Capella is the brighteststar in the northernconstellation ofAuriga. It has theBayer designationα Aurigae, which isLatinised toAlpha Aurigae and abbreviatedAlpha Aur orα Aur. Capella is thesixth-brightest star in thenight sky, and the third-brightest in thenorthern celestial hemisphere afterArcturus andVega. A prominent object in the northern winter sky, it iscircumpolar to observers north of44°N. Its name meaning "little goat" inLatin, Capella depicted the goatAmalthea that suckledZeus in classical mythology. Capella is relatively close, at 42.9light-years (13.2 pc) from theSun. It is one of the brightestX-ray sources in the sky, thought to come primarily from thecorona of Capella Aa.
Although it appears to be a single star to the naked eye, Capella is actually a quadruplestar system organized in twobinary pairs, made up of the stars Capella Aa, Capella Ab, Capella H and Capella L. The primary pair, Capella Aa and Capella Ab, are two bright-yellowgiant stars, both of which are around 2.5 times as massiveas the Sun. The secondary pair, Capella H and Capella L, are around 10,000astronomical units (AU)[note 2] from the first and are two faint, small and relatively coolred dwarfs.
Capella Aa and Capella Ab have exhausted their corehydrogen, and cooled and expanded, moving off themain sequence. They are in a very tight circular orbit about 0.74 AU apart, and orbit each other every 104 days. Capella Aa is the cooler and more luminous of the two withspectral class G8III; it is 78.7±4.2 times theSun's luminosity and 11.98±0.57 timesits radius. An agingred clump star, it is fusinghelium tocarbon andoxygen in its core. Capella Ab is slightly smaller and hotter and of spectral class G0III; it is 72.7±3.6 times as luminous as the Sun and 8.83±0.33 times its radius. It is in theHertzsprung gap, corresponding to a briefsubgiant evolutionary phase as it expands and cools to become ared giant. Several other stars in the same visual field have been catalogued as companions but are physically unrelated.
Capella is the brightest star in the constellation of Auriga (upper left).
α Aurigae (Latinised toAlpha Aurigae) is the star system'sBayer designation. It also has theFlamsteed designation 13 Aurigae. It is listed in several multiple star catalogues asADS 3841,CCDM J05168+4559, andWDS J05167+4600. As a relatively nearby star system, Capella is listed in theGliese-Jahreiss Catalogue with designations GJ 194 for the bright pair of giants and GJ 195 for the faint pair of red dwarfs.
The traditional nameCapella isLatin for(small) female goat; the alternative nameCapra was more commonly used in classical times.[29] It is the translation of the Greek star name Aἴξ (aix) meaning "the Goat". As the sound of the Greek term for the goat (aἴξ) is similar to the sound of the name for the Aegaean Sea, this star has been used for weather rules and determining the seasonal wind direction.[30] In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[31] to catalogue and standardize proper names for stars. The WGSN's first bulletin of July 2016[32] included a table of the first two batches of names approved by the WGSN; which includedCapella for this star. It is now so entered in the IAU Catalog of Star Names.[33] The catalogue of star names listsCapella as applying to the star α Aurigae Aa.[34]
Capella was the brightest star in the night sky from 210,000 years ago to 160,000 years ago, at about −1.8 inapparent magnitude. At −1.1,Aldebaran was brightest before this period; it and Capella were situated rather close to each other in the sky and approximated borealpole stars at the time.[35]
Capella is thought to be mentioned in anAkkadian inscription dating to the 20th century BC.[36] Its goat-associated symbolism dates back toMesopotamia as a constellation called "GAM", "Gamlum" or "MUL.GAM" in the 7th-century BC documentMUL.APIN. GAM represented a scimitar or crook and may have represented the star alone or the constellation of Auriga as a whole. Later,Bedouin astronomers created constellations that were groups of animals, where each star represented one animal. The stars of Auriga comprised a herd of goats, an association also present in Greek mythology.[37] It is sometimes called theShepherd's Star in English literature.[38] Capella was seen as a portent of rain in classical times.[39]
Building J of thepre-Columbian siteMonte Albán inOaxaca state inMexico was built around 275 BC, at a different orientation to other structures in the complex. Its steps are aligned perpendicular to the rising of Capella at that time, so that a person looking out a doorway on the building would have faced it directly. Capella is significant as itsheliacal rising took place within a day of the Sun passing directly overhead over Monte Albán.[40]
ProfessorWilliam Wallace Campbell of theLick Observatory announced that Capella was binary in 1899, based onspectroscopic observations—he noted on photographic plates taken from August 1896 to February 1897 that a secondspectrum appeared superimposed over the first, and that there was adoppler shift toviolet in September and October and tored in November and February—showing that the components were moving toward and away from the Earth (and hence orbiting each other).[41][42] Almost simultaneously, British astronomerHugh Newall had observed its composite spectrum with a four prism spectroscope attached to a 25-inch (64 cm)telescope at Cambridge in July 1899, concluding that it was abinary star system.[43]
Many observers tried to discern the component stars without success.[44] Known as "The Interferometrist's Friend", it was first resolvedinterferometrically in 1919 by John Anderson and Francis Pease atMount Wilson Observatory, who published an orbit in 1920 based on their observations.[45][46] This was the first interferometric measurement of any object outside theSolar System.[47] A high-precision orbit was published in 1994 based on observations by theMark III Stellar Interferometer, again at Mount Wilson Observatory.[48] Capella also became the first astronomical object to be imaged by a separate element optical interferometer when it was imaged by theCambridge Optical Aperture Synthesis Telescope in September 1995.[49]
In 1914, Finnish astronomerRagnar Furuhjelm observed that the spectroscopic binary had a faint companion star, which, as itsproper motion was similar to that of the spectroscopic binary, was probably physically bound to it.[50] In February 1936, Carl L. Stearns observed that this companion appeared to be double itself;[51] this was confirmed in September that year byGerard Kuiper. This pair are designated Capella H and L.[52]
TwoAerobee-Hi rocket flights on September 20, 1962, and March 15, 1963, detected and confirmed anX-ray source in Auriga atRA05h 09mDec +45°, identified as Capella.[53] A major milestone instellar X-ray astronomy happened on April 5, 1974, with the detection of the strongest emission of X-rays up to that time[54] from Capella, measured at more than 10,000 times the x-ray luminosity of the Sun.[55] A rocket flight on that date briefly calibrated its attitude control system when a star sensor pointed the payload axis at Capella. During this period, X-rays in the range 0.2–1.6 keV were detected by an X-ray reflector system co-aligned with the star sensor.[55]
The X-ray luminosity (Lx) of~1024 W (1031 erg/s) is four orders of magnitude above the Sun's X-ray luminosity.[55] Capella's X-rays are thought to be primarily from thecorona of the most massive star.[56] Capella isROSAT X-ray source 1RXS J051642.2+460001. The high temperature of Capella's corona as obtained from the first coronal X-ray spectrum of Capella usingHEAO 1 would require magnetic confinement, unless it is a free-flowing coronal wind.[57]
Annotated night sky image showingAuriga and thePleiades—Capella is the brightest star, towards top left
With an average apparent magnitude of +0.08, Capella is the brightest object in theconstellationAuriga, thesixth-brightest star in the night sky, the third-brightest in thenortherncelestial hemisphere (afterArcturus andVega), and the fourth-brightest visible to the naked eye from the latitude40°N. It appears to be a rich yellowish-white colour, although the yellow colour is more apparent during daylight observation with a telescope, due to the contrast against the blue sky.[58]
Capella is closer to the northcelestial pole than any otherfirst-magnitude star.[59][note 3] Its northern declination is such that it is actually invisible south of latitude44°S—this includes southernmostNew Zealand,Argentina andChile as well as theFalkland Islands. Conversely it iscircumpolar north of44°N: for the whole of theUnited Kingdom andCanada (except for part ofSouthern Ontario), most ofEurope, and the northernmost fringes of thecontiguous United States, the star never sets. Capella and Vega are on opposite sides of the pole, at about the same distance from it, such that an imaginary line between the two stars will nearly pass throughPolaris.[60] Visible halfway betweenOrion's Belt and Polaris, Capella is at its highest in the night sky at midnight in early December and is regarded as a prominent star of the northern winter sky.[61]
A few degrees to the southwest of Capella lie three stars,Epsilon Aurigae,Zeta Aurigae andEta Aurigae, the latter two of which are known as "The Kids", orHaedi. The four form a familiar pattern, orasterism, in the sky.[62]
Based on an annualparallax shift of 76.20milliarcseconds (with a margin of error of 0.46 milliarcseconds) as measured by theHipparcos satellite, this system is estimated to be 42.8light-years (13.12parsecs) fromEarth, with a margin of error of 0.3 light-year (0.09 parsec).[2] An alternative method to determine the distance is via theorbital parallax, which gives a distance of 42.92 light-years (13.159 parsecs) with a margin of error of only 0.1%.[12] Capella is estimated to have been a little closer to theSolar System in the past, passing within 29 light-years distant around 237,000 years ago.[63] At this range, it would have shone at apparent magnitude −0.82, comparable toCanopus today.[64]
In a 1960 paper, American astronomerOlin J. Eggen concluded that Capella was a member of theHyades moving group, a group of stars moving in the same direction as theHyades cluster, after analysing itsproper motion andparallax. Members of the group are of a similar age, and those that are around 2.5 times as massive as the Sun have moved off themain sequence after exhausting their core hydrogen reserves and are expanding and cooling intored giants.[3][65]
There are several stars within a fewarcminutes of Capella and some have been listed as companions in various multiple star catalogues. TheWashington Double Star Catalog lists components A, B, C, D, E, F, G, H, I, L, M, N, O, P, Q, and R, with A being thenaked-eye star. Most are onlyline-of-sight companions,[66] but the close pair ofred dwarfs H and L are at the same distance as the bright component A and moving through space along with it.[67] Capella A is itself aspectroscopic binary with components Aa and Ab, bothgiant stars. The pair of giants is separated from the pair of red dwarfs by 723".[12]
American astronomerRobert Burnham Jr. described a scale model of the system where Capella A was represented by spheres 13 and 7 inches across, separated by ten feet. The red dwarfs were then each 0.7 inch across and they were separated by 420 feet. At this scale, the two pairs are 21 miles apart.[68]
Hertzsprung–Russell diagram showing an evolutionary track for a star of approximately the mass of the two Capella giants. The current states of Capella Aa and Ab are marked.[12]
Capella A consists of two yellowevolved stars that have been calculated to orbit each other every 104.02128±0.00016 days, with asemimajor axis of 111.11±0.10 million km (0.74272±0.00069 AU), roughly the distance betweenVenus and the Sun. The pair is not an eclipsing binary—that is, as seen from Earth, neither star passes in front of the other. The orbit is known extremely accurately and can be used to derive anorbital parallax with far better precision than the one measured directly. The stars are not near enough to each other for theRoche lobe of either star to have been filled and any significant mass transfer to have taken place, even during thered giant stage of the primary star.[12]
Modern convention designates the more luminous cooler star as component Aa and its spectral type has been usually measured between G2 and K0. The hotter secondary Ab has been given various spectral types of late (cooler) F or early (warmer) G. TheMK spectral types of the two stars have been measured a number of times, and they are both consistently assigned a luminosity class of III indicating agiant star.[69] The composite spectrum appears to be dominated by the primary star due to its sharperabsorption lines; the lines from the secondary are broadened and blurred by its rapid rotation.[44] The composite spectral class is given as approximately G3III, but with a specific mention of features due to a cooler component.[9] The most recent specific published types are K0III and G1III,[70] although older values are still widely quoted such as G5IIIe + G0III from theBright Star Catalogue[3] or G8III + G0III by Eggen.[65] Where the context is clear, these two components have been referred to as A and B.[71]
The individualapparent magnitudes of the two component stars cannot be directly measured, but their relative brightness has been measured at various wavelengths. They have very nearly equal brightness in the visible light spectrum, with the hotter secondary component generally being found to be a few tenths of a magnitude brighter.[12] A 2016 measurement gives the magnitude difference between the two stars at a wavelength of 700 nm as 0.00±0.1.[72]
The physical properties of the two stars can be determined with high accuracy. The masses are derived directly from the orbital solution, with Aa being2.5687±0.0074 M☉ and Ab being2.4828±0.0067 M☉}. Their angular radii have been directly measured; in combination with the very accurate distance, this gives11.98±0.57 R☉ and8.83±0.33 R☉ for Aa and Ab, respectively. Theirsurface temperatures can be calculated by comparison of observed and synthetic spectra, direct measurement of their angular diameters and brightnesses, calibration against their observedcolour indices, and disentangling of high resolution spectra. Weighted averages of these four methods give4,970±50 K for Aa and5,730±60 K for Ab. Theirbolometric luminosities are most accurately derived from their apparent magnitudes andbolometric corrections, but are confirmed by calculation from the temperatures and radii of the stars. Aa is 78.7±4.2 times as luminous as the Sun and Ab 72.7±3.6 times as luminous, so the star defined as the primary component is the more luminous when all wavelengths are considered but very slightly less bright at visual wavelengths.[12]
Estimated to be 590 to 650 million years old,[12] the stars were probably at the hot end ofspectral class A during theirmain-sequence lifetime, similar toVega. They have now exhausted their core hydrogen and evolved off the main sequence, their outer layers expanding and cooling.[73] Despite the giant luminosity class, the secondary component is very clearly within theHertzsprung gap on theHertzsprung–Russell diagram, still expanding and cooling towards thered giant branch, making it asubgiant in evolutionary terms. The more massive primary has already passed through this stage, when it reached a maximum radius of 36 to 38 times that of the Sun. It is now ared clump star which is fusinghelium tocarbon andoxygen in its core, a process that has not yet begun for the less massive star. Detailed analysis shows that it is nearing the end of this stage and starting to expand again which will lead it to theasymptotic giant branch.Isotope abundances[note 4] and spin rates confirm this evolutionary difference between the two stars. Heavy element abundances are broadly comparable to those of the Sun and the overallmetallicity is slightly less than the Sun's.[44]
The rotational period of each star can be measured by observing periodic variations in thedoppler shifts of their spectral lines. The absolute rotational velocities of the two stars are known from their inclinations, rotation periods, and sizes, but theprojected equatorial rotational velocities measured usingdoppler broadening of spectral lines are a standard measure and these are generally quoted.[44] Capella Aa has a projected rotational velocity of4.1±0.4 km/s, taking 104±3 days to complete one rotation, while Capella Ab spins much more rapidly at35.0±0.5 km/s, completing a full rotation in only 8.5±0.2 days.Rotational braking occurs in all stars when they expand into giants, and binary stars are alsotidally braked. Capella Aa has slowed until it is rotationally locked to the orbital period, although theory predicts that it should still be rotating more quickly from a starting point of a rapidly-spinning main sequence A star.[12]
Capella has long been suspected to be slightly variable. Its amplitude of about 0.1 magnitudes means that it may at times be brighter or fainter thanRigel,Betelgeuse andVega, which are also variable. The system has been classified as anRS Canum Venaticorum variable,[10] a class of binary stars with activechromospheres that cause hugestarspots, but it is still only listed as a suspected variable in theGeneral Catalogue of Variable Stars.[11] Unusually for RS CVn systems, the hotter star, Capella Ab, has the more active atmosphere because it is located in the Hertzsprung gap—a stage where it is changing its angular momentum and deepening itsconvection zone.[71]
The active atmospheres and closeness of these stars means that they are among the brightestX-ray sources in the sky. However the X-ray emission is due to stable coronal structures and not eruptive flaring activity. Coronal loops larger than the Sun and with temperatures of several million kelvin are likely to be responsible for the majority of the X-rays.[74]
The seventh companion published for Capella, component H, is physically associated with the bright primary star. It is ared dwarf separated from the pair of G-type giants by a distance of around10,000 AU.[67] It has its own close companion, an even fainter red dwarf that was 1.8″ away when it was discovered in 1935. It is component L in double star catalogues. In 2015 the separation had increased to 3.5″, which was sufficient to allowtentative orbital parameters to be derived, 80 years after its discovery.[12][75] TheGliese-Jahreiss Catalogue of nearby stars designates the binary system as GJ 195. The two components are then referred to individually as GJ 195 A and B.[15]
The two stars are reported to have a 3.5-visual-magnitude difference (2.3 mag in the passband of theGaia spacecraft) although the difference is much smaller atinfrared wavelengths. This is unexpected and may indicate further unseen companions.[12]
The mass of the stars can, in principle, be determined from the orbital motion, but uncertainties in the orbit have led to widely varying results. In 1975, an eccentric 388-year orbit gave masses of 0.65 M☉ and 0.13 M☉.[75] A smaller near-circular orbit published in 2015 had a 300-year orbit, benefitting from mass constraints of 0.57 M☉ and 0.53 M☉, respectively, for GJ 195 A and B, based on their infrared magnitudes.[12]
Six visual companions to Capella were discovered before Capella H and are generally known only as Capella B through G. None are thought to be physically associated with Capella, although all appear closer in the sky than the HL pair.[68]
Multiple/double star designation:WDS 05167+4600[26]
Component F is also known as TYC 3358-3142-1. It is listed with a spectral type of K[76] although it is included in a catalogue of OB stars as a distant luminous star.[77]
Component G is BD+45 1076, with a spectral type of F0,[76] at a distance of 401 light-years (123 parsecs).[78] It is identified as a variable member of theGuide Star Catalogue fromChandra observations although it is not known what type of variability.[79] It is known to be an X-ray source with an active corona.[78]
Several other stars have also been catalogued as companions to Capella.[26] Components I, Q and R are 13th-magnitude stars at distances of 92″, 133″ and 134″.[80]V538 Aurigae and its close companion HD 233153 are red dwarfs ten degrees away from Capella; they have very similar space motions but the small difference makes it possible that this is just a coincidence.[81] Two faint stars have been discovered byspeckle imaging in the Capella HL field, around 10″ distant from that pair. These have been catalogued as Capella O and P. It is not known whether they are physically associated with the red dwarf binary.[82]
Capella traditionally marks the left shoulder of the constellation's eponymous charioteer, or, according to the 2nd-century astronomerPtolemy'sAlmagest, the goat that the charioteer is carrying. In Bayer's 1603 workUranometria, Capella marks the charioteer's back.[83] The threeHaedi had been identified as a separate constellation byPliny the Elder andManilius, and were calledCapra,Caper, orHircus, all of which relate to its status as the "goat star".[39]Ptolemy merged the Charioteer and the Goats in the 2nd-centuryAlmagest.[84]
InGreek mythology, the star represented the goatAmalthea that suckledZeus. It was this goat whose horn, after accidentally being broken off by Zeus, was transformed into thecornucopia, or "horn of plenty", which would be filled with whatever its owner desired.[36] Though most often associated with Amalthea, Capella has sometimes been associated with Amalthea's owner, anymph. The myth of the nymph says that the goat's hideous appearance, resembling a Gorgon, was partially responsible for theTitans' defeat, after Zeus skinned the goat and wore it as hisaegis.[85]
In medieval accounts, it bore the uncommon nameAlhajoth (also spelledAlhaior,Althaiot,Alhaiset,Alhatod,Alhojet,Alanac,Alanat,Alioc), which (especially the last) may be a corruption of its Arabic name,العيوق,al-cayyūq.[86]cAyyūq has no clear significance in Arabic,[87] but may be an Arabized form of the Greek αίξaiks "goat"; cf. the modern Greek ΑίγαAiga, the feminine of goat.[86] To theBedouin of theNegev andSinai, Capellaal-'Ayyūq ath-Thurayyā "Capella of thePleiades", from its role as pointing out the position of that asterism.[88] Another name in Arabic wasAl-Rākib "the driver", a translation of the Greek.[86]
To the ancientBalts, Capella was known asPerkūno Ožka "Thunder's Goat", orTikutis.[89] Conversely in Slavic Macedonian folklore, Capella wasJastreb "the hawk", flying high above and ready to pounce on Mother Hen (the Pleiades) and the Rooster (Nath).[90]
InQuechua it was known asColça;[38] the Incas held the star in high regard.[96] The Hawaiians saw Capella as part of an asterismKe ka o Makali'i ("The canoe bailer of Makali'i") that helped them navigate at sea. CalledHoku-lei "star wreath", it formed this asterism withProcyon,Sirius,Castor andPollux.[25] In Tahitian folklore, Capella wasTahi-ari'i, the wife ofFa'a-nui (Auriga) and mother of princeTa'urua (Venus) who sails his canoe across the sky.[97] InInuit astronomy, Capella, along with Menkalinan (Beta Aurigae),Pollux (Beta Geminorum) andCastor (Alpha Geminorum), formed a constellationQuturjuuk, "collar-bones", the two pairs of stars denoting a bone each. Used for navigation and time-keeping at night, the constellation was recognised fromAlaska to western Greenland.[98] TheGwich'in saw Capella and Menkalinan has formingshreets'ą įį vidzee, the right ear of the large circumpolar constellationYahdii, which covered much of the night sky, and whose orientation facilitated navigation and timekeeping.[99]
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