Zeta Geminorum

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Zeta Geminorum
Diagram showing star positions and boundaries of the Gemini constellation and its surroundings
Cercle rouge 100%.svg

Location of ζ Geminorum (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Gemini
Right ascension 07h 04m 06.53079s[1]
Declination +20° 34′ 13.0739″[1]
Apparent magnitude (V) 3.93 (3.68 to 4.16)[2]
Spectral type G0Ibv[3] (F7Ib to G3Ib)[4]
U−B color index +0.55[5]
B−V color index 0.88[5]
Variable type Classical Cepheid
Radial velocity (Rv) +6.7[6] km/s
Proper motion (μ) RA: –7.29[1] mas/yr
Dec.: –0.41[1] mas/yr
Distance 1,183 ± 29(σ2) ± 85(σ) ly
(363 ± 9(σ2) ± 26(σ)[7] pc)
Mass 7.7 ± 0.3[8] M
Radius (65.24 ± 0.20) ± 4.17[9] R
Luminosity 2,900[10] L
Surface gravity (log g) 1.9[11] cgs
Temperature 5,260–5,780[4] K
Metallicity [Fe/H] 0.16[11] dex
Rotational velocity (v sin i) 19[12] km/s
Age 70 ± 25[7] Myr
Other designations
Mekbuda, ζ Gem, 43 Gem, BD +20 1687, FK5 269, HD 52973, HIP 34088, HR 32650, SAO 79031.[3]

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.[13][14][7]

Observation history[edit]

In 1825, German astronomer Julius Schmidt discovered that this star varies in brightness with a period of about 10 days,[15] although it had been suspected of variability as early as 1790.[16] In 1899, American astronomer W. W. Campbell announced that Zeta Geminorum has a variable radial velocity.[17] (This variation was independently discovered by Russian astronomer Aristarkh Belopolsky, published in 1901.)[16] 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.[18] 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.[16]

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.[19]


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.[2] This period of variation is decreasing at the rate of 3.1 seconds per year, or 0.085 seconds per cycle.[19] It has a nominal stellar classification of G0 Ibv,[3] 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,[4] while the radius varies from 61 to 69 times the Sun's radius.[9] On average, it is radiating about 2,900 times the luminosity of the Sun.[10]

There is no evidence of a stellar companion to Zeta Geminorum.[19] However, the star was recently discovered to belong to a star cluster.[7] Delta Cephei, the prototypical classical Cepheid variables, is also a member of a star cluster.[20][21] 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.[7] 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.[15] 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.[22] Consequently, ζ Geminorum itself is known as 井宿七 (Jǐng Su qī, English: the Seventh Star of Well.)[23]


  1. ^ a b c d van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. 
  2. ^ a b Klagyivik, P.; Szabados, L. (September 2009), "Observational studies of Cepheid amplitudes. I. Period-amplitude relationships for Galactic Cepheids and interrelation of amplitudes", Astronomy and Astrophysics 504 (3): 959–972, arXiv:0908.3561, Bibcode:2009A&A...504..959K, doi:10.1051/0004-6361/200811464 
  3. ^ a b c "V* zet Gem -- Classical Cepheid (delta Cep type)", SIMBAD (Centre de Données astronomiques de Strasbourg), retrieved 2012-01-01 
  4. ^ a b c Kervella, P. et al. (March 2001), The angular diameter and distance of the Cepheid ? Geminorum 367, pp. 876–883, arXiv:astro-ph/0102359, Bibcode:2001A&A...367..876K, doi:10.1051/0004-6361:20000490 
  5. ^ a b Nicolet, B. (1978), "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System", Observatory, Bibcode:1978ppch.book.....N 
  6. ^ Wielen, R. et al. (1999), Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions (35), Astronomisches Rechen-Institut Heidelberg, Bibcode:1999VeARI..35....1W 
  7. ^ a b c d e Majaess, D. et al. (2012), "Discovery of the Host Cluster for the Fundamental Cepheid Calibrator Zeta Geminorum", Astrophysical Journal Letters 748 (1): L9, arXiv:1202.2363, Bibcode:2012ApJ...748L...9M, doi:10.1088/2041-8205/748/1/L9 
  8. ^ Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x 
  9. ^ a b Groenewegen, M. A. T. (November 2007), "The projection factor, period-radius relation, and surface-brightness colour relation in classical cepheids", Astronomy and Astrophysics 474 (3): 975–981, Bibcode:2007A&A...474..975G, doi:10.1051/0004-6361:20078225 
  10. ^ a b Mallik, Sushma V. (December 1999), "Lithium abundance and mass", Astronomy and Astrophysics 352: 495–507, Bibcode:1999A&A...352..495M 
  11. ^ a b Mallik, Sushma V. (October 1998), "Chromospheric activity in cool stars and the lithium abundance", Astronomy and Astrophysics 338: 623–636, Bibcode:1998A&A...338..623M 
  12. ^ Uesugi, Akira; Fukuda, Ichiro (1970), "Catalogue of rotational velocities of the stars", Contributions from the Institute of Astrophysics and Kwasan Observatory (University of Kyoto), Bibcode:1970crvs.book.....U 
  13. ^ Benedict, G. Fritz et al. (April 2007), "Hubble Space Telescope Fine Guidance Sensor Parallaxes of Galactic Cepheid Variable Stars: Period-Luminosity Relations", Astronomical Journal 133 (4): 1810–1827, arXiv:astro-ph/0612465, Bibcode:2007AJ....133.1810B, doi:10.1086/511980. 
  14. ^ van Leeuwen, Floor et al. (August 2007), "Cepheid parallaxes and the Hubble constant", Monthly Notices of the Royal Astronomical Society 379 (2): 723–737, arXiv:0705.1592, Bibcode:2007MNRAS.379..723V, doi:10.1111/j.1365-2966.2007.11972.x. 
  15. ^ a b Allen, Richard Hinckley (1899), Star-names and their meanings, G. E. Stechert, p. 235 
  16. ^ a b c Henroteau, F. (1925), "A study of zeta Geminorum, I.", Publications of the Dominion Observatory Ottawa 9: 105–116, Bibcode:1925PDO.....9..105H 
  17. ^ Campbell, W. W. (February 1899), "The variable velocity of zeta Geminorum in the line of sight.", Astrophysical Journal 9: 86, Bibcode:1899ApJ.....9...86C, doi:10.1086/140556 
  18. ^ Campbell, W. W. (January 1901), "The motion of zeta Geminorum in the line of sight", Astrophysical Journal 13: 90–97, Bibcode:1901ApJ....13...90C, doi:10.1086/140792 
  19. ^ a b c Abt, Helmut A.; Levy, Saul G. (March 1974), "Period Variation of the Cepheid Zeta Geminorum", Astrophysical Journal 188: L75, Bibcode:1974ApJ...188L..75A, doi:10.1086/181436 
  20. ^ de Zeeuw, P. T. et al. (1999), "A HIPPARCOS Census of the Nearby OB Associations", Astronomical Journal 117 (1): 354–399, arXiv:astro-ph/9809227, Bibcode:1999AJ....117..354D, doi:10.1086/300682. 
  21. ^ Majaess, D.; Turner, D.; Gieren, W. (2012), "New Evidence Supporting Cluster Membership for the Keystone Calibrator Delta Cephei", Astrophysical Journal 747 (2): 145, arXiv:1201.0993, Bibcode:2012ApJ...747..145M, doi:10.1088/0004-637X/747/2/145. 
  22. ^ (Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
  23. ^ (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line November 23, 2010.

Coordinates: Sky map 07h 04m 06.53079s, +20° 34′ 13.0739″