Iota Orionis (ι Orionis, abbreviated ι Ori) is a

multiple star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a ''star cluster'' or ''galaxy'', although, broadly speaking ...

in the equatorial constellation of Orion the hunter. It is the eighth-brightest member of Orion with an apparent visual magnitude of 2.77 and also the brightest member of the asterism known as Orion's Sword. It is a member of the NGC 1980

open cluster An open cluster is a type of star cluster made of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way galaxy, and ...

. From

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different lines of sight and is measured by the angle or semi-angle of inclination between those two lines. Due to foreshortening, nearby object ...

measurements, it is located at a distance of roughly from the Sun. The system has three visible components designated Iota Orionis A, B and C. Iota Orionis A has also been resolved using speckle interferometry and is also a massive spectroscopic binary, with components Iota Orionis Aa1 (officially named Hatysa ), Aa2, and Ab.

Nomenclature

''ι Orionis'' ( Latinised to ''Iota Orionis'') is the system's

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. T ...

. The designations of the three constituents as ''Iota Orionis A'', ''B'' and ''C'', and those of ''A's'' components - ''Iota Orionis Aa1'', ''Aa2'', and ''Ab'' - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...

(IAU). The system has the traditional name ''Nair al Saif'', from the Arabic نير السيف ''nayyir as-sayf'' "the Bright One of the Sword", though this is little used. Since Bečvář's 1951 '' Atlas Coeli'', it has borne the proper name ''Hatysa''. Kunitzsch was unable to find an older source for the latter name. In 2016, the IAU organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize List of proper names of stars, proper names for stars for the international astronomical community. It operates under ...

(WGSN) to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems. It approved the name ''Hatysa'' for the component Iota Orionis Aa on 5 September 2017 and it is now so included in the List of IAU-approved Star Names. Iota Orionis B is a variable star and in 2011 it was given the

variable star designation In astronomy, a variable star designation is a unique identifier given to variable stars. It uses a variation on the Bayer designation format, with an identifying label (as described below) preceding the Latin genitive of the name of the constel ...

V2451 Orionis.

Distance

Iota Orionis has a parallax of in the

Hipparcos new reduction ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial obj ...

, indicating a distance around . The previous published

Hipparcos ''Hipparcos'' was a scientific satellite of the European Space Agency (ESA), launched in 1989 and operated until 1993. It was the first space experiment devoted to precision astrometry, the accurate measurement of the positions of celestial obj ...

parallax was , suggesting a closer distance.

Gaia Data Release 2 The ''Gaia'' catalogues are star catalogues created using the results obtained by ''Gaia'' space telescope. The catalogues are released in stages that will contain increasing amounts of information; the early releases also miss some stars, especia ...

has individual parallaxes for the two fainter components of the Iota Orionis

star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or ''galaxy'', although, broadly speakin ...

of and , indicating distances of and respectively, with margins of error of just a few parsecs. There is little doubt that all three stars are at the same distance. Iota Orionis is generally assumed to be associated with the open cluster NGC 1980, which is at a distance of around . However, they may not lie at exactly the same distance and Iota Orionis may have a complex history involving stellar encounters and runaway stars. NGC 1980 contains few bright stars other than Iota Orionis. Only eighteen other stars are considered members in a survey down to 14th magnitude, most of them around 9th magnitude but including the 5th magnitude stars

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and 1887.

Properties

Iota Orionis is dominated by the multiple star Iota Orionis A. It is clearly identified as a double-lined spectroscopic binary whose components are a stellar class O9 III star ( blue giant) and a class B0.8 III/IV star about 2 magnitudes fainter. The combined spectral type has long been accepted as O9 III and it was listed as a standard star for that type. The collision of the

stellar wind A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric. ...

s from this pair makes the system a strong

X-ray X-rays (or rarely, ''X-radiation'') are a form of high-energy electromagnetic radiation. In many languages, it is referred to as Röntgen radiation, after the German scientist Wilhelm Conrad Röntgen, who discovered it in 1895 and named it ' ...

source. Oddly, the two objects of this system appear to have different ages, with the secondary being about double the age of the primary. In combination with the high eccentricity (e=0.764) of their 29-day orbit, this suggests that the binary system was created through a capture, rather than by being formed together and undergoing a mass transfer. This capture may have occurred, for example, through an encounter between two binary systems, with one star being donated from each binary and two runaway stars being ejected. A third component away has been identified using speckle interferometry and is probably a B2 subgiant. The primary component of Iota Orionis A is a class O

giant star A giant star is a star with substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moore, New York: Oxford University Press ...

with a mass of about . It has a surface

temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied on ...

of and radius of , resulting in a bolometric luminosity of . It is calculated to be around nine million years old. The secondary star of the spectroscopic binary pair is a class B giant or

subgiant A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution ...

with a mass of about . It has a temperature of and radius of , resulting in it radiating over 8,000 times as much energy as the sun. Iota Orionis B is a B8 giant at 11" (approximately 5,000 AU) which has been shown to be variable, and likely to be a young stellar object. It is also a

helium-weak Helium-weak stars are chemically peculiar stars which have a weak helium lines for their spectral type. Their helium lines place them in a later (i.e. cooler) spectral type then their hydrogen lines. List of helium-weak stars This is a non-exte ...

chemically peculiar star. The fainter Iota Orionis C is an A0 star at 49".

References

External links

Iota Orionis
by Dr. Jim Kaler.

{{DEFAULTSORT:Iota Orionis O-type giants B-type giants B-type subgiants A-type stars Orion variables Spectroscopic binaries 4 Orion (constellation) J05352645-0554445 Orionis, Iota Orionis, 44

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026241 037043 Durchmusterung objects Orionis, V2451 Hatysa