Alpha Trianguli

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Alpha Trianguli
(incl. Mothallah)
Diagram showing star positions and boundaries of the Triangulum constellation and its surroundings
Cercle rouge 100%.svg
Location of α Trianguli (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Triangulum
Right ascension  01h 53m 04.90710s[1]
Declination +29° 34′ 43.7801″[1]
Apparent magnitude (V) +3.42[2]
Spectral type F5III[3] to F6IV[4] + M[5]
U−B color index +0.06[2]
B−V color index +0.48[2]
Variable type Ellipsoidal variable[4]
Radial velocity (Rv)–12.6[6] km/s
Proper motion (μ) RA: 10.82[1] mas/yr
Dec.: –234.24[1] mas/yr
Parallax (π)51.50 ± 0.23[1] mas
Distance63.3 ± 0.3 ly
(19.42 ± 0.09 pc)
Absolute magnitude (MV)1.98[7]
α Tri A
Mass1.70[5] M
Radius3.22[5] R
Surface gravity (log g)3.91[8] cgs
Temperature6,288[8] K
Metallicity [Fe/H]–0.09[7] to 0.00[8] dex
Rotational velocity (v sin i)81.6[9] km/s
Age1.6[7] Gyr
α Tri B
Mass0.11[5] M
Other designations
Mothallah, Ras al Muthallah, Caput Trianguli, α Trianguli,[10] 2 Trianguli, HR 544, HD 11443, BD+28° 312, FK5 64, HIP 8796, SAO 74996.[11]
Database references

Alpha Trianguli (α Trianguli, abbreviated Alpha Tri, α Tri) is a spectroscopic binary star in the constellation of Triangulum. Based on parallax measurements obtained during the Hipparcos mission, it is approximately 63.3 light-years (19.4 parsecs) distant from the Sun.[1] The brighter or primary component is named Mothallah /məˈθælə/.[12]


α Trianguli (Latinised to Alpha Trianguli) is the system's Bayer designation.

The system bore the traditional names Ras al Muthallah or Mothallah and Caput Trianguli derived from the Arabic رأس المثلث raʼs al-muthallath "the head of the triangle" and its Latin translation.[10] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[13] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[14] It approved the name Mothallah for this star on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.[12] For such names relating to members of multiple star systems, and where a component letter (from e.g. Washington Double Star Catalog) is not explicitly listed, the WGSN says that the name should be understood to be attributed to the brightest component by visual brightness.[15]

In combination with Beta Trianguli, these stars were called Al Mīzān, which is Arabic for "The Scale Beam".[16]

In Babylonian astronomy, Alpha Trianguli is listed as UR.BAR.RA "The Wolf", bearing the epithet "the seeder of the plough" in the MUL.APIN, listed after "The Plough", the name for a constellation formed of Triangulum plus Gamma Andromedae.[17]


Estimates of the combined stellar classification for this system range from F5III[3] to F6IV,[4] with the luminosity class of 'IV' or 'III' indicating the primary component is a subgiant or giant star, respectively. It is a member of a close binary system—a spectroscopic binary—whose components complete an orbit about their center of mass once every 1.736 days. Because the primary star is rotating rapidly, it has assumed the shape of an oblate spheroid. The ellipsoidal profile of the star, as viewed from Earth, varies over the course of an orbit causing the luminosity to vary in magnitude during the same period.[4] Such stars are termed ellipsoidal variables. Within a few million years, as the primary continues to evolve into a red giant star, the system may become a semi-detached binary with the Roche lobe becoming filled to overflowing.[5]

The mean apparent magnitude of +3.42[2] for this pair is bright enough to be readily seen with the naked eye. It forms the second brightest star or star system in this generally faint constellation, following Beta Trianguli. The effective temperature of the primary's outer envelope is 6,288 K,[8] giving it a yellow-white hue typical of F-type stars.[18] It has a mean radius about three times the radius of the Sun.[4] The system is an estimated 1.6 billion years old.[7]


  1. ^ a b c d e f 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 c d Johnson, H. L.; et al. (1966), "UBVRIJKL photometry of the bright stars", Communications of the Lunar and Planetary Laboratory, 4 (99), Bibcode:1966CoLPL...4...99J
  3. ^ a b Abt, Helmut A. (January 2009), "MK Classifications of Spectroscopic Binaries", The Astrophysical Journal Supplement, 180 (1): 117–118, Bibcode:2009ApJS..180..117A, doi:10.1088/0067-0049/180/1/117
  4. ^ a b c d e Fekel, Francis C.; Warner, Phillip B.; Kaye, Anthony B. (April 2003), "Spectroscopy of Early F Stars: γ Doradus Candidates and Possible Metallic Shell Stars", The Astronomical Journal, 125 (4): 2196–2214, Bibcode:2003AJ....125.2196F, doi:10.1086/368239
  5. ^ a b c d e Fuhrmann, Klaus (February 2008), "Nearby stars of the Galactic disc and halo - IV", Monthly Notices of the Royal Astronomical Society, 384 (1): 173–224, Bibcode:2008MNRAS.384..173F, doi:10.1111/j.1365-2966.2007.12671.x
  6. ^ Wilson, Ralph Elmer (1953), General catalogue of stellar radial velocities, Carnegie Institution of Washington, Bibcode:1953GCRV..C......0W
  7. ^ 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
  8. ^ a b c d Balachandran, Suchitra (May 1990), "Lithium depletion and rotation in main-sequence stars", Astrophysical Journal, Part 1, 354: 310–332, Bibcode:1990ApJ...354..310B, doi:10.1086/168691
  9. ^ Schröder, C.; Reiners, A.; Schmitt, J. H. M. M. (January 2009), "Ca II HK emission in rapidly rotating stars. Evidence for an onset of the solar-type dynamo" (PDF), Astronomy and Astrophysics, 493 (3): 1099–1107, Bibcode:2009A&A...493.1099S, doi:10.1051/0004-6361:200810377
  10. ^ a b Allen, R. H. (1899). Star Names: Their Lore and Meaning. G. E. Stechert. p. 416.
  11. ^ "V* alf Tri -- Ellipsoidal variable Star", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2011-12-12
  12. ^ a b "Naming Stars". Retrieved 18 June 2018.
  13. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  14. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  15. ^ "Bulletin of the IAU Working Group on Star Names, No. 2" (PDF). Retrieved 16 December 2017.
  16. ^ Garfinkle, Robert A. (1997), Star-Hopping: Your Visa to Viewing the Universe, Cambridge University Press, p. 238, ISBN 0-521-59889-3
  17. ^ Rogers, J. H. (February 1998). "Origins of the ancient constellations: I. The Mesopotamian traditions". Journal of the British Astronomical Association. 108 (1): 9–28. Bibcode:1998JBAA..108....9R.
  18. ^ "The Colour of Stars", Australia Telescope, Outreach and Education, Commonwealth Scientific and Industrial Research Organisation, December 21, 2004, archived from the original on March 10, 2012, retrieved 2012-01-16

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