Eta Orionis

Last updated
η Orionis
Orion constellation map.svg
Red circle.svg
Location of η Orionis (circled)
Observation data
Epoch J2000        Equinox J2000
Constellation Orion
Right ascension 05h 24m 28.61672s [1]
Declination −02° 23′ 49.7311″ [1]
Apparent magnitude  (V)3.42 [2] (4.50 + 5.90 + 5.65 + 4.95) [3]
Characteristics
Spectral type Aa: B1 V
Ab: B3 V
Ac: B3 V
B: B2 V [3]
U−B color index −0.90 [2]
B−V color index −0.17 [2]
Variable type Algol [4]
Astrometry
Radial velocity (Rv)+19.8 [5] km/s
Proper motion (μ)RA: −0.71 [1] mas/yr
Dec.: −3.46 [1] mas/yr
Parallax (π)3.34±1.07  mas [1]
Distance approx. 1,000  ly
(approx. 300  pc)
Orbit
PrimaryAa
CompanionAb
Period (P)7.98763(22) [6] days
Eccentricity (e)0.0095±0.0010 [6]
Inclination (i)87.62±0.42 [6] °
Argument of periastron (ω)
(primary)		164±18 [6] °
Semi-amplitude (K1)
(primary)		145.5±0.03 [7] km/s
Semi-amplitude (K2)
(secondary)		150±3 [7] km/s
Orbit [3]
CompanionAc
Period (P)9.442±0.012 yr
Semi-major axis (a)0.0441±0.0015″
Eccentricity (e)0.45±0.02
Inclination (i)102.8±1.8°
Details
η Ori Aa
Mass 10.87±0.44 [6]   M☉
Radius 6.477±0.073 [6]   R☉
Surface gravity (log g)3.851±0.010 [6]   cgs
Temperature 26,600 [7]   K
Rotational velocity (v sin i)20 [7]  km/s
η Ori Ab
Mass 10.54±0.22 [6]   M☉
Radius 4.79±0.10 [6]   R☉
Surface gravity (log g)4.100±0.016 [6]   cgs
Temperature 25,950 [6]   K
Rotational velocity (v sin i)130 [7]  km/s
η Ori Ac
Mass 6.78 [8]   M☉
η Ori B
Mass 8.7 [8]   M☉
Other designations
η Ori, 28 Orionis, BD−02°1235, HD 35411, HIP 25281, HR 1788, SAO 132071 [9]
Database references
SIMBAD data

Eta Orionis is a multiple star in the constellation Orion. It lies a little to the west of Orion's Belt between Delta Orionis and Rigel, being closer to Delta Orionis than to Rigel. It lies at a distance of around 1,000 light-years from Earth and is part of the Orion OB1 association.

Contents

Nomenclature

Eta Orionis, Latinized from η Orionis, is the star's Bayer designation. It has the traditional Arabic name Saif al Jabbar, the Sword of the Giant, but this name is now used for another star, Saiph (Kappa Orionis). [10] It has also sometimes been called by the Latin name Ensis, [10] and Algjebbah. [11]

System

Eta Orionis is listed in multiple star catalogues as having two companions: a bright component B less than 2″ away; and a faint component C nearly 2′ away. [12] The two are estimated to orbit every 1,800 years. [8]

The primary star, Eta Orionis A, is itself a spectroscopic triple star, known from multiple spectral lines with varying radial velocities. [7] The most distant component Ac, has been resolved using speckle interferometry, at a separation of about 0.04″. It orbits the other two in 9.4 years. [3] The two closest stars, Aa and Ab, are separated by only about a tenth of an astronomical unit and orbit in just under eight days. [7]

The system lies within the Orion OB1 association, a group of massive stars that includes most of the bright stars of Orion. [13] It is assigned to the oldest and closest part of the association, known as OB1a. [14]

Variability

A light curve for Eta Orionis, plotted from TESS data, showing both eclipses and the 0.432 day variability. EtaOriLightCurve.png
A light curve for Eta Orionis, plotted from TESS data, showing both eclipses and the 0.432 day variability.

Eta Orionis drops in brightness every four days from a combined apparent magnitude of 3.31 to about magnitude 3.6. This is due to eclipses between the two closest components, Aa and Ab. [4] The primary and secondary eclipses are very similar, 0.24 and 0.23 magnitudes deep, respectively. [16]

It has also been suggested that component Ab is intrinsically variable with a period of 0.3 days and a very small amplitude. This star has unusual variable spectral lines and lies with the β Cephei variable instability strip. [7] However, it is now thought that the variable component is either B and Ac, possibly due to an unseen companion or rotational modulation. The actual period is 0.432 days and the 0.3-day period was an alias. [16]

See also

References

  1. 1 2 3 4 5 van Leeuwen, F. (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. S2CID   18759600.
  2. 1 2 3 Crawford, D. L.; Barnes, J. V.; Golson, J. C. (1971). "Four-color, H-beta, and UBV photometry for bright B-type stars in the northern hemisphere". The Astronomical Journal. 76: 1058. Bibcode:1971AJ.....76.1058C. doi:10.1086/111220.
  3. 1 2 3 4 Balega, I. I; Balega, Yu. Yu; Hofmann, K. -H; Tokovinin, A. A; Weigelt, G. P (1999). "Parameters of four multiple systems from speckle interferometry". Astronomy Letters. 25 (12): 797. Bibcode:1999AstL...25..797B.
  4. 1 2 Watson; et al. (2006–2012). "AAVSO International Variable Star Index VSX". VizieR. Retrieved 2020-09-24.
  5. ↑ Evans, D. S. (June 20–24, 1966). Batten, Alan Henry; Heard, John Frederick (eds.). "The Revision of the General Catalogue of Radial Velocities". Determination of Radial Velocities and Their Applications, Proceedings from IAU Symposium No. 30. 30: 57. Bibcode:1967IAUS...30...57E.
  6. 1 2 3 4 5 6 7 8 9 10 11 Southworth, John; Bowman, Dominic M. (July 2022). "High-mass pulsators in eclipsing binaries observed using TESS". Monthly Notices of the Royal Astronomical Society . 513 (3): 3191–3209. arXiv: 2203.15365 . Bibcode:2022MNRAS.513.3191S. doi: 10.1093/mnras/stac875 .
  7. 1 2 3 4 5 6 7 8 De Mey, K.; Aerts, C.; Waelkens, C.; Van Winckel, H. (1996). "The early-type multiple system η Orionis. II. Line profile variations in component Ab". Astronomy and Astrophysics. 310: 164. Bibcode:1996A&A...310..164D.
  8. 1 2 3 "HIP 25281". Multiple Star Catalog. Archived from the original on 2020-10-03. Retrieved 2020-09-24.
  9. ↑ "CCDM J05245-0223AB -- Double or multiple star". SIMBAD. Centre de Données astronomiques de Strasbourg . Retrieved 2012-02-09.
  10. 1 2 Allen, R.H. (1899), Star Names: Their Lore and Meaning, p. 316
  11. ↑ Darling, David. "Orion (constellation)" . Retrieved 10 June 2025.
  12. ↑ 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 .
  13. ↑ Abt, H. A; Levato, H (1977). "Spectral types in the Orion OB1 association". Publications of the Astronomical Society of the Pacific. 89: 797. Bibcode:1977PASP...89..797A. doi: 10.1086/130230 .
  14. ↑ Warren, W. H. Jr; Hesser, J. E (1978). "A photometric study of the Orion OB 1 association. III - Subgroup analyses". The Astrophysical Journal Supplement Series. 36: 497. Bibcode:1978ApJS...36..497W. doi:10.1086/190510.
  15. ↑ "MAST: Barbara A. Mikulski Archive for Space Telescopes". Space Telescope Science Institute. Retrieved 8 December 2021.
  16. 1 2 Waelkens, C.; Lampens, P. (1988). "The early-type multiple system eta Orionis. I. Photometric variability and rediscussion of the physical parameters of the components". Astronomy and Astrophysics. 194: 143. Bibcode:1988A&A...194..143W.
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