LAMBDA ARI (Lambda Arietis). A pair of degrees due west of Aries' luminary, exact-on second magnitude
(2.00) Alpha Arietis (Hamal), lies fainter
fifth magnitude (but at 4.6 just barely) Lambda Arietis. The
telescope quickly reveals it to be nicely double, consisting of fifth
magnitude (4.79) Lambda Ari A and seventh magnitude (6.65) Lambda
B. (Though not without controversy, as "B" is also given magnitude
7.6 to 7.8). The separation of 30 seconds of arc has remained
constant since Herschel's time (1777), so the two seem to be
tracking each other and really together. "A fine double star" say
19th century Smythe and Chambers, "A 5 1/2, yellowish-white; B 8,
blue." Their color for "A" is close, as it is indeed a yellow-
white class F (F0) dwarf, but it is way off on "B," which is a
cooler and yellower F7 dwarf, the apparent "blue" color coming from
contrast effects as seen by the human eye. The distance is
accurately measured at 129 light years give or take just 2, while
temperatures are given as 7200 Kelvin for Lambda A, 5930 for "B,"
which actually better supports a cooler G1 class, adding to the
confusion. Nevertheless, we adopt here the brighter magnitude and
warmer class. Respective luminosities (which require practically
no corrections for ultraviolet or infrared light) then come in at
14.5 and a modest 2.85 times that of the Sun, the radii at 2.45 and 1.60 solar. A
fairly fast projected equatorial rotation speed of 103 kilometers
per second gives "A" a rotation period of under 1.2 days, while
that of "B" is unknown. Theory indicates a mass for "A" of 1.8
solar masses and suggests that the star is well along in its 1.6
billion year hydrogen fusing lifetime. With a lesser mass of just
1.25 Suns, "B" will live on in dwarfhood more than twice as long.
The angular separation translates into an orbit of Lambda B around
A (two really orbiting mutually) with a radius of at least 1500
Astronomical Units, which, given the masses, would require a period
of more than 33,000 years. With these numbers, from the lesser
more solar "B," the chief star would glow with the light of some 3
times that of the full Moon. Off in the distance, at 189 and 271
seconds separation, lie tenth magnitude Lambda Ari C and D, their
motions showing them to lie merely in the line of sight. As Alpha
Ari is a guide to Lambda, so Lambda is to the modest eclipsing
variable RR Ari (Flamsteed 7) that falls half a degree due west of
Lambda. It's listed as a sixth magnitude K1 giant 560 light years away. An
eclipsing binary, RR Ari is
seen to drop twice in brightness by a few tenths of a magnitude
every 47.9 days as first one component and then the other gets in
the way.
Written by Jim Kaler 11/30/12. Return to STARS.