78 VIR (78 Virginis) = CW VIR. It would be dull if stars behaved
themselves as they ought. It's more fun when they do not. In the
intro textbooks, they all (ok, most) have the same chemical
compositions, in which hydrogen dominates at 90 percent, helium
comes in at 10 percent, and all the rest -- "metals" in astro-
jargon -- descend in minor abundance just as they do in the Sun. The Sun and stars of its kind are also in
a high state of outer convection, which generate magnetic fields
and all sorts of activity. In hotter stars, the convection
disappears and their magnetic fields should also. But nobody told
these things to 78 Virginis, a fifth magnitude (4.94) white class
A (A1) dwarf that falls into a special category of "A-peculiar"
("Ap") stars (making it "A1p"), whose prototype is Cor Caroli, Alpha-2 Canum Venaticorum,
the luminary of Canes Venatici (the
Hunting Dogs). Located in northern Virgo just over four degrees north of prominent Zeta Virginis, and slightly variable (4.91 to
4.99), 78 Vir is also known by its variable-star name of CW
Virginis. First, the usual statistics, then the oddities.
Temperature measures are scattered, but average around 9300 Kelvin,
about right for the class. From a distance of 185 light years
(plus or minus 2), the star shines with the light of 30 Suns, which
gives it a radius of 2.1 solar and a mass of 2.2 times that of the
Sun, showing it to be a relatively young dwarf that still has a
long way to go to finish out its hydrogen-fusing lifetime of 900
The A1p spectral class is just the beginning. The whole thing
reads "A1pSrCrEu" to imply huge overabundances of the chemical
elements strontium, chromium, and europium, typical of Ap stars.
That of europium (number 63 in the periodic table and a "rare
earth") has been measured at 30,000 times the solar abundance.
Such stars (and their cousins, the "metallic-
line stars"nn and the "mercury-manganese
stars") suffer from diffusion of elements in quiet atmospheres
(the stirring effect at a projected rotation speed of 16 km/s
minimal), in which some elements like underabundant calcium and
scandium fall under the effect of gravity, while others are lofted
upwards by the power of stellar radiation, thus grossly altering
the outer compositions. The Ap stars add another dimension with
strong magnetic fields that concentrate the elements in "starspots"
near the poles. As the spots swing in and out of view, the
starlight varies. 78 Virginis is particularly famous for being the
first star, other than the Sun, known to have a magnetic field, the
discovery made (through effects on the spectrum) in 1947. The field is tilted
against the rotation axis, so that it regularly varies, but it has
been measured at greater than 3000 times that of Earth. All the
variations allow a measure of rotation period, which in the case of
78 Vir is 3.772 days. Combination with measured rotation velocity
gives a rotation-axis tilt of 34 degrees to the line of sight.
While the odd abundances are more or less understood, nobody really
knows what creates the Ap stars' strong magnetism, presenting us
with one of the greater of stellar mysteries.
Written by Jim Kaler 7/16/10. Return to STARS.