38 LYN (38 Lyncis). Double
stars, sprinkled all over the sky, fascinate. Though coming in
a great many varieties, the ones we best know love are those that
can be seen directly by eye through the telescope. Among the many
favorites are Mizar, Albireo, Almach
(Gamma Andromedae), and Algieba (Gamma
Leonis). Of their many charms is a visual effect that strongly
enhances slight differences in the colors of stars that lie next to
each other. Nineteenth century astronomers were eloquent in
ascribing double-star colors, even thinking that binaries were
really colored differently from single stars. We need not stick to
classical constellations to find such gems. Between Auriga and Ursa
Major sprawls a long string of stars that represents Lynx, the (no surprise here) Lynx. The
luminary, Alpha Lyncis, lies almost due
south of the most western of the three pairs of stars that
represents the Great Bear's paws (the Arab's three "leaps of the gazelle"), made of Iota and Kappa
Ursae Majoris. Immediately to the north of Alpha Lyn find
fourth magnitude (3.8) 38 Lyncis. Even a modest telescope splits
it into a close pair of stars 2.6 seconds of arc apart that
consists of a brighter fourth magnitude (3.92) class A3 hydrogen-
fusing dwarf coupled to a somewhat mysterious, lesser, cooler sixth
magnitude (6.09) secondary. Though both are really white (38 Lyn
B somewhat less so), the proximity effect led to early colors given
as "silvery white and lilac." With a temperature of 8400 Kelvin,
39 Lyncis A radiates at a rate 31 times that of the Sun, which in turn leads to a radius of 2.4
times solar and a mass 2.2 times solar, the star about midway
through its 900 million year hydrogen-fusing lifetime. A very
rapid equatorial rotation of at least 190 kilometers per second
gives the star a rotation period of under 15 hours! Now the small
mystery. 38 Lyn B is classed as an A4 or A6 dwarf. But such a
star would be expected to be more than a magnitude brighter than it
is. Either the pair is a line-of-sight coincidence, or the class
is way off, as 38-B shines more like an F4 star. Given that the
stars have been tracking each other since the discovery of
duplicity by William Herschel in the 1700s, they almost have to be
gravitationally bound, leading to an error of classification that
perhaps might be linked to a peculiar chemical abundance (38 A
oddly sometimes called a Lambda Bootis
star, one deficient in metals). If really cooler than expected,
then 38 B is a 1.4 solar mass star glowing with the light of 4 Suns
from a 6800 Kelvin surface. Now the mystery deepens.
Interferometry shows 30 Lyn B also to be double with a close
separation of (at last look) 0.23 seconds of arc, which corresponds
to 7.5 Astronomical Units; 38 A might be similarly duplicitous with
the two at less than half that separation. So the star might
really be quadruple. With 11th magnitude "companions" at
separations of about one and three minutes of arc, how about
sextuple? No: their motions show that these are clearly just lying
along the line of sight, leaving the inner system to itself.
(Thanks to Bill Hartkopf for discussion.)
Written by Jim Kaler 12/26/08. Return to STARS.