LAMBDA CEP (Lambda Cephei). Two and a half degrees northwest of, and almost completely overshadowed by, the famed variable Delta Cephei (of Cepheus, the celestial King) lies an example of one the rarer stellar types of the naked eye sky, or for that matter of the entire "normal" stellar population. Upon closer examination, rather anonymous looking fifth magnitude (5.04) Lambda Cephei reveals itself to be a crystalline blue-white class O (O6) supergiant with a flowing wind whose spectrum overlays that of the star itself, producing emissions from helium and nitrogen. At a great distance of 1980 light years (give or take a hefty 265), the star is rather heavily obscured by interstellar dust in the Milky Way. Were the line of sight clear, Lambda Cep would be an obvious third magnitude (3.3). For that matter, were it at Vega's distance of 25 light years it would shine at magnitude -6, four times the light of Venus at her best. A mean temperature of 36,400 Kelvin makes it a bit cool for the class. Stars like this one are so hot that they radiate most of their light in the ultraviolet. In the visual spectrum, Lambda Cep is some 15,000 times brighter than the Sun. In total radiation, however, the figure climbs to a whopping 440,000 Suns. It produces so much light per unit area that in spite of it being called a supergiant, Lambda Cep is only about 15 times bigger than the Sun. A high projected equatorial rotation speed of at least 240 kilometers per second gives it a rotation period of under three days. Of most interest is Lambda Cep's huge mass. Luminosity and temperature, along with the theory of stellar structure and evolution, give it 45 times the bulk of the Sun. Other estimates come as high as 60. In spite of its supergiant status, theory suggests that it is really a hydrogen-fusing dwarf. But not for long, as core hydrogen fusion at this mass lasts a mere 5 million years, a thousandth the present age of then Sun. After giving up hydrogen fusion, it will expand to true supergiant proportions: if the mass-losing wind, blowing at more than a millionth of a solar mass per year (a hundred million times the flow rate of the solar wind), lets it get that far. Certainly one of the grander stars of the Galaxy, Lambda Cephei seems to have little choice but to blow up as a supernova to create a neutron star or even to collapse into a black hole. The star seems single. But perhaps in once was not. Lambda Cep's motion indicates that about 2.5 million years ago it became a "runaway" from the Cepheus OB 3 association of hot stars, which centers about 2800 light years from us. (B or OB associations are expanding systems of massive and other stars that share giant birthclouds.) Indeed, it's moving at a good pace of 83 kilometers per second relative to us, some five times higher than normal.

Lambda Cephei As Lambda Cephei, indicated by the circle, moves down and to the right, it sends a shock wave in front of it through the surounding gases. (From a paper by V. V. Gvaramadze and A. Gualandris in the Monthly Notices of the Royal Astronomical Society.)

Runaways seem to be caused by one of two mechanisms. Such a star perhaps once had a more massive companion that blew up first, a sideways kick to the supernova sending them screaming away from each other. Or it could have been ejected by interaction within a multiple star, rather like what happened to Mu Columbae and AE Aurigae. Lambda's motion actually produces a shock wave in the surrounding gases something like the bow wave off a speeding boat or the sonic boom that follows a supersonic aircraft, leaving little question as to its course. (Thanks to Bas Verhagen, who suggested this star.)
Written by Jim Kaler 9/14/12. Return to STARS.