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.
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.)
