Epoch J2000 Equinox J2000
|Right ascension||07h 04m 06.53079s|
|Declination||+20° 34′ 13.0739″|
|Apparent magnitude (V)||3.93 (3.68 to 4.16)|
|Spectral type||G0Ibv (F7Ib to G3Ib)|
|U−B color index||+0.55|
|B−V color index||0.88|
|Variable type||Classical Cepheid|
|Radial velocity (Rv)||+6.7 km/s|
|Proper motion (μ)||RA: –7.29 mas/yr
Dec.: –0.41 mas/yr
|Distance||1,183 ± 29(σ2) ± 85(σ) ly
(363 ± 9(σ2) ± 26(σ) pc)
|Mass||7.7 ± 0.3 M☉|
|Radius||(65.24 ± 0.20) ± 4.17 R☉|
|Surface gravity (log g)||1.9|
|Metallicity [Fe/H]||0.16 dex|
|Rotational velocity (v sin i)||19 km/s|
|Age||70 ± 25 Myr|
Zeta Geminorum (ζ Gem, ζ Geminorum) is the Bayer designation for a star in the zodiac constellation Gemini, on the outstretched left 'leg' of the twin Pollux. It has the traditional name Mekbuda, meaning the lion's folded paw. As a member of the category of variable stars known as classical Cepheids, it has a regular pulsation frequency that is determined by its mass. Because the mass allows the luminosity to be directly determined, the star serves as an important calibrator for the cosmic distance ladder.
Observation history 
In 1825, German astronomer Julius Schmidt discovered that this star varies in brightness with a period of about 10 days, although it had been suspected of variability as early as 1790. In 1899, American astronomer W. W. Campbell announced that Zeta Geminorum has a variable radial velocity. (This variation was independently discovered by Russian astronomer Aristarkh Belopolsky, published in 1901.) Based on his observations, Campbell later published orbital elements for the binary. However, he found that the curve departed from a keplerian orbit and even suggested that it was a triple star system in order to explain the irregularities. The periodic variation in radial velocity was subsequently explained as the result of radial pulsations that occur in a class of variable stars known as Cepheid variables—named after Delta Cephei.
The periodicity of the star is itself variable, a trend first noted by German astronomer Paul Guthnick in 1920, who suspected that the period change was the result of an orbiting companion. In 1930, Danish astronomer Axel Nielsen suggested that the change was instead the result in a steady decrease of about 3.6 seconds per year in the period.
This star is a Classical Cepheid variable that undergoes regular, periodic variation in luminosity because of radial pulsations. In the V band, the apparent magnitude varies between a high of 3.68 and a low of 4.16 (with a mean of 3.93) over a period of 10.148 days. This period of variation is decreasing at the rate of 3.1 seconds per year, or 0.085 seconds per cycle. It has a nominal stellar classification of G0 Ibv, with the luminosity class of Ib indicating that this is an intermediate luminous supergiant. The 'v' suffix means that it has variable spectral features, with the actual classification changing between F7Ib and G3Ib over the course of a pulsation cycle. Likewise the effective temperature of the outer envelope varies between 5,780 K and 5,260 K, while the radius varies from 61 to 69 times the Sun's radius. On average, it is radiating about 2,900 times the luminosity of the Sun.
There is no evidence of a stellar companion to Zeta Geminorum. However, the star was recently discovered to belong to a star cluster. Delta Cephei, the prototypical classical Cepheid variables, is also a member of a star cluster. The Cepheid's cluster membership, along with recent Hubble Space Telescope and Hipparcos parallaxes, place strong constraints on the star's distance: 363 ± 9(σ2) ± 26(σ) parsecs. Zeta Geminorum is thus an important calibrator for establishing the cosmic distance ladder.
This star has the traditional name Mekbuda, which comes from ancient Arabic where it and the star Mebsuta (Epsilon Geminorum) were the paws of a lion. Mekbuda comes from a phrase meaning the lion's folded paw. In Chinese, 井宿 (Jǐng Su), meaning Well (asterism), refers to an asterism consisting of ζ Geminorum, μ Geminorum, γ Geminorum, ν Geminorum, ξ Geminorum, ε Geminorum, 36 Geminorum and λ Geminorum. Consequently, ζ Geminorum itself is known as 井宿七 (Jǐng Su qī, English: the Seventh Star of Well.)
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