Phi Orionis

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Phi Orionis
Observational Data
Designation φ Orionis
37 Orionis
Right ascension 05h 34m 49.2380s[1][2]
Declination +09° 29′ 22.4878″[1][2]
Constellation Orion
Type of object Main sequence star
Magnitude Apparent Mag: +4.39[3]
Absolute Mag: -3.01 / -3.22[3]
Distance from Earth 1087.21 ly[3]
Radial velocity 33.2 ± 0.9 km/s[1][4]
Proper motion RA: 0.27 mas/yr[1][2]
Dec: -2.26 mas/yr[1][2]
Parallax 3.00 ± 0.25 mas[1][2]

Phi Orionis (37 Orionis, φ Orionis) is a main sequence star in the constellation of Orion.[5] The name "Phi Orion" actually refers to two unrelated stars in Orion that happen to appear near each other, designated Phi-1 Orion and Phi-2 Orion (sometimes written with superscripts as Phi1 Orionis and Phi2 Orionis.)[6] They are separated by 0.71° degrees in the night sky and along with the star Meissa, form a lesser known triangle.[7] The two stars along with Lambda Orionis are situated in a red emission nebula designated Sh2-264.[8] Here we mainly consider Phi-1 Orion.

Phi-1 Orionis is located nine times further away from Earth than Phi-2 Orionis at a distance of 1087.21 light years.[3] The star is significantly larger than the Sun with a mass of 14 solar masses and a radius 6.9 times larger than Sol's.[5] Its extraordinarily high surface temperature of 28,900 kelvin means it radiates vast quantities of radiation in the ultraviolet portion of the electromagnetic spectrum. It also contributes to Phi-1 Orionis' high luminosity of 29,650 times larger than the Sun's.[5] It's equator rotates at 28 kilometers per second, or equivalently the star rotates with a period of just under 12.3 days, fairly slow compared to most stars. The star is known from spectroscopy to have a companion star, making it a binary system.[9] They orbit each other with a period of 8.4 years and an orbital separation of 10 Astronomical units.[5]

Phi-2 Orionis

Phi-2 Orionis (Flamsteed designation 40 Orionis) is a hypergiant star and is situated much closer than Phi-1 Orionis, at around 117.48 light years.[10][11] It is slightly brighter in the night sky than its counterpart at magnitude +4.1 as it is much closer; intrinsically it is only 34.8 times brighter than the Sun.[12] It is 6.95 times the size of the Sun and has a surface temperature of 4,948 Kelvin.


  1. 1.0 1.1 1.2 1.3 1.4 1.5 37 Orionis. Simbad Astronomical Database. Retrieved on July 09, 2020.
  2. 2.0 2.1 2.2 2.3 2.4 van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy & Astrophysics 474 (2): 653-664. doi:10.1051/0004-6361:20078357. Bibcode2007A&A...474..653V.  arXiv:0708.1752
  3. 3.0 3.1 3.2 3.3 Phi1 Orionis (37 Orionis) Star Facts. Retrieved on July 09, 2020.
  4. Wilson, Raplh Elmer (1953). "General Catalogue of Stellar Radial Velocities". Washington, [Carnegie Institution of Washington]. Bibcode1953GCRV..C......0W. 
  5. 5.0 5.1 5.2 5.3 Jim Kaller (February 02, 2012). PHI-1 ORI. Stars. Retrieved on July 09, 2020.
  6. Phi1 Orionis. Retrieved on July 09, 2020.
  7. Orion Constellation. constellation-guide. Retrieved on July 09, 2020.
  8. CCD optical image of nebula around Lambda Orionis. Retrieved on July 09, 2020.
  9. Struve, O. (1926). "Preliminary orbit of the long-period spectroscopic binary 37 phi-1 Orionis.". Astrophysical Journal 63: 60-66. doi:10.1086/142950. Bibcode1926ApJ....63...60S. 
  10. φ Orionis. Retrieved on July 09, 2020.
  11. φ2 Orionis (phi2 Orionis). Retrieved on July 09, 2020.
  12. Phi2 Orionis (40 Orionis) Star Facts. Retrieved on July 09, 2020.