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Delta Geminorum

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Delta Geminorum
Location of δ Geminorum (circled)
Observation data
Epoch J2000      Equinox ICRS
Constellation Gemini
δ Gem A
Right ascension 07h 20m 07.3746s[1]
Declination +21° 58′ 56.295″[1]
Apparent magnitude (V) +3.53[2]
δ Gem B
Right ascension 07h 20m 07.0768s[3]
Declination +21° 58′ 52.183″[3]
Apparent magnitude (V) +8.18[4]
Characteristics
δ Gem A
Evolutionary stage main sequence[5]
Spectral type F2VkF0mF0[6]
U−B color index +0.04[2]
B−V color index +0.34[2]
δ Gem B
Spectral type K6V[7]
Astrometry
δ Gem A
Radial velocity (Rv)−15.3±1.5[8] km/s
Proper motion (μ) RA: –27.722 mas/yr[1]
Dec.: –14.702 mas/yr[1]
Parallax (π)53.7692±0.2403 mas[1]
Distance60.7 ± 0.3 ly
(18.60 ± 0.08 pc)
Absolute magnitude (MV)+1.95[9]
δ Gem B
Proper motion (μ) RA: −28.196 mas/yr[3]
Dec.: +13.306 mas/yr[3]
Parallax (π)53.9507±0.0452 mas[3]
Distance60.45 ± 0.05 ly
(18.54 ± 0.02 pc)
Orbit[10]
Period (P)550±450 yr
Semi-major axis (a)13.8±1.6″
Eccentricity (e)0.98±0.30
Inclination (i)75±10°
Longitude of the node (Ω)128±51°
Periastron epoch (T)2136±57
Argument of periastron (ω)
(secondary)		266±32°
Details
δ Gem A
Mass1.57[11] M☉
Radius2.06±0.25[12] R☉
Luminosity9.88[13] L☉
Temperature6,900[14] K
Metallicity [Fe/H]–0.26[14] dex
Rotational velocity (v sin i)129.7[15] km/s
Age1.6[14] Gyr
δ Gem B
Radius0.59[16] R☉
Luminosity0.189[16] L☉
Temperature4,952[16] K
Other designations
Wasat, 55 Geminorum, BD+22°1645, FK5 279, Gl 271, HD 56986, HIP 35550, HR 2777, SAO 79294[17]
Database references
SIMBADδ Gem A
δ Gem B

Delta Geminorum (δ Geminorum, abbreviated Delta Gem, δ Gem), formally named Wasat /ˈweɪsət/,[18][19] is a binary star system in the constellation of Gemini.

Nomenclature

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δ Geminorum (Latinised to Delta Geminorum) is the system's Bayer designation.

It bore the traditional name Wasat, which derives from the Arabic word for "middle".[20][21] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[22] to catalogue and standardize proper names for stars. The WGSN approved the name Wasat for Delta Geminorum A on 21 August 2016 and it is now so entered in the IAU Catalog of Star Names.[19]

In Chinese, 天樽 (Tiān Zūn), meaning Celestial Wine Cup, refers to an asterism consisting of Delta Geminorum, 57 Geminorum and Omega Geminorum.[23] Consequently, Delta Geminorum itself is known as 天樽二 (Tiān Zūn èr, English: the Second Star of Celestial Wine Cup.).[24] From this Chinese name, the name Ta Tsun has appeared.[25]

Properties

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Delta Geminorum is the bright star next to Jupiter.[26] Jupiter is ~280× brighter.

Delta Geminorum A is a main-sequence star with the stellar classification F2VkF0mF0.[6] This star has 1.57 times the mass of the Sun[11] and is rotating rapidly with a projected rotational velocity of 129.7 km s−1.[15] The estimated age is 1.6 billion years.[14]

It has an apparent visual magnitude of +3.53,[2] allowing it to be seen with the naked eye. It is 0.18 degree south of the ecliptic so it is occasionally occulted by the Sun, Moon and, rarely, by a planet; and is eclipsed by the sun from about 10-12 July.[27] Thus the star can be viewed the whole night, crossing the sky, in mid-January. The last occultation by a planet was by Saturn on June 30, 1857, and the next will be by Venus on August 12, 2420.[citation needed] In 1930, the dwarf planet Pluto was discovered about 0.5° to the east of this star by American astronomer Clyde Tombaugh.[28]

Delta Geminorum A has been reported to be a single-lined spectroscopic binary in 1964, with a preliminary orbital solution indicating an orbital period of 2,238.6 days, a minimum semi-major axis of 5.22 au and an eccentricity of 0.353,[29] although these orbital elements could not be confirmed with proper motion measurements[30] and the companion was never directly detected,[31][32] making it unlikely that Delta Geminorum A is a binary system.[31] The binary mass function, together with the mass of the visible star, would imply a secondary mass of at least 6 solar masses, making it a black hole.[32]

The companion is a cooler class K star,[7] not apparent to the naked eye, but clearly visible in a small telescope.[20] The orbital period is uncertain, 550±450 years, and the eccentricity is also uncertain, at 0.98±0.30. The angular semi-major axis is somewhat better-determined, at 13.8±1.6".[10] The star has 59% the radius of the Sun, 18.9% the luminosity and an effective temperature of 4,952 K.[16]

The system is now approaching the Solar System at a radial velocity of −15.3 km/s, and is expected to reach its closest distance in 1.1 million years, at about 6.7 ly (2.1 pc).[8]

References

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  1. ^ a b c d Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  2. ^ a b c d Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99): 99, Bibcode:1966CoLPL...4...99J
  3. ^ a b c d Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  4. ^ Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (December 2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal. 122 (6): 3466–3471. doi:10.1086/323920. ISSN 0004-6256.
  5. ^ Zorec, J.; Royer, F. (2012), "Rotational velocities of A-type stars. IV. Evolution of rotational velocities", Astronomy and Astrophysics, 537: A120, arXiv:1201.2052, Bibcode:2012A&A...537A.120Z, doi:10.1051/0004-6361/201117691
  6. ^ a b Gray, R. O.; Corbally, C. J.; Garrison, R. F.; McFadden, M. T.; Robinson, P. E. (2003), "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.", The Astronomical Journal, 126 (4): 2048, arXiv:astro-ph/0308182, Bibcode:2003AJ....126.2048G, doi:10.1086/378365
  7. ^ a b Adams, Walter S.; Joy, Alfred H.; Humason, Milton L.; Brayton, Ada Margaret (April 1935), "The Spectroscopic Absolute Magnitudes and Parallaxes of 4179 Stars", The Astrophysical Journal, 81: 187, Bibcode:1935ApJ....81..187A, doi:10.1086/143628, ISSN 0004-637X
  8. ^ a b García-Sánchez, J.; et al. (November 2001), "Stellar encounters with the solar system", Astronomy and Astrophysics, 379 (2): 634–659, Bibcode:2001A&A...379..634G, doi:10.1051/0004-6361:20011330
  9. ^ Reiners, Ansgar (January 2006), "Rotation- and temperature-dependence of stellar latitudinal differential rotation", Astronomy and Astrophysics, 446 (1): 267–277, arXiv:astro-ph/0509399, Bibcode:2006A&A...446..267R, doi:10.1051/0004-6361:20053911, S2CID 8642707
  10. ^ a b Izmailov, Igor; Khovritchev, Maxim (January 2025), "New Orbital Parameters of 850 Wide Visual Binary Stars and Their Statistical Properties", Research in Astronomy and Astrophysics, 25 (1): 015016, Bibcode:2025RAA....25a5016I, doi:10.1088/1674-4527/ad9da3, ISSN 1674-4527
  11. ^ a b Shaya, Ed J.; Olling, Rob P. (January 2011), "Very Wide Binaries and Other Comoving Stellar Companions: A Bayesian Analysis of the Hipparcos Catalogue", The Astrophysical Journal Supplement, 192 (1): 2, arXiv:1007.0425, Bibcode:2011ApJS..192....2S, doi:10.1088/0067-0049/192/1/2, S2CID 119226823
  12. ^ Baines, Ellyn K.; et al. (May 7, 2025), "Vintage NPOI: New and Updated Angular Diameters for 145 Stars", The Astronomical Journal, 169 (6): 293, arXiv:2506.02912, Bibcode:2025AJ....169..293B, doi:10.3847/1538-3881/adc930, ISSN 1538-3881
  13. ^ Gáspár, András; Rieke, George H.; Ballering, Nicholas (Aug 2016), "The Correlation between Metallicity and Debris Disk Mass", The Astrophysical Journal, 826 (2): 171, arXiv:1604.07403, Bibcode:2016ApJ...826..171G, doi:10.3847/0004-637X/826/2/171, ISSN 0004-637X
  14. ^ a b c d Nordström, B.; et al. (May 2004), "The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of ˜14 000 F and G dwarfs", Astronomy and Astrophysics, 418: 989–1019, arXiv:astro-ph/0405198, Bibcode:2004A&A...418..989N, doi:10.1051/0004-6361:20035959, S2CID 11027621
  15. ^ a b Schröder, C.; Reiners, Ansgar; Schmitt, Jürgen H. M. M. (January 2009), "Ca II HK emission in rapidly rotating stars. Evidence for an onset of the solar-type dynamo", Astronomy and Astrophysics, 493 (3): 1099–1107, Bibcode:2009A&A...493.1099S, doi:10.1051/0004-6361:200810377
  16. ^ a b c d Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
  17. ^ "Delta Geminorum (Wasat)", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2011-06-03
  18. ^ 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
  19. ^ a b IAU Catalog of Star Names, retrieved 28 July 2016
  20. ^ a b Kaler, Jim (n.d.), "WASAT (Delta Geminorum)", Stars (University of Illinois sponsored website), retrieved July 29, 2014, The name is a mess, "Wasat" meaning "middle" in Arabic, but the middle of WHAT is not clear, whether the middle of Gemini, of the sky, or of the neighboring constellation Orion (which the Arabs referred to as the "Central One"), the star name improperly applied to our Delta.
  21. ^ Allen, Richard (1889). "The history of the star: Wasat, from p.234 of Star Names, Richard Hinckley Allen, 1889". Constellations of Words website. Retrieved July 29, 2014. Wasat and Wesat are from Al Wasat, the Middle, i.e. of the constellation; but some have referred this to the position of the star very near to the ecliptic, the central circle.
  22. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, archived from the original on 26 June 2019, retrieved 22 May 2016
  23. ^ (in 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.
  24. ^ (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived August 19, 2010, at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
  25. ^ Richard Hinckley Allen: Star Names — Their Lore and Meaning: Gemini
  26. ^ King, Bob (12 December 2013), "To Delta Geminorum by way of Jupiter and Pluto", Astro Bob, Areavoices.com, archived from the original on 13 December 2013, retrieved 13 December 2013
  27. ^ In the Sky Earth astronomy reference utility showing the ecliptic and relevant date as at J2000 - present
  28. ^ O'Meara, Stephen James (2002), The Caldwell objects, Deep-sky companions, Cambridge University Press, p. 156, ISBN 0-521-82796-5
  29. ^ Abt, Helmut (March 1961), "The Frequency of Binaries among A-Type Stars.", The Astronomical Journal, 66: 277, Bibcode:1961AJ.....66Q.277A, doi:10.1086/108502, ISSN 0004-6256
  30. ^ Abt, H. A.; Levy, S. G. (April 1974), "Reinvestigation of certain long-period A-type binaries.", The Astrophysical Journal, 188: 291–294, Bibcode:1974ApJ...188..291A, doi:10.1086/152716, ISSN 0004-637X
  31. ^ a b Tremaine, S. D.; Groth, E. J.; Nelson, M. R. (May 1974), "Observations of a lunar occultation of delta Geminorum.", The Astronomical Journal, 79: 649–650, Bibcode:1974AJ.....79..649T, doi:10.1086/111590, ISSN 0004-6256
  32. ^ a b Rowan, D. M.; Kraus, S.; Thompson, Todd A. (2025-12-01), "Hidden in Plain Sight II: Characterizing the luminous companion to Kappa Velorum with VLTI/GRAVITY", arXiv:2512.02116 [astro-ph.SR]
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