Lambda Cas

LAMBDA CAS (Lambda Cassiopeiae). Good things can come in faint packages. Fifth magnitude (4.73) Lambda Cassiopeiae appears almost anonymous just south of the seat of Cassiopeia's Chair and just barely northwest of Zeta Cas.

Zeta Cassiopeiae, the bright star just below top center, is set into a dense part of the Milky Way. The brighter of the two stars down and to the left of it is Lambda Cas. They lie to the north (and a bit west) of Shedar, Alpha Cas, seen toward the bottom. To the right of Shedar is a slightly fuzzy patch, the nebula NGC 281.

To the contrary, this class B (B8) star is actually notable as a fine very close pair of fairly massive class B (B8 plus B9) dwarfs that are under a second of arc apart, but far enough from each other to allow a decent orbit to be constructed. Measurements fitted with the best ellipse show the two to go around each other every 536.5 years at a mean separation of 135 Astronomical Units (as derived from a good distance of 378 plus or minus 19 light years). A quite-large eccentricity of 0.82 takes them from as far as 245 AU apart to close as 25, which is about where they will be (or were) in mid-2012. They are far enough apart that gravitational interaction has not circularized the orbit. From Kepler's Laws we then derive a combined mass of 8.6 times that of the Sun, which seems high for class B dwarfs and, from the following further analysis, indeed is.

The B9 dwarf Lambda Cas B orbits the B8 dwarf Lambda Cas A on an eccentric path that takes 536.5 years to complete, the two averaging 135 AU (or less) apart. In reality, they both orbit a common center of mass that lies between them. The orbital major axis is severely offset from the apparent ellipse because of the tilt and orientation of the orbital plane against the plane of the sky. The scale is in seconds of arc. Note how close the stars are to each other, which makes them difficult to separate. (W. I. Hartkopf and B. D. Mason, Sixth Catalog of Orbits of Visual Binary Stars, US Naval Observatory Double Star Catalog, 2006.)

Lambda Cas A and B are respectively found to shine at magnitudes 5.33 and 5.62, which give a combined magnitude of 4.71, which, given the difficulty of separation of two very close points of light, is remarkably close to that found from observation as if Lambda were single (4.73). The spectral classes (plus one other estimate) suggest temperatures (needed to find the amount of ultraviolet light emitted by the stars) of 12,200 and 11,000 Kelvin. From these and the distance, the two appear to radiate at rates of 153 and 92 times solar, which give radii of 2.8 and 2.7 times that of the Sun. A projected equatorial rotation speed of 220 kilometers per second (presumably for Lambda Cas A alone) yields a rotation period of under 0.64 days (which if the star's equator is tipped to the plane of the sky through the same 76 degree angle as is that of the orbit, is very close to the actual value). The theory of stellar structure and evolution then lead to masses of 3.0 and 2.8 times that of the Sun, which sums to 5.8 solar, considerably less than that derived from the orbit. The latter, though, can be reconciled by reducing the orbital size by a bit over 10 percent, to 119 AU. Early orbital data gave even higher masses; more of the currently-precise observations over the years will refine the orbit even further, and hopefully will bring agreement. Lambda A is now reaching the end of its life as a dwarf (that is, as a core hydrogen-burner), which for a three solar mass star takes 360 million years. Lambda B is following with a dwarf lifetime of 450 million years. The couple appears to be a member of the vast Cassiopeia-Taurus association of hot stars and that spreads between the two constellations. Some 4000 light years across, Cas-Tau is one of the largest "structures" of related stars in the Galaxy.

Written by Jim Kaler 12/02/11. Return to STARS.