Eta Geminorum

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Eta Geminorum
Gemini constellation map.svg
Red circle.svg
Location of η Geminorum (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Gemini
Right ascension  06h 14m 52.657s[1]
Declination +22° 30′ 24.48″[1]
Apparent magnitude (V) 3.15 - 3.90[2]
Right ascension  06h 14m 52.569s[1]
Declination +22° 30′ 24.31″[1]
Apparent magnitude (V) 6.04[1]
Spectral type M2 IIIa[3] + G0 III[4]
Variable type SRa + EA[2]
Proper motion (μ) RA: −62.46 ± 1.06[5] mas/yr
Dec.: −12.12 ± 0.70[5] mas/yr
Parallax (π)8.48 ± 1.23[5] mas
Distanceapprox. 380 ly
(approx. 120 pc)
Absolute magnitude (MV)−1.87[6]
Period (P)8.17 yr
Semi-major axis (a)0.077″
Eccentricity (e)0.53
Period (P)473.7 yr
Semi-major axis (a)1.08″
Eccentricity (e)0.54
Mass2.5[4] M
Radius153[9] R
Luminosity3,162[9] L
Temperature3,548[9] K
Metallicity [Fe/H]+0.04[10] dex
Age0.81[4] Gyr
Other designations
Propus,[11] Praepes, Tejat Prior, Pish Pai,[12] η Geminorum, 7 Geminorum, HD 42995, HR 2216, BD+22°1241, HIP 29655, SAO 78135, CCDM J06149+2230, ADS 4841
Database references

Eta Geminorum (η Geminorum, abbreviated Eta Gem, η Gem), formally named Propus /ˈprpəs/,[13][11] is a triple star system in the constellation of Gemini. It is a naked-eye variable star around 380 light years from the Sun.


Eta Geminorum is the star's Bayer designation. The traditional names Tejat Prior, Propus (from the Greek, meaning forward foot) and Praepes and Pish Pai (from the Persian Pīshpāy, پیش‌پای, meaning foreleg). In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[14] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[15] included a table of the first two batches of names approved by the WGSN; which included Propus for this star.

This star, along with γ Gem (Alhena), μ Gem (Tejat Posterior), ν Gem and ξ Gem (Alzirr) were Al Han'ah, "the brand" (on the neck of the camel). They also were associated in Al Nuḥātai, the dual form of Al Nuḥāt, "a Camel's Hump".[16]

In Chinese lunar mansion, Tejat Prior is the only member of the lunar mansion 钺 (Pinyin: Yuè, Chinese 'Battle Axe').[17][16]


η Gem (right), μ Gem (left), and the nebulosity between them

η Geminorum lies at the foot of the Castor side of Gemini, about two degrees west of μ Geminorum and two degrees southeast of the bright open cluster M35. Between the two stars are several faint areas of nebulosity. η Gem just to the west of the supernova remnant shell IC 443. Further east around μ Gem is the emission nebula S249. In between is the small faint emission nebula IC 444 around the 7th magnitude 12 Geminorum.

η Geminorum is 0.9 degree south of the ecliptic, so it can be occulted by the Moon[18] and, rarely, by planets. The last occultation by a planet took place on July 27, 1910, by Venus,[19] and the next to last on July 11, 1837, by Mercury.[citation needed]


In 1865, Julius Schmidt first reported that η Geminorum was a variable star. The light variations were described by Schmidt and other observers as having long maxima of constant brightness, minima of greatly varying size and shape, and a period around 231 days.[20] The star was classified as both a semiregular variable and an eclipsing variable. The eclipse period has been set at about eight years, corresponding to the orbit of an unseen companion. The eclipses have been questioned many times, but special observations are still been made at the times of predicted eclipses.[21][22]

The semi-regular variations have been classified as type SRa, indicating relatively predictable periodicity with some variations in amplitude and light curve shape. These types of variable are considered to be very similar to Mira variables, but with smaller amplitudes.[2] Many long-period variables show long secondary periods, typically ten times longer than the main period, but these changes have not been detected for η Geminorum. The main period has been refined to an average of 234 days.[23]


η Gem is the bright star lying just outside the supernova remnant IC 443 (WISE infrared image)

η Geminorum is a triple system, with the luminous class M star having a close companion known only from radial velocity variations, and a more distant companion resolved visually.

In 1881, Burnham observed that η Geminorum had a close companion (η Gem B). At that time the separation was measured to be 1.08".[24] This has now increased to 1.65" and an orbit has been calculated to be 474 years long and rather eccentric.[20] Little is known about the companion, although it is 6th magnitude. It is given a G0 spectral type and is assumed to be a giant on the basis of its brightness.[4]

In 1902, William Wallace Campbell reported that η Geminorum A showed radial velocity variations. The assumption was that the star was a spectroscopic binary, although no period or other orbital parameters were determined.[25] An orbit calculated in 1944 is essentially unchanged today, with a period of 2,983 days and an eccentricity of 0.53. Observations were made looking for sign of eclipses corresponding to the derived orbit, but the evidence was regarded as inconclusive.[26] Due to the appearance of the spectrum, the spectroscopic companion is suspected to be a fainter M-class star.[4]


The luminous main component of η Geminorum is an asymptotic giant branch star, a highly evolved cool luminous star that was originally 2-8 M on the main sequence.[27] How the system contains 3 evolved stars with very different masses remains to be seen.


USS Propus (AK-132) was a United States Navy Crater class cargo ship named after the star.


  1. ^ a b c d e Høg, E.; Fabricius, C.; Makarov, V. V.; Urban, S.; Corbin, T.; Wycoff, G.; Bastian, U.; Schwekendiek, P.; Wicenec, A. (2000). "The Tycho-2 catalogue of the 2.5 million brightest stars". Astronomy and Astrophysics. 355: L27. Bibcode:2000A&A...355L..27H. doi:10.1888/0333750888/2862.
  2. ^ a b c 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.
  3. ^ Keenan, Philip C.; McNeil, Raymond C. (1989). "The Perkins catalog of revised MK types for the cooler stars". Astrophysical Journal Supplement Series. 71: 245. Bibcode:1989ApJS...71..245K. doi:10.1086/191373.
  4. ^ a b c d e Hunsch, Matthias; Schmitt, Jurgen H. M. M.; Schroder, Klaus-Peter; Zickgraf, Franz-Josef (1998). "On the X-ray emission from M-type giants". Astronomy and Astrophysics. 330: 225. Bibcode:1998A&A...330..225H.
  5. ^ a b c Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357.
  6. ^ Ryon, Jenna; et al. (August 2009), "Comparing the Ca ii H and K Emission Lines in Red Giant Stars", Publications of the Astronomical Society of the Pacific, 121 (882): 842, arXiv:0907.3346, Bibcode:2009PASP..121..842R, doi:10.1086/605456.
  7. ^ Malkov, O. Yu.; Tamazian, V. S.; Docobo, J. A.; Chulkov, D. A. (2012). "Dynamical masses of a selected sample of orbital binaries". Astronomy & Astrophysics. 546: A69. Bibcode:2012A&A...546A..69M. doi:10.1051/0004-6361/201219774.
  8. ^ Tokovinin, A. A. (1997). "MSC - a catalogue of physical multiple stars". Astronomy and Astrophysics Supplement Series. 124: 75. Bibcode:1997A&AS..124...75T. doi:10.1051/aas:1997181.
  9. ^ a b c Jorissen, A.; Frankowski, A.; Famaey, B.; Van Eck, S. (2009). "Spectroscopic binaries among Hipparcos M giants. III. The eccentricity - period diagram and mass-transfer signatures". Astronomy and Astrophysics. 498 (2): 489. arXiv:0901.0938. Bibcode:2009A&A...498..489J. doi:10.1051/0004-6361/200810703.
  10. ^ Huang, Y.; Liu, X.-W.; Yuan, H.-B.; Xiang, M.-S.; Chen, B.-Q.; Zhang, H.-W. (2015). "Empirical metallicity-dependent calibrations of effective temperature against colours for dwarfs and giants based on interferometric data". Monthly Notices of the Royal Astronomical Society. 454 (3): 2863. arXiv:1508.06080. Bibcode:2015MNRAS.454.2863H. doi:10.1093/mnras/stv1991.
  11. ^ a b "IAU Catalog of Star Names". Retrieved 28 July 2016.
  12. ^ p. 235, Star-names and Their Meanings, Richard Hinckley Allen, G. E. Stechert, 1899.
  13. ^ Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
  14. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  15. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016.
  16. ^ a b Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York, NY: Dover Publications Inc. p. 234. ISBN 0-486-21079-0. Retrieved 2010-12-12.
  17. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 26 日
  18. ^ White, Nathaniel M.; Feierman, Barry H. (September 1987), "A Catalog of Stellar Angular Diameters Measured by Lunar Occultation", Astronomical Journal, 94: 751, Bibcode:1987AJ.....94..751W, doi:10.1086/114513.
  19. ^ Können, G. P.; Van Maanen, J. (April 1981). "Planetary occultations of bright stars". Journal of the British Astronomical Association. 91: 148–157. Bibcode:1981JBAA...91..148K.
  20. ^ a b Hassforther, B. (2007). "Eta Geminorum - wirklich ein Bedeckungsveraenderlicher?". BAV Rundbrief. 56: 205. Bibcode:2007BAVSR..56..205H.
  21. ^ "The 2004 eclipse of Eta Geminorum" (PDF). Retrieved August 12, 2016.
  22. ^ "The 2004 eclipse of Eta Geminorum" (PDF). Retrieved August 12, 2016.
  23. ^ Percy, J. R.; Nasui, C. O.; Henry, G. W. (2008). "Long-Term Photometric Variability of 13 Bright Pulsating Red Giants". The Journal of the American Association of Variable Star Observers. 36: 139. Bibcode:2008JAVSO..36..139P.
  24. ^ Clerke, A. M. (1902). "The system of eta Geminorum". The Observatory. 25: 389. Bibcode:1902Obs....25..389C.
  25. ^ Campbell, W. W. (1902). "Six stars whose velocities in the line of sight are variable". Astrophysical Journal. 16: 114. Bibcode:1902ApJ....16..114C. doi:10.1086/140954.
  26. ^ McLaughlin, Dean B.; Van Dijke, Suzanne E. A. (1944). "The Spectrographic Orbit and Light-Variations of η Geminorum". Astrophysical Journal. 100: 63. Bibcode:1944ApJ...100...63M. doi:10.1086/144637.
  27. ^ Eggen, Olin J. (1992). "Asymptotic giant branch stars near the sun". Astronomical Journal. 104: 275. Bibcode:1992AJ....104..275E. doi:10.1086/116239.