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.)