ALPHA VOL (Alpha Volantis). We immediately assume that the "alpha"
of anything is the one on top. Stars usually follow the rule, the
brightest indeed often being "Alpha." But the rule is also often
broken, as witness such constellations as
Gemini, Sagittarius, and several others that include a modern
figure of the southern hemisphere, Volans
(the Flying Fish), in which Gamma Vol
tops the list. (A couple -- Leo Minor and
Norma -- do not even HAVE Alpha stars, a
quirk of boundaries and naming.) While Alpha Volantis is not at
the top of its domain, it does have some style in that it is the
very definition of a fourth magnitude star (coming in right at
4.00) and that it also illustrates some severe difficulties in
classification. Nominally, it's a white class A (A2.5) subgiant
that lies 124 light years away. That and a rather ill-defined
temperature of 8430 Kelvin lead to a luminosity 29 times that of
the Sun, a radius of 2.5 solar, a mass of
2.2 solar, and a realization that the star is hardly a subgiant
(which implies cessation of core hydrogen fusion), but a dwarf
about half-way through its 890-million-year H-burning lifetime.
The problem is that it is another one of these slowly rotating (for
its class) metallic-line stars in which chemical elements are
separated by diffusion in a quiet atmosphere, some lofted upward
via radiation, others settling downward under the force of
gravity. The measured equatorial rotation speed of 34 kilometers
per second (and a rotation period of under 3.7 days) is apparently
not sufficient to keep things stirred up. The phenomenon then
makes a mess out of spectral
classification, which is based on a chemistry of "normal" solar
proportions. Depending on what chemical elements are used, we can
get anything from the nominal class A2.5 to much cooler A7. Wait,
there's more confusion. Many class A stars (Vega, Fomalhaut)
are surrounded by dusty disks that imply some kind of real (or
failed) planetary systems. Some sources say Alpha Vol is one of
their number, others say no. The star is also referred to as a
double, one that is observed only via the spectrum. But the reference goes
back to 1905 and is probably the result of not-surprisingly poor
observations. Such confusion, however, should delight, as it
provides new opportunities for study and research.
Written by Jim Kaler 5/18/07. Return to STARS.