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In the west of Andromeda, near the point where that constellation meets Cassiopeia and Lacerta, lies an arrangement of stars shaped like a downward-pointing 'V'. Lambda Andromedae is the brightest of this group, forming the upper end of the western arm of the 'V' shape. It is also the closest of these stars to the Solar System, at a distance of about eighty-six light years.

Lambda Andromedae is a busy stellar system with several components, at the heart of which lies a yellow subgiant that is in the process of evolving from a hydrogen-burning main sequence star to a helium-burning giant. This massive star has a much smaller and fainter companion in a very close orbit of about 0.24 AU (considerably closer to the primary star than Mercury is to the Sun).

This close companion interacts with the magnetic field of the subgiant star, creating dense starspots on its surface and so causing its brightness to fluctuate. Close binaries of this type are characterised as RS Canum Venaticorum variables, though Lambda Andromedae is something of an oddity within this class. Typically, systems like this show changes in brightness over a period tied to the orbit of the companion star, but Lambda Andromeda does not conform to this pattern. The subgiant's companion orbits every 20.5 days, but the star's periodic changes in brightness occur over a period of 54 days. The mechanism behind this disparity is not yet fully understood.

The inner subgiant and its close companion are not the only stars in the system. Much, much farther out from the main yellow star lie a trio of red dwarfs following millennia-long orbits around the system. The closest of these three is at least 1,300 AU from the central star (that is, about three hundred times more distant than the inner companion). At least four times farther still from the central subgiant lie two more dwarfs that orbit together, forming a binary pair of their own on the outskirts of the Lambda Andromedae system.


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