature of about and a radius nearly three times the Sun, leading to a bolometric luminosity 45 times that of the Sun.
Epsilon Boötis may turn out to be a quadruple system. A widely-separated brown dwarf appears to have similar proper motions with the inner pair, and thus astronomers believe both bodies may be gravitationally related. The estimated separation is of 4,900 astronomical units, but this is actually a projected separation, thus a lower limit. In addition, there is another proper motion companion at a much wider projected separation of 186,000 AU.
By the time the smaller main sequence star reaches the current point of the primary in its evolution, the larger star will have lost much of its mass in a planetary nebula and will have evolved into a white dwarf. The pair will have essentially changed roles: the brighter star becoming the dim dwarf, while the lesser companion will shine as a giant star.