LAMBDA AND (Lambda Andromedae). Radiating at just under third magnitude (3.52), Lambda Andromedae is modestly bright, but it is so far north of the major part of the outline of the constellation Andromeda that it hardly seems a part of it, and is thereby largely ignored by the backyard observer. Not, however, by the research astronomer, as Lambda And is a rather unusual critter, one of the brightest "RS Canum Venaticorum" (RS CVn) variable stars in the sky. Glowing as a "yellow" class G (G8) 4900 Kelvin giant-subgiant, Lambda And resides at a relatively nearby distance of 84 light years. A "spectroscopic" double, its companion, which cannot be directly detected, orbits with a period of 20.5212 days, the bulk of the light coming from the primary star itself, which shines with the light of 22 Suns, the star's radius 6.5 times solar. These parameters give us a mass close to double solar. About a billion years old, with a now-dead helium core, the star is now making its transition to becoming a true giant. Taking a wild guess at the companion's mass, say equal to that of the Sun, the separation between the two would be only 0.24 Astronomical Units (AU). Like others of the "RS CVn" class, the two stars of Lambda interact tidally (a tide being a gravitational stretching effect) to affect each others' rotations. Rotation and convection in the Sun cause concentrations of magnetic fields that give rise to sunspots and related magnetic and thus starspot activity. The interaction in Lambda (and other RS CVn stars) causes increased rotation and thus vastly enhanced magnetic activity that gives it a powerful outer corona with temperatures that range between 10 and 40 million Kelvin (as compared with the Sun's 2 million Kelvin corona). As a result of either huge starspots or bright active zones that swing in and out of sight, Lambda varies by about a tenth of a magnitude with its rotation period of 54 days. Lambda And is a "long period" RS CVn star, an unusual one in that the orbital and rotation periods are not synchronized such that the two stars always face each other. Apparently, the companion is still "spinning up" the larger giant via tidal action. There is also some evidence for a long activity cycle akin to the solar 11 year cycle, one that lasts from 5 to 14 years. Lambda has two other facets that recommend it. Intense magnetic activity makes it visible to radio telescopes. As a result it is one of the stars that can help correlate precise radio coordinate positions of celestial objects with optical positions of stars made by satellite. The double that makes Lambda And proper is also surrounded by a quartet of low mass class M dwarfs that make something of a double-double with the bright star (at least as long as there are no line-of-sight coincidences involved). 48 seconds of arc (at least 1300 AU) away is a 13th magnitude component that would take at least 25,000 years to orbit. 217 seconds (at least 5600 AU) away is a pair of 11th magnitude M stars separated by 69 seconds that would take at least 96,000 years to orbit each other, the outer double taking at least 200,000 years to go around the inner trio, the whole system a fascinating quintuple star.
Written by Jim Kaler. Return to STARS.