Magnitude 4.01 Apparent magnitude (V) 4.01 | ||
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Similar Epsilon Geminorum, Delta Geminorum, Eta Geminorum, Mu Geminorum, Gamma Geminorum | ||
Zeta geminorum occultation
Zeta Geminorum (ζ Geminorum, abbreviated Zeta Gem, ζ Gem), also named Mekbuda, is a star in the zodiac constellation of Gemini, on the outstretched left 'leg' of the twin Pollux. 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.
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Nomenclature
ζ Geminorum (Latinised to Zeta Geminorum) is the star's Bayer designation.
It bore the traditional name Mekbuda, from an Arabic phrase meaning "the lion's folded paw" (Zeta and Epsilon Geminorum (Mebsuta) were the paws of a lion). In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars. The WGSN approved the name Mekbuda for this star on 12 September 2016 and it is now so entered in the IAU Catalog of Star Names.
In Chinese, 井宿 (Jǐng Su), meaning Well (asterism), refers to an asterism consisting of Zeta Geminorum, Mu Geminorum, Gamma Geminorum, Nu Geminorum, Xi Geminorum, Epsilon Geminorum, 36 Geminorum and Lambda Geminorum. Consequently, Zeta Geminorum itself is known as 井宿七 (Jǐng Su qī, English: the Seventh Star of Well.)
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.
Properties
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. The spectral classification varies 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.
Zeta Geminorum has a visual companion 1.4' distant, which shares a common space velocity and may be physically associated. Zeta Gem B appears as an F4 main sequence star and is a spectroscopic binary. Zeta Geminorum 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.