Sigma Orionis or Sigma Ori (σ Orionis, σ 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 constellation Orion, consisting of the brightest members of a young

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 ...

. It is found at the eastern end of

the belt ''The Belt'' ( it, La cintura) is a 1989 Italian erotic drama film directed by Giuliana Gamba. It is based on the drama play with the same name by Alberto Moravia. Cast * Eleonora Brigliadori: Bianca Ravelli *James Russo: Vittorio Di Simone *Gi ...

, south west of Alnitak and west of the Horsehead Nebula which it partially illuminates. The combined brightness of the component stars is magnitude 3.80.

History

σ Orionis is a naked eye star at the eastern end of Orion's Belt, and has been known since antiquity, but it was not included in

Ptolemy Claudius Ptolemy (; grc-gre, Πτολεμαῖος, ; la, Claudius Ptolemaeus; AD) was a mathematician, astronomer, astrologer, geographer, and music theorist, who wrote about a dozen scientific treatises, three of which were of import ...

Almagest The ''Almagest'' is a 2nd-century Greek-language mathematical and astronomical treatise on the apparent motions of the stars and planetary paths, written by Claudius Ptolemy ( ). One of the most influential scientific texts in history, it cano ...

. It was referred to by Al Sufi, but not formally listed in his catalogue. In more modern times, it was measured by

Tycho Brahe Tycho Brahe ( ; born Tyge Ottesen Brahe; generally called Tycho (14 December 154624 October 1601) was a Danish astronomer, known for his comprehensive astronomical observations, generally considered to be the most accurate of his time. He was ...

and included in his catalogue. In Kepler's extension it is described as "Quae ultimam baltei praecedit ad austr." (preceding the outermost of the belt, to the south). It was then recorded by

Johann Bayer Johann Bayer (1572 – 7 March 1625) was a German lawyer and uranographer (celestial cartographer). He was born in Rain, Lower Bavaria, in 1572. At twenty, in 1592 he began his study of philosophy and law at the University of Ingolstadt, ...

in his Uranometria as a single star with the Greek letter σ (sigma). He described it as "in enſe, prima" (in the sword, first). It was also given the

Flamsteed designation A Flamsteed designation is a combination of a number and constellation name that uniquely identifies most naked eye stars in the modern constellations visible from southern England. They are named for John Flamsteed who first used them while c ...

48. In 1776,

Christian Mayer Christian Mayer may refer to: * Christian Mayer (astronomer) (1719–1783), Czech astronomer and teacher *Christian Mayer (skier) (born 1972), Austrian former alpine skier *Christian Mayer (Wisconsin politician) (1827–1910), Wisconsin manufacture ...

described σ Ori as a triple star, having seen components AB and E, and suspected another between the two. Component D was confirmed by FGW Struve who also added a fourth (C), published in 1876. In 1892 Sherburne Wesley Burnham reported that σ Ori A was itself a very close double, although a number of later observers failed to confirm it. In the second half of the twentieth century, the orbit of σ Ori A/B was solved and at the time was one of the most massive binaries known. σ Ori A was discovered to have a variable

radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the rate of change of the distance or range between the two points. It is equivalent to the vector projection ...

in 1904, considered to indicate a single-lined spectroscopic binary. The

spectral line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...

s of the secondary were elusive and often not seen at all, possibly because they are broadened by rapid rotation. There was confusion over whether the reported spectroscopic binary status actually referred to the known visual companion B. Finally in 2011, it was confirmed that the system is triple, with an inner spectroscopic pair and a wider visual companion. The inner pair was resolved

interferometrically Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber opti ...

in 2013. σ Ori E was identified as helium-rich in 1956, having variable radial velocity in 1959, having variable emission features in 1974, having an abnormally strong magnetic field in 1978, being photometrically variable in 1977, and formally classified as a variable star in 1979. In 1996, a large number of low-mass pre-main sequence stars were identified in the region of Orion's Belt. A particular close grouping was discovered to lie around σ Orionis. A large number of

brown dwarf Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen (hydrogen-1, 1H) into helium in their cores, unlike a main sequence, main-sequence star. Instead, they have ...

s were found in the same area and at the same distance as the bright σ Orionis stars. Optical, infrared, and x-ray objects in the cluster, including 115 non-members lying in the same direction, were listed in the ''Mayrit Catalogue'' with a running number, except for the central star which was listed simply as Mayrit AB.

Cluster

The σ Orionis cluster is part of the Ori OB1b

stellar association A stellar association is a very loose star cluster, looser than both open clusters and globular clusters. Stellar associations will normally contain from 10 to 100 or more stars. The stars share a common origin, but have become gravitationally ...

, commonly referred to as Orion's Belt. The cluster was not recognised until 1996 when a population of pre-main sequence stars was discovered around σ Ori. Since then it has been extensively studied because of its closeness and the lack of interstellar extinction. It has been calculated that

star formation Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includ ...

in the cluster began 3 million years (myr) ago and it is approximately 360 pc away. In the central arc-minute of the cluster five particularly bright stars are visible, labelled A to E in order of distance from the brightest component σ Ori A. The closest pair AB are only separated by 0.2" - 0.3" but were discovered with a 12" telescope. An

infrared Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from aroun ...

and

radio Radio is the technology of signaling and communicating using radio waves. Radio waves are electromagnetic waves of frequency between 30 hertz (Hz) and 300 gigahertz (GHz). They are generated by an electronic device called a transm ...

source, IRS1, 3.3" from σ Ori A that was considered to be a patch of nebulosity has been resolved into two subsolar stars. There is an associated variable

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 that is assumed to be a T Tauri star. The cluster is considered to include a number of other stars of spectral class A or B: * HD 37699, an outlying B5

giant In folklore, giants (from Ancient Greek: '' gigas'', cognate giga-) are beings of human-like appearance, but are at times prodigious in size and strength or bear an otherwise notable appearance. The word ''giant'' is first attested in 1297 fr ...

very close to the Horsehead Nebula * HD 37525, a B5

main sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

star and spectroscopic binary * HD 294271, a B5 young stellar object with two low mass companions * HD 294272, a binary containing two B class young stellar objects * HD 37333, a peculiar A1 main sequence star * HD 37564, an A8 young stellar object * V1147 Ori, a B9.5 giant and α² CVn variable * HD 37686, a B9.5 main sequence star close to HD 37699 * HD 37545, an outlying B9 main sequence * HD 294273, an A8 young stellar object * 2MASS J05374178-0229081, an A9 young stellar object HD 294271 and HD 294272 make up the "double" star Struve 761 (or STF 761). It is three arc minutes from σ Orionis, which is also known as Struve 762. Over 30 other probable cluster members have been detected within an arc minute of the central star, mostly brown dwarfs and planetary mass objects such as

S Ori 70 S Ori 70 or S Ori J053810.1-023626 is a mid-T type astronomical object in the foreground of the σ Orionis cluster, which is approximately 1,150 light-years from Earth. It was discovered on November 24, 2002 by M. R. Zapatero-Osorio and ...

, but including the early M

red dwarf ''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave (TV channel), Dave since 2009, gaining a ...

s 2MASS J05384746-0235252 and 2MASS J05384301-0236145. In total, several hundred low mass objects are thought to be cluster members, including around a hundred spectroscopically measured class M stars, around 40 K class stars, and a handful of G and F class objects. Many are grouped in a central core, but there is a halo of associated objects scattered across more than 10 arc-minutes.

σ Orionis AB

The brightest member of the σ Orionis system appears as a late O class star, but is actually made up of three stars, designated Aa, Ab, and B. The inner pair complete a highly eccentric orbit every 143 days, while the outer star completes its near-circular orbit once every 157 years. It has not yet completed a full orbit since it was first discovered to be a double star. All three are very young main sequence stars with masses between .

Components

The primary component Aa is the class O9.5 star, with a temperature of 35,000 K and a luminosity over . Lines representing a B0.5 main sequence star have been shown to belong to its close companion Ab, which has a temperature of 31,000 K and a luminosity of . Their separation varies from less than half an astronomical unit to around two AU. Although they cannot be directly imaged with conventional single mirror telescopes, their respective visual magnitudes have been calculated at 4.61 and 5.20. The two components of σ Orionis A have been resolved

using the CHARA array, and the combination of interferometric and visual observations yields a very accurate orbit. The spectrum of component B, the outer star of the triple, cannot be detected. The luminosity contribution from σ Ori B can be measured and it is likely to be a B0-2 main sequence star. Its visual magnitude of 5.31 is similar to σ Ori Ab and so it should be easily visible, but it is speculated that its spectral lines are highly broadened and invisible against the backdrop of the other two stars. The orbit of component B has been calculated precisely using the NPOI and CHARA arrays. The combined orbits of the three stars together give a parallax significantly more precise than the

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. The inclinations of the two orbits are known accurately enough to calculate their relative inclination. The two orbital planes are within 30° of being

orthogonal In mathematics, orthogonality is the generalization of the geometric notion of '' perpendicularity''. By extension, orthogonality is also used to refer to the separation of specific features of a system. The term also has specialized meanings in ...

, with the inner orbit being prograde and the outer retrograde. Although slightly surprising, this situation is not necessarily rare in triple systems.

Mass discrepancy

The masses of these three component stars can be calculated using: spectroscopic calculation of the

surface gravity The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration due to gravity experien ...

and hence a ''spectroscopic mass''; comparison of evolutionary models to the observed physical properties to determine an ''evolutionary mass'' as well as the age of the stars; or determination of a ''dynamical mass'' from the orbital motions of the stars. The spectroscopic masses found for each component of σ Orionis have large margins of error, but the dynamical and spectroscopic masses are considered accurate to about , and the dynamical masses of the two components of σ Orionis A are known to within about . However, the dynamical masses are all larger than the evolutionary masses by more than their margins of error, indicating a systemic problem. This type of mass discrepancy is a common and long-standing problem found in many stars.

Ages

Comparison of the observed or calculated physical properties of each star with theoretical stellar evolutionary tracks allows the age of the star to be estimated. The estimated ages of the components Aa, Ab, and B, are respectively Myr, Myr, and Myr. Within their large margins of error, these can all be considered to be consistent with each other, although it is harder to reconcile them with the 2-3 Myr estimated age of the σ Orionis cluster as a whole.

σ Orionis C

The faintest member of the main σ Orionis stars is component C. It is also the closest to σ Ori AB at 11", corresponding to 3,960

astronomical unit The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbi ...

s. It is an A-type main sequence star. σ Ori C has a faint companion 2" away, referred to as Cb and MAD-4. Cb is five magnitudes fainter than σ Ori Ca at infrared wavelengths, K band magnitude 14.07, and is likely to be a brown dwarf.

σ Orionis D

Component D is a fairly typical B2 main sequence star of magnitude 6.62. It is 13" from σ Ori AB, corresponding to 4,680 AU. Its size, temperature, and brightness are very similar to σ Ori E but it shows none of the unusual spectral features or variability of that star.

σ Orionis E

Component E is an unusual variable star, classified as an

SX Arietis variable SX Arietis variables are a class of variable stars. They are generally B-type main sequence stars of spectral types B0p to B9p— high-temperature analogues of Alpha2 Canum Venaticorum variables—and exhibit strong magnetic fields and intense He& ...

and also known as V1030 Orionis. It is helium-rich, has a strong magnetic field, and varies between magnitudes 6.61 and 6.77 during a 1.19 day period of rotation. It has a spectral type of B2 Vpe. The variability is believed to be due to large-scale variations in surface brightness caused by the magnetic field. The rotational period is slowing due to magnetic braking; it is one of the few magnetic stars to have its rotation period change directly measured. σ Ori E is 41" from σ Ori AB, approximately 15,000 AU. The magnetic field is highly variable from −2,300 to +3,100

gauss Johann Carl Friedrich Gauss (; german: Gauß ; la, Carolus Fridericus Gauss; 30 April 177723 February 1855) was a German mathematician and physicist who made significant contributions to many fields in mathematics and science. Sometimes refer ...

, matching the brightness variations and the likely rotational period. This requires a magnetic dipole of at least 10,000 G. Around minimum brightness, a shell type spectrum appears, attributed to plasma clouds rotating above the photosphere. The helium enhancement in the spectrum may be due to hydrogen being preferentially trapped towards the magnetic poles leaving excess helium near the equator. It was at one point suggested that σ Ori E could be further away and older than the other members of the cluster, from modelling its evolutionary age and size. However, ''Gaia'' parallaxes place σ Ori E within the cluster, and later modelling has suggested that it is very young, at less than a million years old. σ Ori E has a faint companion about a third of an arc-second away. It is about 5 magnitudes fainter than the helium-rich primary, about magnitude 10-11 at K band infrared wavelengths. It is presumed to be a low mass star .

σ Orionis IRS1

The infrared source IRS1 is close to σ Ori A. It has been resolved to a pair of low mass objects, a proplyd, and a possible third object. The brighter object has an M1 spectral class, a mass around , and appears to be a relatively normal low mass star. The fainter object is very unusual, showing a diluted M7 or M8 absorption spectrum with emission lines of hydrogen and helium. The interpretation is that it is a

embedded within a proplyd that is being photoevaporated by σ Ori A.

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 ' ...

emission from IRS1 suggests the presence of an accretion disc around a T Tauri star, but it is unclear how this can fit with the proplyd scenario.

Dust wave

In infrared images, a prominent arc is visible centred on σ Ori AB. It is about 50" away from the class O star, around 0.1 parsecs at its distance. It is directed towards IC434, the Horesehead Nebula, in line with the space motion of the star. The appearance is similar to a

bowshock In astrophysics, a bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at which the speed of th ...

, but the type of radiation shows that it is not a bowshock. The observed infrared emission, peaking at around 45 microns, can be modelled by two approximately black-body components, one at 68K and one at 197 K. These are thought to be produced by two different sizes of dust grains. The material of the arc is theorised to be produced by photoevaporation from the

molecular cloud A molecular cloud, sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydroge ...

around the Horsehead Nebula. The dust becomes decoupled from the gas that carried it away from the molecular cloud by

radiation pressure Radiation pressure is the mechanical pressure exerted upon any surface due to the exchange of momentum between the object and the electromagnetic field. This includes the momentum of light or electromagnetic radiation of any wavelength that is a ...

from the hot stars at the centre of the σ Orionis cluster. The dust accumulates into a denser region that is heated and forms the visible infrared shape. The term "dust wave" is applied when the dust piles up but the gas is largely unaffected, as opposed to a "bow wave" where both dust and gas are stopped. Dust waves occur when the interstellar medium is sufficiently dense and the stellar wind sufficiently weak that the dust stand-off distance is larger than the stand-off distance of a bow shock. This would clearly be more likely for slow-moving stars, but slow-moving luminous stars may not have lifetimes long enough to produce a bow wave. Low luminosity late class O stars should commonly produce bow waves if this model is correct.

Distance

The distance to σ Orionis and the cluster of stars around it has historically been uncertain.

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 ...

parallaxes were available for several presumed members, but with very high uncertainties for the σ Orionis components. Published distance estimates ranged from to . A dynamical parallax of has been derived using the orbits of the two central stars, giving a distance of .

Gaia In Greek mythology, Gaia (; from Ancient Greek , a poetical form of , 'land' or 'earth'),, , . also spelled Gaea , is the personification of the Earth and one of the Greek primordial deities. Gaia is the ancestral mother—sometimes parthe ...

has published parallaxes for hundreds of cluster members, including

s, and thousands of other stars in the field of the cluster. The cluster has been found to be quite extended, but around an average distance of . Gaia Early Data Release 3 parallaxes for components C, D, and E are , , and respectively. These have low statistical uncertainties although significant astrometric excess noise. No Gaia parallax has been published for the central AB component. Corresponding distances are , , and for components C, D, and E respectively.

References

External links

Turn Left at Orion
Naked eye and telescopic views
σ Orionis
Researchers page at Instituto de Astrofísica de Andalucía
January double star
Observing guide from Astronomical Society of Southern Africa
Bright and Multiple Stars Gallery
taken at Fresno State's Campus Observatory, largely by students
A Quintuple Star in the Constellation Orion
for simpler explanation and pictures {{DEFAULTSORT:Sigma Orionis Orion (constellation) Orion molecular cloud complex 5 O-type main-sequence stars B-type main-sequence stars A-type main-sequence stars Orionis, Sigma Orionis, 48

1931 Events January * January 2 – South Dakota native Ernest Lawrence invents the cyclotron, used to accelerate particles to study nuclear physics. * January 4 – German pilot Elly Beinhorn begins her flight to Africa. * January 22 – Sir I ...

026549 037468 Durchmusterung objects SX Arietis variables Orionis, V1030 Open clusters Emission-line stars J05384476-0236001