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
86.1 light years (give or take a half). 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 23 Suns, the star's
radius 6.6 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. In addition, the double that makes Lambda And proper
is surrounded by a trio of faint apparent companions. 48 seconds of
arc away is a 13th magnitude component that if really joined (which
it probably isn't) would be more than 1300 AU distant and would take
at least 25,000 years to orbit. 217 seconds away is a pair of 11th-12th
magnitude stars separated by 66 seconds that probably orbit each other,
but we have no idea of how far away they are as they are almost certainly
a line of sight coincidence with the bright star.
Written byJim Kaler 3/19/04; revised 9/09/15.
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