Eta Orionis is a

multiple star A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. It may sometimes be used to refer to a single star. A large group of stars bound by gravitation is generally called a ''st ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Orion. It lies a little to the west of

Orion's Belt Orion's Belt is an asterism in the constellation of Orion. Other names include the Belt of Orion, the Three Kings, and the Three Sisters. The belt consists of three bright and easily identifiable collinear star systems – Alnitak, Alnilam, ...

between Delta Orionis and

Rigel Rigel is a blue supergiant star in the constellation of Orion. It has the Bayer designation β Orionis, which is Latinized to Beta Orionis and abbreviated Beta Ori or β Ori. Rigel is the brightest and most massive componentand ...

, being closer to Delta Orionis than to Rigel. It lies at a distance of around 1,000

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s from

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

and is part of the Orion OB1 association.

Nomenclature

Eta Orionis, Latinized from η Orionis, is the star's

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

. It has the traditional Arabic name ''Saif al Jabbar'', the Sword of the Giant, but this name is now used for another star,

Saiph Saiph , designation Kappa Orionis (κ Orionis, abbreviated Kappa Ori, κ Ori) and 53 Orionis (53 Ori), is a blue supergiant star and the sixth-brightest star in the constellation of Orion. Of the four bright stars that compose Orion's mai ...

(Kappa Orionis). It has also sometimes been called by the Latin name ''Ensis'', and ''Algjebbah''.

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. The two are estimated to orbit every 1,800 years. The primary star, Eta Orionis A, is itself a

spectroscopic Spectroscopy is the field of study that measures and interprets electromagnetic spectra. In narrower contexts, spectroscopy is the precise study of color as generalized from visible light to all bands of the electromagnetic spectrum. Spectrosc ...

triple star, known from multiple

spectral line A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission (electromagnetic radiation), emission or absorption (electromagnetic radiation), absorption of light in a narrow frequency ...

s with varying

radial velocities The radial velocity or line-of-sight velocity of a target with respect to an observer is the rate of change of the vector displacement between the two points. It is formulated as the vector projection of the target-observer relative velocity on ...

. The most distant component Ac, has been resolved using

speckle interferometry Speckle imaging comprises a range of high-resolution Astrophotography, astronomical imaging techniques based on the analysis of large numbers of short Exposure (photography), exposures that freeze the variation of Astronomical seeing, atmospheri ...

, at a separation of about 0.04″. It orbits the other two in 9.4 years. The two closest stars, Aa and Ab, are separated by only about a tenth of an

astronomical unit The astronomical unit (symbol: au or AU) is a unit of length defined to be exactly equal to . Historically, the astronomical unit was conceived as the average Earth-Sun distance (the average of Earth's aphelion and perihelion), before its m ...

and orbit in just under eight days. The system lies within the Orion OB1 association, a group of massive stars that includes most of the bright stars of Orion. It is assigned to the oldest and closest part of the association, known as OB1a.

Variability

Eta Orionis drops in brightness every four days from a combined

apparent magnitude Apparent magnitude () is a measure of the Irradiance, brightness of a star, astronomical object or other celestial objects like artificial satellites. Its value depends on its intrinsic luminosity, its distance, and any extinction (astronomy), ...

of 3.31 to about magnitude 3.6. This is due to eclipses between the two closest components, Aa and Ab. The primary and secondary eclipses are very similar, 0.24 and 0.23 magnitudes deep, respectively. 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 Beta (, ; uppercase , lowercase , or cursive ; or ) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Ancient Greek, beta represented the voiced bilabial plosive . In Modern Greek, it represe ...

instability strip. 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.

References

{{DEFAULTSORT:Eta Orionis B-type main-sequence stars Algol variables Orion (constellation) Orionis, Eta BD-02 1235 Orionis, 28 035411 025281

1788 Events January–March * January 1 – The first edition of ''The Times'', previously ''The Daily Universal Register'', is published in London. * January 2 – Georgia ratifies the United States Constitution, and becomes the fourth U.S ...

Nomenclature

System

Variability

See also

References