| absmag_v =
}}{{Starbox orbit
| reference = [13]
| primary = Aa
| name = Ab
| period_unitless = {{val|7.989255|0.000005}} days
| eccentricity = {{val|0.011|0.0002}}
| inclination = 85
| k1 = {{val|145.5|0.03}}
| k2 = {{val|150|3}}
}}{{Starbox orbit
| reference = [4]
| name = Ac
| period = {{val|9.442|0.012}}
| eccentricity = {{val|0.45|0.02}}
| axis = {{val|0.0441|0.0015}}
| inclination = {{val|102.8|1.8}}
}}{{Starbox orbit
| reference = [15]
| primary = A
| name = B
| period = {{val|704.8|127.7}}
| eccentricity = {{val|0.914|0.165}}
| axis = {{val|0.970|0.331}}
| inclination = {{val|96.9|13.8}}
}}{{Starbox detail
| component1 = η Ori Aa
| mass = {{val|11.0|0.5}}[13]
| luminosity =
| temperature = 26,600[13]
| radius = {{val|6.3|0.6}}[13]
| rotational_velocity = 20[13]
| component2 = η Ori Ab
| mass2 = {{val|10.6|0.7}}[13]
| luminosity2 =
| temperature2 = 26,600[13]
| radius2 = {{val|5.2|0.4}}[13]
| rotational_velocity2 = 130[13]
}}{{Starbox catalog
| names = η Ori, 28 Orionis, BD−02°1235, HD 35411, HIP 25281, HR 1788, SAO 132071.[24]
}}{{Starbox reference
| Simbad=HR+1788
}}{{Starbox end}}Eta Orionis (η 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.
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.[25]
The primary star, Eta Orionis A, is itself a spectroscopic triple star, known from multiple spectral lines with varying radial velocities.[13] 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.[4] 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.[13]
The system lies within the Orion OB1 association, a group of massive stars that includes most of the bright stars of Orion.[29] It is assigned to the oldest and closest part of the association, known as OB1a.[30]
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.[8] The primary and secondary eclipses are very similar, 0.24 and 0.23 magnitudes deep respectively.[32]
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.[13] 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.[32]
See also
References
1. ^1 {{Cite journal | last=Evans | first=D. S. | date=June 20–24, 1966 | editor1=Batten, Alan Henry |editor2= Heard, John Frederick | title=The Revision of the General Catalogue of Radial Velocities | journal=Determination of Radial Velocities and Their Applications | volume=30 | pages=57 | booktitle=Determination of Radial Velocities and their Applications, Proceedings from IAU Symposium no. 30 | bibcode=1967IAUS...30...57E }}
2. ^1 2 3 {{cite journal | last1=Crawford | first1=D. L. | last2=Barnes | first2=J. V. | last3=Golson | first3=J. C. | title=Four-color, H-beta, and UBV photometry for bright B-type stars in the northern hemisphere | journal=The Astronomical Journal | year=1971 | pages=1058 | volume=76 | bibcode=1971AJ.....76.1058C | doi=10.1086/111220 }}
3. ^1 {{citation | title=CCDM J05245-0223AB -- Double or multiple star | work=SIMBAD | publisher=Centre de Données astronomiques de Strasbourg | url=http://simbad.u-strasbg.fr/simbad/sim-id?Ident=Eta+Orionis | accessdate=2012-02-09 }}
4. ^1 {{citation | display-authors=1 | last1=Zasche | first1=P. | last2=Wolf | first2=M. | last3=Hartkopf | first3=W. I. | last4=Svoboda | first4=P. | last5=Uhlař | first5=R. | last6=Liakos | first6=A. | last7=Gazeas | first7=K. | title=A Catalog of Visual Double and Multiple Stars With Eclipsing Components | journal=The Astronomical Journal | volume=138 | issue=2 | pages=664–679 |date=August 2009 | doi=10.1088/0004-6256/138/2/664 | bibcode=2009AJ....138..664Z |arxiv = 0907.5172 }}
5. ^1 2 3 4 5 6 7 8 9 10 11 12 {{cite journal|bibcode=1996A&A...310..164D|title=The early-type multiple system η Orionis. II. Line profile variations in component Ab|journal=Astronomy and Astrophysics|volume=310|pages=164|last1=De Mey|first1=K.|last2=Aerts|first2=C.|last3=Waelkens|first3=C.|last4=Van Winckel|first4=H.|year=1996}}
6. ^1 2 3 4 {{cite journal|bibcode=1999AstL...25..797B|title=Parameters of four multiple systems from speckle interferometry|journal=Astronomy Letters|volume=25|issue=12|pages=797|last1=Balega|first1=I. I|last2=Balega|first2=Yu. Yu|last3=Hofmann|first3=K. -H|last4=Tokovinin|first4=A. A|last5=Weigelt|first5=G. P|year=1999}}
7. ^1 {{cite journal|bibcode=2001AJ....122.3466M|title=The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog|journal=The Astronomical Journal|volume=122|issue=6|pages=3466|last1=Mason|first1=Brian D|last2=Wycoff|first2=Gary L|last3=Hartkopf|first3=William I|last4=Douglass|first4=Geoffrey G|last5=Worley|first5=Charles E|year=2001}}
8. ^1 2 AAVSO International Variable Star Index VSX (Watson+, 2006-2012) VizieR
9. ^1 2 {{cite journal|bibcode=1988A&A...194..143W|title=The early-type multiple system eta Orionis. I. Photometric variability and rediscussion of the physical parameters of the components|journal=Astronomy and Astrophysics|volume=194|pages=143|last1=Waelkens|first1=C.|last2=Lampens|first2=P.|year=1988}}
10. ^1 {{cite journal|bibcode=1977PASP...89..797A|title=Spectral types in the Orion OB1 association|journal=Publications of the Astronomical Society of the Pacific|volume=89|pages=797|last1=Abt|first1=H. A|last2=Levato|first2=H|year=1977}}
11. ^1 {{cite journal|doi=10.1086/190510|title=A photometric study of the Orion OB 1 association. III - Subgroup analyses|journal=The Astrophysical Journal Supplement Series|volume=36|pages=497|year=1978|last1=Warren|first1=W. H., Jr|last2=Hesser|first2=J. E}}