LAMBDA SER (Lambda Serpentis). Without getting into the historical details, constellations are generally singular, self-contained units, but there are exceptions. Two pairs share a star, Andromeda one with Pegasus, Taurus one with Auriga. And then there is Serpens, the Serpent, the only one that is divided into two parts, Serpens Caput (the Head, to the west) and Serpens Cauda (the Tail, the eastern part), with Ophiuchus (the Serpent Bearer) in the middle. The two parts are still considered as a unit, however, with the Greek letters divided between them. Alpha (Unukalhai) through Epsilon are in Ser Caput, while for Zeta, Eta, and Theta (Alya) we switch over to Ser Cauda. For fourth magnitude (4.01) Lambda Serpentis, however, we go back to Serpens Caput, where it falls just a degree north-northeast of Alpha. Not only is Lambda in an exceptional constellation, by itself it's something of an exceptional star. Most of those that make our constellation figures (though Lambda is not usually part of the connect-the-dots pattern) are considerably more luminous than the Sun. Here, however, we find a quite sunlike class G (G0) dwarf that lies a mere 39.5 light years (accurately known to just 0.2) away. Put it at 100 light years (not much farther than the middle stars of the Big Dipper) and it would be invisible to the naked eye. Even so, it still tops the Sun with a temperature of 5890 Kelvin, a luminosity of 2.1 Suns, a radius of 1.4 solar, and from theory a mass 1.1 times solar. One of the reasons for the extra brightness is age (dwarfs getting slowly brighter as they process their core hydrogen into helium), various estimates running 7 or 8 billion years as opposed to the Sun's 4.5 billion. Indeed, it is old enough that it could be called a subgiant that has used most if not all of its internal fuel. With a rather poorly known equatorial rotation speed of at least 4 kilometers per second, the star completes a rotation in under 17 days (compared to 25 for the Sun), fast enough to generate a magnetic outer chromosphere and X-rays. Given its solar status, we might expect Lambda Ser to have one or more planets, but none has yet been found. One might think that the lack could be due to a binary companion once announced with a period 5.0 years, which would place it about 3 Astronomical Units away. But that too does not seem to exist, as it was just a product of inadequate data and/or analysis. What Lambda Ser does have going for it is a speed relative to the Sun of 67 kilometers per second, some 4 to 5 times "normal," suggesting a visit from a different part of the Galaxy (the metal content however quite solar). More interesting, most of the velocity is radial, the star coming almost right at us at 66 km/s. We'll get a better look 166,000 years from now when Lambda Ser passes just 7.4 light years away, glowing at a bright first magnitude.

Written by Jim Kaler 7/05/13. Return to STARS.