DELTA CEP (Delta Cephei). Surely among the most famed of all
stars, fourth magnitude (4.1 or thereabouts) Delta Cephei, set at
the southeastern corner of dim Cepheus
(the King), does not even have a proper name. It is, however, the
only star that has given its constellation name over to represent
a whole class of stars, the "
Cepheids." While most stars look down
steadily upon us, Delta Cep is one of the few easily-visible
variables, its magnitude changing from 3.5 to 4.3 and back over an
amazingly regular period of 5 days 8 hours 47 minutes and 32
seconds, the star acting like a natural clock. We can't even pin
down the class. Listed as a yellow-white class F (F5) supergiant,
the star actually changes from F to cooler G in synchrony with its
light variations, the temperature changing from 6800 Kelvin at the
warmest to 5500 at the coolest. The term "supergiant" is apt, as
at a distance between about 900 and 1000 light years, the star pours an average
of 2000 solar luminosities into space from
a surface swollen to some 40 solar diameters. At the pinnacle of
a vast class of stars, Delta Cep has a few naked-eye cousins that
include Mekbuda (Zeta Geminorum) and Eta Aquilae (actually the first of
the breed to be discovered), both of which are just somewhat
brighter. All Cepheids are dying high mass stars (Delta Cep around
5 solar masses) with helium cores that have lost a sense of
equilibrium, and regularly expand and contract, pulsing like
celestial hearts. Their deep astronomical importance lies in the
observation that their luminosities are directly related to their
periods of pulsation (which run from about a day to over 50 days).
Since the period gives the luminosity, we need only measure the
apparent luminosity (the visual magnitude) to find the distance.
Cepheids are so luminous that they are easily seen in nearby
galaxies, their presence then giving the galaxies' distances.
Edwin Hubble's discovery of a Cepheid in the Andromeda Galaxy allowed the first firm distance to be
determined. The observation of Cepheids in more distant systems
was a "key project" of the Hubble Space Telescope, one that has
allowed the establishment of an accurate cosmological distance
scale. Delta Cephei does not pulse in lonely splendor, but has a
6th magnitude (6.3) class B (probably B7) 500 solar-luminosity
companion 41 seconds of arc away. Less than 100 million years ago,
Delta Cep began its journey as a hotter and more-massive star than
its four-solar mass neighbor, which is still a normal hydrogen-
burner, but which will soon follow Delta Cep's path when its
internal hydrogen gives out. Separated by at least 12,000
Astronomical Units, the two take at least half a million years to
orbit each other. Even at that distance, the companion would shine
in Delta's sky about as bright as our Venus. Watching Delta vary
by over a full magnitude from the companion would be a fascinating
sight, not there would be anybody to watch, the stars being much too
youthful to have any life on any planets that are probably not
there in the first place.