AVIOR (Epsilon Carinae). If there is a stellar category of "bright stars getting no respect," Avior probably holds the record. One would think all the brighter stars in the sky would have been studied in great detail. One would be wrong. Bright second magnitude (1.86) Avior, the 39th brightest star in the sky, ranking 17th in second magnitude, is something of a mystery, in part because hardly anyone seems to bother to look at it in spite of it being a bit of a curiosity. Third brightest star in Carina, the Keel (of the Ship Argo, of which Canopus is King), Avior is called Epsilon, but logically so, as the Greek letters were first assigned to the whole Ship, and when it was broken up into Vela, Carina, and Puppis, Carina got Alpha (Canopus)and Beta, while Vela got Gamma and Delta. Avior's problem is its placement. Not quite 60 degrees below the celestial equator, it clears the horizon only south of 30 degrees north latitude, and is out of sight in most of the north where traditional astronomy has (unfortunately) been done. As bright as Alkaid at the end of the handle of the Big Dipper, if Avior had been in the north, it would have been examined in great detail. Even its name is a mystery. Too far south to have received a classical Greek or Arabic name, "Avior," made up during World War II, refers to the star's use in aircraft navigation. Avior is perhaps best known as the brightest member of the "False Cross," which is sometimes confused with Crux, the Southern Cross, and which also includes Aspidiske (Iota Carinae), Delta Velorum, and Markeb (Kappa Velorum). At least we know its class and distance, but that is where the line is drawn. Some 630 light years away, Avior is a wondrous double star, but one that, in spite of sophisticated effort, has never been resolved into two visible stars. We know of its duplicity only because two classes are simultaneously visible in the spectrum. One star is a hot blue class B (B2) hydrogen-fusing dwarf, the other a dying class K (K3) orange giant. Together they shine with a visual brilliance 6000 times that of the Sun. It is hard to say which star produces most of the light, however. The blue B star should be brighter of the two, but the color of the star suggests the opposite. Theory shows that the B dwarf should have a mass some 7 times that of the Sun. Since high mass stars begin to die first (as a result of much faster fuel consumption), the orange giant must once have been even more massive. There is a meager suggestion that the stars may eclipse each other, producing a slight 30 percent dip in brightness every 2.2 years. but typical of Avior, nobody really seems to know. If so, from the period and masses of the stars, they should be only about 4 astronomical units apart, less than the distance of Jupiter from the Sun. Unlike Algol, which has roughly the same kinds of stars, the components are too far apart for mass transfer to take place. Yet separated by a mere 0.02 seconds of arc, it is no wonder that Avior's stars cannot be seen as separate. Planets could hardly orbit either star. If one were to orbit the double, it would have to be twice as far as Pluto from the Sun to receive our level of daylight, and perhaps four times as far to keep from being fried by the infrared heat and ultraviolet from the stunning duo at the center of the orbit. No one, of course, yet knows if such planets could possibly exist. If nothing else, Avior presents a great opportunity for study. (Thanks to Jeff Bryan, who suggested this star, and to Michael Smith for pointing out the source of the proper name.)
Written by Jim Kaler. Return to STARS.