MEKBUDA (Zeta Geminorum).
More widely known as the Zeta star in Gemini (Zeta Geminorum), the Arabic name Mekbuda refers
to "lion's paw," a larger Arabic figure that overlaid our
constellations, and at one time may have more referred to one of
Gemini's luminaries Castor or Pollux. At mid-fourth magnitude, Mekbuda
(just southwest of brighter Wasat, Delta
Geminorum) shines with the yellow-white light of the Sun, the
temperature of the two about the same. That, however, is about
where the similarity ends. Mekbuda -- known affectionately as
"Zeta Gem" -- is a supergiant, albeit as supergiants go a modest
one. Its great distance of 1200 light years leads to a luminosity
almost 3000 times that of the
Sun and a diameter 60 solar. If our
Sun were Zeta Gem, it would appear some 30 degrees across, half
again as big as the constellation Gemini itself. Through a small
telescope, Mekbuda appears to have a faint companion a minute of
arc away, but it is a line-of-sight accident. The bright star
itself, however, does seem to have a true faint companion that
cannot be resolved by eye at the telescope. None of these facts
give the star any particular claim to fame. What does is its
changeability, for Zeta Gem is one of the sky's few easily visible
"Cepheid
variable stars." Cepheids are dying supergiants that have
lost their sense of stability, and pulsate, changing their
diameters, temperatures (5300 Kelvin to 5800 for Mekbuda), classes
(G to F), and magnitudes. The second of these discovered was
Delta
Cephei, after which the class of stars was named. It,
Eta Aquilae (the first-known of the breed),
and the southern hemisphere's Beta Doradus are all about the same
apparent brightness, Mekbuda changing from magnitude 3.7 to 4.2 and
back every 10.2 days. Cepheids are enormously valuable stars, as
their periods of pulsation are firmly linked to their true
luminosities: the longer the period, the more luminous the star.
Once we know the long-established relationship, we can find the
true luminosity of a Cepheid from only its period. From its
luminosity and its apparent brightness, we can calculate its
distance. Cepheids allow us to find the distances of other
galaxies, helping us to explore the Universe. One of the key
projects for the Hubble Space Telescope is the measurement of the
expansion rate of the Universe using distant Cepheids to help
measure distance. Though the four brightest Cepheids all look
similar to the eye, Mekbuda has the longest period, almost double
that of Delta Cephei, making it the most luminous of them and a
true "gem" of a star. (Thanks to Jason Pero, who helped research
this star.)