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||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
|Parallax (π)||2.78 ± 0.18 mas|
|Distance||1,183 ± 29(σ2) ± 85(σ) ly
(363 ± 9(σ2) ± 26(σ) pc)
|Absolute magnitude (MV)||-3.99|
|Mass||7.7 ± 0.3 M☉|
|Radius||(65.24 ± 0.20) ± 4.17 R☉|
|Surface gravity (log g)||1.9 cgs|
|Metallicity [Fe/H]||0.16 dex|
|Rotational velocity (v sin i)||19 km/s|
|Age||70 ± 25 Myr|
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 closely related to the luminosity and the star serves as an important calibrator for the cosmic distance ladder.
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.)
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.
ζ Geminorum has three companions listed in the Washington Double Star Catalog. The closest is ζ Gem D, a 12th magnitude star measured to be 67.8" away in 2008. It was 80" distant when first measured in 1905. ζ Gem C is the magnitude 7.6 G1 main sequence star HD 268518, 91.9" away when discovered in 1779 and 101.3" distant in 2008. ζ Gem B is an 11th magnitude star76.0" distant in 1831 and 87.4" in 2008.
ζ Geminorum B is itself a spectroscopic binary, although little is known about the two components. The combined spectrum is of an F4 main sequence star. It is thought to be physically associated with the supergiant primary and a member of a loose cluster of stars around ζ Geminorum.
A combination of photometry, spectroscopy, and astrometry has identified 26 stars approximately 355 parsecs away, which are likely to be members of the birth cluster of ζ Geminorum. The brightest are late B and early A giant stars such as the 7th magnitude stars HD 49381 and HD 50634, while the faintest detected cluster members are 12th magnitude class F main sequence stars including ζ Geminorum B.
The star ζ Geminorum A is a Classical Cepheid variable that undergoes regular, periodic variation in brightness 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.
Membership of a cluster provides independent validation of distances determined using recent Hubble Space Telescope and Hipparcos parallaxes. This strongly constrains the star's distance: 363 ± 9(σ2) ± 26(σ) parsecs. Zeta Geminorum is thus an important calibrator for the Cepheid period-luminosity relation used for establishing the cosmic distance ladder.
- van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv: . Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
- 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: , Bibcode:2009A&A...504..959K, doi:10.1051/0004-6361/200811464
- Kervella, P.; et al. (March 2001), "The angular diameter and distance of the Cepheid ? Geminorum", Astronomy and Astrophysics, 367 (3): 876–883, arXiv: , Bibcode:2001A&A...367..876K, doi:10.1051/0004-6361:20000490
- Nicolet, B. (1978), "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System", Observatory, Bibcode:1978ppch.book.....N
- Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. Originally published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
- Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, Astronomisches Rechen-Institut Heidelberg, 35 (35), Bibcode:1999VeARI..35....1W
- 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: , Bibcode:2007AJ....133.1810B, doi:10.1086/511980.
- Majaess, D.; et al. (2012), "Discovery of the Host Cluster for the Fundamental Cepheid Calibrator Zeta Geminorum", Astrophysical Journal Letters, 748 (1): L9, arXiv: , Bibcode:2012ApJ...748L...9M, doi:10.1088/2041-8205/748/1/L9
- Turner, D. G. (2010). "The PL calibration for Milky Way Cepheids and its implications for the distance scale". Astrophysics and Space Science. 326 (2): 219. arXiv: . Bibcode:2010Ap&SS.326..219T. doi:10.1007/s10509-009-0258-5.
- 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: , Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x
- 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
- Mallik, Sushma V. (December 1999), "Lithium abundance and mass", Astronomy and Astrophysics, 352: 495–507, Bibcode:1999A&A...352..495M
- 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
- 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
- "V* zet Gem -- Classical Cepheid (delta Cep type)", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-01
- "IAU Catalog of Star Names". Retrieved 28 July 2016.
- Allen, Richard Hinckley (1899), "Star-names and their meanings", New York, G. E. Stechert: 235, Bibcode:1899sntm.book.....A
- IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
- (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.
- (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived January 30, 2011, at the Wayback Machine., Hong Kong Space Museum. Accessed on line November 23, 2010.
- Henroteau, F. (1925), "A study of zeta Geminorum, I.", Publications of the Dominion Observatory Ottawa, 9: 105–116, Bibcode:1925PDO.....9..105H
- 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
- 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
- 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
- Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal. 122 (6): 3466. Bibcode:2001AJ....122.3466M. doi:10.1086/323920.
- de Zeeuw, P. T.; et al. (1999), "A HIPPARCOS Census of the Nearby OB Associations", Astronomical Journal, 117 (1): 354–399, arXiv: , Bibcode:1999AJ....117..354D, doi:10.1086/300682.
- Majaess, D.; Turner, D.; Gieren, W. (2012), "New Evidence Supporting Cluster Membership for the Keystone Calibrator Delta Cephei", Astrophysical Journal, 747 (2): 145, arXiv: , Bibcode:2012ApJ...747..145M, doi:10.1088/0004-637X/747/2/145.