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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 first of these discovered was Delta Cephei, after which the class of stars was named. It, Eta Aquilae, and the southern hemisphere's Beta Doradus are all about the same apparent brightness, Mekbuda changing from magnitude 3.7 to 4.1 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.)