A star ofspectral type B8Ia, Rigel is calculated to be anywhere from 61,500 to 363,000 timesas luminous as the Sun, and 18 to 24 timesas massive, depending on the method and assumptions used. Its radius is more than seventy timesthat of the Sun, and itssurface temperature is12,100 K. Due to itsstellar wind, Rigel'smass-loss is estimated to be ten million times that of the Sun. With an estimated age of seven to nine million years, Rigel has exhausted its core hydrogen fuel, expanded, and cooled to become asupergiant. It is expected to end its life as atypeIIsupernova, leaving aneutron star or ablack hole as a final remnant, depending on the initial mass of the star.
Rigel varies slightly in brightness, itsapparent magnitude ranging from 0.05 to 0.18. It is classified as anAlpha Cygni variable due to the amplitude and periodicity of its brightness variation, as well as its spectral type. Itsintrinsic variability is caused by pulsations in its unstable atmosphere. Rigel is generally theseventh-brightest star in thenight sky and the brightest star in Orion, though it is occasionally outshone byBetelgeuse, which varies over a larger range.
A triple-star system is separated from Rigel by an angle of9.5 arc seconds. It has an apparent magnitude of 6.7, making it 1/400th as bright as Rigel. Two stars in the system can be seen by large telescopes, and the brighter of the two is aspectroscopic binary. These three stars are allblue-white main-sequence stars, each three to four times as massive as the Sun. Rigel and the triple system orbit a common center of gravity with a period estimated to be 24,000 years. The inner stars of the triple system orbit each other every 10 days, and the outer star orbits the inner pair every 63 years. A much fainter star, separated from Rigel and the others by nearly anarc minute, may be part of the same star system.
Orion, with Rigel at bottom right, at optical wavelengths plus the Hα (hydrogen-alpha) spectral line to emphasize gas clouds
In 2016, theInternational Astronomical Union (IAU) included the name "Rigel" in the IAU Catalog of Star Names.[22][23] According to the IAU, this proper name applies only to the primary component A of the Rigel system. The system is listed variously in historicalastronomical catalogs asHII33,Σ668,β555, orADS3823. For simplicity, Rigel's companions are referred to as Rigel B,[23] C, and D;[24][25] the IAU describes such names as "useful nicknames" that are "unofficial".[23] In modern comprehensive catalogs, the whole multiple star system is known asWDS 05145-0812 orCCDM 05145–0812.[8][26]
The designation of Rigel as β Orionis (Latinized to Beta Orionis) was made byJohann Bayer in 1603. The "beta" designation is commonly given to the second-brightest star in each constellation, but Rigel is almost always brighter than α Orionis (Betelgeuse).[27] AstronomerJames B. Kaler has speculated that Rigel was designated by Bayer during a rare period when it was outshone by the variable star Betelgeuse, resulting in the latter star being designated "alpha" and Rigel designated "beta".[24] Bayer did not strictly order the stars by brightness, instead grouping them by magnitude.[28] Rigel and Betelgeuse were both considered to be of the first magnitude class, and in Orion the stars of each class are thought to have been ordered north to south.[29] Rigel is included in theGeneral Catalogue of Variable Stars, but since it already has aBayer designation it has no separatevariable star designation.[30]
Rigel A and Rigel B as they appear in a small telescope
Rigel is anintrinsic variable star with anapparent magnitude ranging from 0.05 to 0.18.[5] It is typically the seventh-brightest star in thecelestial sphere, excluding the Sun, although occasionally fainter than Betelgeuse.[32] It is fainter thanCapella, which may also vary slightly in brightness.[33] Rigel appears slightly blue-white and has aB-V color index of −0.06.[34] It contrasts strongly with reddish Betelgeuse.[35]
Culminating every year at midnight on 12 December, and at 9:00pm on 24 January, Rigel is visible on winter evenings in theNorthern Hemisphere and on summer evenings in theSouthern Hemisphere.[27] In the Southern Hemisphere, Rigel is the first bright star of Orion visible as the constellation rises.[36] Correspondingly, it is also the first star of Orion to set in most of the Northern Hemisphere. The star is a vertex of the "Winter Hexagon", anasterism that includesAldebaran, Capella,Pollux,Procyon, andSirius. Rigel is a prominentequatorial navigation star, being easily located and readily visible in all the world's oceans (the exception is the area north of the82nd parallel north).[37]
Rigel'sspectral type is a defining point of the classification sequence for supergiants.[38][39] The overall spectrum is typical for alate B class star, with strongabsorption lines of the hydrogenBalmer series as well as neutral helium lines and some of heavier elements such as oxygen, calcium, and magnesium.[40] Theluminosity class for B8 stars is estimated from the strength and narrowness of the hydrogen spectral lines, and Rigel is assigned to thebright supergiant class Ia.[41] Variations in the spectrum have resulted in the assignment of different classes to Rigel, such as B8 Ia, B8 Iab, and B8 Iae.[16][42]
As early as 1888, the heliocentricradial velocity of Rigel, as estimated from theDoppler shifts of its spectral lines, was seen to vary. This was confirmed and interpreted at the time as being due to a spectroscopic companion with a period of about 22 days.[43] The radial velocity has since been measured to vary by about10 km/s around a mean of21.5 km/s.[44]
In 1933, theHα line in Rigel's spectrum was seen to be unusually weak and shifted0.1 nm towards shorter wavelengths, while there was a narrowemission spike about1.5 nm to the long wavelength side of the main absorption line.[45] This is now known as aP Cygni profile after a star that shows this feature strongly in its spectrum. It is associated withmass loss where there is simultaneously emission from a dense wind close to the star and absorption from circumstellar material expanding away from the star.[45]
The unusual Hα line profile is observed to vary unpredictably. It is a normal absorption line around a third of the time. About a quarter of the time, it is a double-peaked line, that is, an absorption line with an emission core or an emission line with an absorption core. About a quarter of the time it has a P Cygni profile; most of the rest of the time, the line has an inverse P Cygni profile, where the emission component is on the short wavelength side of the line. Rarely, there is a pure emission Hα line.[44] The line profile changes are interpreted as variations in the quantity and velocity of material being expelled from the star. Occasional very high-velocity outflows have been inferred, and, more rarely, infalling material. The overall picture is one of largelooping structures arising from thephotosphere and driven by magnetic fields.[46]
Alight curve for Rigel, adapted from Moravvejiet al. (2012)[18]
Rigel has been known to vary in brightness since at least 1930. The small amplitude of Rigel's brightness variation requiresphotoelectric orCCD photometry to be reliably detected. This brightness variation has no obvious period. Observations over 18 nights in 1984 showed variations at red, blue, and yellow wavelengths of up to 0.13 magnitudes on timescales of a few hours to several days, but again no clear period. Rigel'scolor index varies slightly, but this is not significantly correlated with its brightness variations.[47]
From analysis ofHipparcos satellite photometry, Rigel is identified as belonging to theAlpha Cygni class of variable stars,[48] defined as "non-radially pulsating supergiants of the Bep–AepIa spectral types".[33] In those spectral types, the 'e' indicates that it displays emission lines in its spectrum, while the 'p' means it has an unspecified spectral peculiarity. Alpha Cygni type variables are generally considered to be irregular[49] or havequasi-periods.[50] Rigel was added to the General Catalogue of Variable Stars in the 74th name-list of variable stars on the basis of the Hipparcos photometry,[51] which showed variations with a photographic amplitude of 0.039 magnitudes and a possible period of 2.075 days.[52] Rigel was observed with the CanadianMOST satellite for nearly 28 days in 2009. Milli-magnitude variations were observed, and gradual changes in flux suggest the presence of long-period pulsation modes.[18]
From observations of the variable Hα spectral line, Rigel's mass-loss rate due to stellar wind is estimated be(1.5±0.4)×10−7 solar masses per year (M☉/yr)—about ten million times more than the mass-loss rate from theSun.[53] More detailed optical andKband infrared spectroscopic observations, together withVLTI interferometry, were taken from 2006 to 2010. Analysis of the Hα andHγ line profiles, and measurement of the regions producing the lines, show that Rigel's stellar wind varies greatly in structure and strength. Loop and arm structures were also detected within the wind. Calculations of mass loss from the Hγ line give(9.4±0.9)×10−7M☉/yr in 2006-7 and(7.6±1.1)×10−7M☉/yr in 2009–10. Calculations using the Hα line give lower results, around1.5×10−7M☉/yr. The terminal wind velocity is300 km/s.[54] It is estimated that Rigel has lost about three solar masses (M☉) since beginning life as a star of24±3 M☉ seven to nine million years ago.[9]
Rigel's distance from the Sun is somewhat uncertain, different estimates being obtained by different methods. Old estimates placed it 166 parsecs (or 541 light years) away from the Sun.[55] The 2007Hipparcos new reduction of Rigel'sparallax is3.78±0.34 mas, giving a distance of 863 light-years (265 parsecs) with amargin of error of about 9%.[3] Rigel B, usually considered to be physically associated with Rigel and at the same distance, has aGaia Data Release 3 parallax of3.2352±0.0553 mas, suggesting a distance around 1,000 light-years (310 parsecs). However, the measurements for this object may be unreliable.[15]
Indirect distance estimation methods have also been employed. For example, Rigel is believed to be in a region ofnebulosity, its radiation illuminating several nearby clouds. Most notable of these is the 5°-longIC 2118 (Witch Head Nebula),[56][57] located at anangular separation of 2.5° from the star,[56] or a projected distance of 39 light-years (12 parsecs) away.[24] From measures of other nebula-embedded stars, IC2118's distance is estimated to be 949 ± 7 light-years (291 ± 2 parsecs).[58]
Rigel is an outlying member of theOrion OB1 association, which is located at a distance of up to 1,600 light-years (500 parsecs) from Earth. It is a member of the loosely definedTaurus-Orion R1 Association, somewhat closer at 1,200 light-years (360 parsecs).[31][59] Rigel is thought to be considerably closer than most of the members of Orion OB1 and theOrion Nebula. Betelgeuse andSaiph lie at a similar distance to Rigel, although Betelgeuse is arunaway star with a complex history and might have originally formed in the main body of the association.[42]
Thestar system of which Rigel is a part has at least four components. Rigel (sometimes called Rigel A to distinguish from the other components) has avisual companion, which is likely a close triple-star system. A fainter star at a wider separation might be a fifth component of the Rigel system.
William Herschel discovered Rigel to be a visual double star on 1 October 1781, cataloguing it as star 33 in the "second class of double stars" in his Catalogue of Double Stars,[20] usually abbreviated to HII33, or as H233 in the Washington Double Star Catalogue.[8]Friedrich Georg Wilhelm von Struve first measured the relative position of the companion in 1822, cataloguing the visual pair as Σ 668.[60][61] The secondary star is often referred to as Rigel B or β Orionis B. The angular separation of Rigel B from Rigel A is 9.5 arc seconds to its south alongposition angle 204°.[8][62] Although not particularly faint atvisual magnitude 6.7, the overall difference in brightness from Rigel A (about 6.6 magnitudes or 440 times fainter) makes it a challenging target for telescope apertures smaller than 15 cm (6 in).[7]
At Rigel's estimated distance, Rigel B'sprojected separation from Rigel A is over 2,200astronomical units (AU). Since its discovery, there has been no sign of orbital motion, although both stars share a similarcommon proper motion.[57][63] The pair would have an estimated orbital period of 24,000years.[12] Gaia Data Release 3(DR3) contains a probably unreliable parallax for Rigel B, placing it at about 1,010 light-years (309 parsecs), further away than the Hipparcos distance for Rigel, but similar to the Taurus-Orion R1 association. There is no parallax for Rigel in Gaia DR3. The Gaia DR3 proper motions for Rigel B and the Hipparcos proper motions for Rigel are both small, although not quite the same.[15]
In 1871,Sherburne Wesley Burnham suspected Rigel B to be a binary system, and in 1878, he resolved it into two components.[64] This visual companion is designated as component C (Rigel C), with a measured separation from component B that varies from less than0.1″ to around0.3″.[8][64] In 2009,speckle interferometry showed the two almost identical components separated by0.124″,[65] with visual magnitudes of 7.5 and 7.6, respectively.[8] Their estimated orbital period is 63years.[12] Burnham listed the Rigel multiple system as β555 in hisdouble star catalog[64] or BU555 in modern use.[8]
Component B is a double-linedspectroscopic binary system, which shows two sets ofspectral lines combined within its singlestellar spectrum. Periodic changes observed in relative positions of these lines indicate an orbital period of 9.86days. The two spectroscopic components Rigel Ba and Rigel Bb cannot be resolved in optical telescopes but are known to both be hot stars of spectral type around B9. This spectroscopic binary, together with the close visual component Rigel C, is likely a physical triple-star system,[63] although Rigel C cannot be detected in the spectrum, which is inconsistent with its observed brightness.[7]
In 1878, Burnham found another possibly associated star of approximately 13th magnitude. He listed it as component D of β555,[64] although it is unclear whether it is physically related or a coincidental alignment. Its 2017 separation from Rigel was44.5″, almost due north at a position angle of 1°.[8] Gaia DR2 finds it to be a 12th magnitude sunlike star at approximately the same distance as Rigel.[66] Likely aK-type main-sequence star, this star would have an orbital period of around 250,000 years, if it is part of the Rigel system.[24]
A spectroscopic companion to Rigel was reported on the basis of radial velocity variations, and its orbit was even calculated, but subsequent work suggests the star does not exist and that observed pulsations are intrinsic to Rigel itself.[63]
Rigel is ablue supergiant that has exhausted the hydrogen fuel in its core, expanded and cooled as it moved away from themain sequence across the upper part of theHertzsprung–Russell diagram.[5][67] When it was on the main sequence, itseffective temperature would have been around30,000 K.[68] Rigel's complex variability atvisual wavelengths is caused bystellar pulsations similar to those ofDeneb. Further observations of radial velocity variations indicate that it simultaneously oscillates in at least 19 non-radial modes with periods ranging from about 1.2 to 74 days.[18]
Estimation of many physical characteristics of blue supergiant stars, including Rigel, is challenging due to their rarity and uncertainty about how far they are from the Sun. As such, their characteristics are mainly estimated from theoreticalstellar evolution models.[69] Its effective temperature can be estimated from the spectral type and color to be around12,100 K.[19] A mass of21±3 M☉ at an age of8±1million years has been estimated by comparing evolutionary tracks, while atmospheric modeling from the spectrum gives a mass of24±8M☉.[9]
Although Rigel is often considered the most luminous star within 1,000 light-years of the Sun,[27][32] its energy output is poorly known. Using the Hipparcos distance of 860 light-years (264 parsecs), the estimated relative luminosity for Rigel is about 120,000 times that of the Sun (L☉),[18] but another recently published distance of 1,170 ± 130 light-years (360 ± 40 parsecs) suggests an even higher luminosity of 219,000 L☉.[9] Other calculations based on theoretical stellar evolutionary models of Rigel's atmosphere give luminosities anywhere between 83,000 L☉ and 363,000 L☉,[31] while summing thespectral energy distribution from historical photometry with the Hipparcos distance suggests a luminosity as low as61,515±11,486 L☉.[17] A 2018 study using theNavy Precision Optical Interferometer measured theangular diameter as2.526 mas. After correcting forlimb darkening, the angular diameter is found to be2.606±0.009 mas, yielding a radius of74.1+6.1 −7.3R☉.[17] An older measurement of the angular diameter gives2.75±0.01 mas,[70] equivalent to a radius of 78.9 R☉ at264 pc.[18] These radii are calculated assuming the Hipparcos distance of264 pc; adopting a distance of360 pc leads to a significantly larger size.[54] Older distance estimates were mostly far lower than modern estimates, leading to lower radius estimates; a 1922 estimate byJohn Stanley Plaskett gave Rigel a diameter of 25 million miles, or approximately 28.9 R☉, smaller than its neighborAldebaran.[71]
Due to their closeness to each other and ambiguity of the spectrum, little is known about the intrinsic properties of the members of the Rigel BC triple system. All three stars seem to be near equally hotB-type main-sequence stars that are three to four times as massive as the Sun.[12]
Stellar evolution models suggest the pulsations of Rigel are powered by nuclear reactions in a hydrogen-burning shell that is at least partially non-convective. These pulsations are stronger and more numerous in stars that have evolved through ared supergiant phase and then increased in temperature to again become a blue supergiant. This is due to the decreased mass and increased levels of fusion products at the surface of the star.[68]
Rigel is likely to befusing helium in its core.[11] Due to strong convection of helium produced in the core while Rigel was on the main sequence and in the hydrogen-burning shell since it became a supergiant, the fraction of helium at the surface has increased from 26.6% when the star formed to 32% now. The surface abundances of carbon, nitrogen, and oxygen seen in the spectrum are compatible with a post-red supergiant star only if its internal convection zones are modeled using non-homogeneous chemical conditions known as theLedoux Criteria.[68]
Rigel is expected to eventually end its stellar life as atype II supernova.[11] It is one of the closest known potentialsupernova progenitors to Earth,[18] and would be expected to have a maximum apparent magnitude of around−11 (about the same brightness as a quarter Moon or around 300 times brighter than Venus ever gets).[5] The supernova would leave behind either a black hole or a neutron star.[11]
Orion illustrated in a copy ofAbd al-Rahman al-Sufi'sBook of Fixed Stars. The foot on the left is annotatedrijl al-jauza al-yusra, the Arabic name from whichRigel is derived.[a]
The earliest known recording of the nameRigel is in theAlfonsine tables of 1521. It is derived from the Arabic nameRijl Jauzah al Yusrā, "the left leg (foot) of Jauzah" (i.e.rijl meaning "leg, foot"),[73] which can be traced to the 10th century.[74] "Jauzah" was a proper name for Orion; an alternative Arabic name wasرجل الجبارrijl al-jabbār, "the foot of the great one", from which stems the rarely used variant namesAlgebar orElgebar. TheAlphonsine tables saw its name split into "Rigel" and "Algebar", with the note,et dicitur Algebar. Nominatur etiam Rigel.[b][75] Alternate spellings from the 17th century includeRegel by Italian astronomerGiovanni Battista Riccioli,Riglon by German astronomerWilhelm Schickard, andRigel Algeuze orAlgibbar by English scholarEdmund Chilmead.[73]
With the constellation representing the mythological Greek huntsmanOrion, Rigel is his knee or (as its name suggests) foot; with the nearby starBeta Eridani marking Orion's footstool.[27] Rigel is presumably the star known as "Aurvandil's toe" inNorse mythology.[76] In the Caribbean, Rigel represented the severed leg of the folkloric figureTrois Rois, himself represented by the three stars of Orion's Belt. The leg had been severed with a cutlass by the maidenBįhi (Sirius).[77] TheLacandon people of southern Mexico knew it astunsel ("little woodpecker").[78]
Rigel was known asYerrerdet-kurrk to theWotjobalukkoori of southeastern Australia, and held to be the mother-in-law ofTotyerguil (Altair). The distance between them signified the taboo preventing a man from approaching his mother-in-law.[79] The indigenousBoorong people of northwestern Victoria named Rigel asCollowgullouric Warepil.[80] TheWardaman people of northern Australia know Rigel as the RedKangaroo LeaderUnumburrgu and chief conductor of ceremonies in a songline when Orion is high in the sky.Eridanus, the river, marks a line of stars in the sky leading to it, and the other stars of Orion are his ceremonial tools and entourage. Betelgeuse isYa-jungin "Owl Eyes Flicking", watching the ceremonies.[81]
TheMāori people of New Zealand named Rigel asPuanga, said to be a daughter ofRehua (Antares), the chief of all-stars.[82] Itsheliacal rising presages the appearance ofMatariki (thePleiades) in the dawn sky, marking the Māori New Year in late May or early June. TheMoriori people of theChatham Islands, as well as some Maori groups in New Zealand, mark the start of their New Year with Rigel rather than the Pleiades.[83]Puaka is asouthern name variant used in the South Island.[84]
In Japan, the Minamoto orGenji clan chose Rigel and its white color as its symbol, calling the starGenji-boshi (源氏星), while the Taira orHeike clan adopted Betelgeuse and its red color. The two powerful families fought theGenpei War; the stars were seen as facing off against each other and kept apart only by the three stars ofOrion's Belt.[85][86][87]
TheMSRigel was originally a Norwegian ship, built in Copenhagen in 1924. It was requisitioned by the Germans duringWorld War II and sunk in 1944 while being used to transport prisoners of war.[88] Two US Navy ships have borne the nameUSSRigel.[89][90][91] TheSSM-N-6 Rigel was acruise missile program for theUS Navy that was cancelled in 1953 before reaching deployment.[92]
TheRigel Skerries are a chain of small islands inAntarctica, renamed after originally being called Utskjera. They were given their current name as Rigel was used as anastrofix.[93]Mount Rigel, elevation 1,910 m (6,270 ft), is also in Antarctica.[94]
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