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.