Capella

Coordinates: Sky map 05h 16m 41.3591s, 45° 59′ 52.768″
Source: Wikipedia, the free encyclopedia.
For other uses, see Capella (disambiguation).

Capella
Location of Capella Aa, as the brightest star (circled)
Observation data
ICRS
)
Constellation Auriga
Pronunciation
 /kəˈpɛlə/[1]
A
Right ascension 05h 16m 41.35871s[2][note 1]
Declination +45° 59′ 52.7693″[2][note 1]
Apparent magnitude (V) +0.08[3] (+0.03 – +0.16[4])
H
Right ascension 05h 17m 23.728s[5]
Declination +45° 50′ 22.97″[5]
Apparent magnitude (V) 10.16[6]
L
Right ascension 05h 17m 23.943s[7]
Declination +45° 50′ 19.84″[7]
Apparent magnitude (V) 13.7[8]
Characteristics
A
Spectral type G3III:[9]
U−B color index +0.44[3]
B−V color index +0.80[3]
V−R color index −0.3[3]
R−I color index +0.44[3]
Variable type RS CVn[10] (suspected[11])
Aa
Evolutionary stage Red clump[12]
Spectral type K0III[13]
Ab
Evolutionary stage Subgiant[12]
Spectral type G1III[13]
H
Evolutionary stage Main sequence (red dwarf)[14]
Spectral type M2.5 V[15]
U−B color index 1.24[16]
B−V color index 1.50[14]
R−I color index 0.91[14]
L
Evolutionary stage Main sequence (red dwarf)
Spectral type M4:[17]
Distance
42.919±0.049 ly
(13.159±0.015[12] pc)
Aa
Absolute magnitude (MV)+0.296[12]
Ab
Absolute magnitude (MV)+0.167[12]
HL
Radial velocity (Rv)31.63±0.14[12] km/s
H
Distance
43.38 ± 0.03 ly
(13.301 ± 0.009 pc)
Absolute magnitude (MV)13.1[23]
Semi-amplitude
(K1)
(primary)		25.9611±0.0044 km/s
Semi-amplitude (K2)
(secondary)		26.860±0.0017 km/s
Argument of periastron
(ω)
(secondary)		88°
Details
Myr
Aa
Rotational velocity
(v sin i)		35.0±0.5 km/s
H
dex
L
Mass0.53[12] M☉
NLTT 14788, 2MASS J05172386+4550229[28]
L: 2MASS J05172394+4550198[29]
Database references
SIMBADCapella
Capella H
Capella L

Capella is the brightest

44°N. Its name meaning "little goat" in Latin, Capella depicted the goat Amalthea that suckled Zeus in classical mythology. Capella is relatively close, at 42.9 light-years (13.2 pc) from the Sun. It is one of the brightest X-ray sources in the sky, thought to come primarily from the corona
of Capella Aa.

Although it appears to be a single star to the naked eye, Capella is actually a quadruple star system organized in two binary 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-yellow giant stars, both of which are around 2.5 times as massive as the Sun. The secondary pair, Capella H and Capella L, are around 10,000 astronomical units (AU)[note 2] from the first and are two faint, small and relatively cool red dwarfs.

Capella Aa and Capella Ab have exhausted their core

Sun's luminosity and 11.98 ± 0.57 times its radius. An aging red clump star, it is fusing helium to carbon and oxygen in its core. Capella Ab is slightly smaller and hotter and of spectral class G1III; it is 72.7 ± 3.6 times as luminous as the Sun and 8.83 ± 0.33 times its radius. It is in the Hertzsprung gap, corresponding to a brief subgiant evolutionary phase as it expands and cools to become a red giant
. Several other stars in the same visual field have been catalogued as companions but are physically unrelated.

Nomenclature

Capella is the brightest star in the constellation of Auriga (upper left).

α Aurigae (

Gliese-Jahreiss Catalogue
with designations GJ 194 for the bright pair of giants and GJ 195 for the faint pair of red dwarfs.

The traditional name Capella is

Working Group on Star Names (WGSN)[32] to catalogue and standardize proper names for stars. The WGSN's first bulletin of July 2016[33] included a table of the first two batches of names approved by the WGSN; which included Capella for this star. It is now so entered in the IAU Catalog of Star Names.[34] The catalogue of star names lists Capella as applying to the star α Aurigae Aa.[35]

Observational history

Capella was the brightest star in the night sky from 210,000 years ago to 160,000 years ago, at about −1.8 in apparent 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 boreal pole stars at the time.[36]

some ancient stone ruins of buildings in a sandy area
Building J (foreground) at Monte Albán

Capella is thought to be mentioned in an Akkadian inscription dating to the 20th century BC.[37] Its goat-associated symbolism dates back to Mesopotamia as a constellation called "GAM", "Gamlum" or "MUL.GAM" in the 7th-century BC document MUL.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.[38] It is sometimes called the Shepherd's Star in English literature.[39] Capella was seen as a portent of rain in classical times.[40]

Building J of the

pre-Columbian site Monte Albán in Oaxaca state in Mexico 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 its heliacal rising took place within a day of the Sun passing directly overhead over Monte Albán.[41]

Multiple status

Professor

violet in September and October and to red in November and February—showing that the components were moving toward and away from the Earth (and hence orbiting each other).[42][43] Almost simultaneously, British astronomer Hugh 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 a binary star system.[44]

Many observers tried to discern the component stars without success.[45] Known as "The Interferometrist's Friend", it was first resolved interferometrically in 1919 by John Anderson and Francis Pease at Mount Wilson Observatory, who published an orbit in 1920 based on their observations.[46][47] This was the first interferometric measurement of any object outside the Solar System.[48] A high-precision orbit was published in 1994 based on observations by the Mark III Stellar Interferometer, again at Mount Wilson Observatory.[49] Capella also became the first astronomical object to be imaged by a separate element optical interferometer when it was imaged by the Cambridge Optical Aperture Synthesis Telescope in September 1995.[50]

In 1914, Finnish astronomer Ragnar Furuhjelm observed that the spectroscopic binary had a faint companion star, which, as its proper motion was similar to that of the spectroscopic binary, was probably physically bound to it.[51] In February 1936, Carl L. Stearns observed that this companion appeared to be double itself;[52] this was confirmed in September that year by Gerard Kuiper. This pair are designated Capella H and L.[53]

X-ray source

Two

stellar X-ray astronomy happened on April 5, 1974, with the detection of the strongest emission of X-rays up to that time[55] from Capella, measured at more than 10,000 times the x-ray luminosity of the Sun.[56] 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.[56]

The X-ray luminosity (Lx) of ~1024 W (1031 erg s−1) is four orders of magnitude above the Sun's X-ray luminosity.

HEAO 1 would require magnetic confinement, unless it is a free-flowing coronal wind.[58]

Observation

photograph of night sky above a dimly-lit horizon
Annotated night sky image showing Auriga and the Pleiades—Capella is the brightest star, towards top left

With an average apparent magnitude of +0.08, Capella is the brightest object in the

40°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.[59]

Capella is closer to the north

44°N: for the whole of the United Kingdom and Canada (except for part of Southern Ontario), most of Europe, and the northernmost fringes of the contiguous 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 through Polaris.[61] Visible halfway between Orion'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.[62]

A few degrees to the southwest of Capella lie three stars, Epsilon Aurigae, Zeta Aurigae and Eta Aurigae, the latter two of which are known as "The Kids", or Haedi. The four form a familiar pattern, or asterism, in the sky.[63]

Distance

Based on an annual

milliarcseconds (with a margin of error of 0.46 milliarcseconds) as measured by the Hipparcos satellite, this system is estimated to be 42.8 light-years (13.12 parsecs) from Earth, with a margin of error of 0.3 light-year (0.09 parsec).[2] An alternative method to determine the distance is via the orbital 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 the Solar System in the past, passing within 29 light-years distant around 237,000 years ago.[64] At this range, it would have shone at apparent magnitude −0.82, comparable to Canopus today.[65]

In a 1960 paper, American astronomer

Hyades cluster, after analysing its proper motion and parallax. 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 the main sequence after exhausting their core hydrogen reserves and are expanding and cooling into red giants.[3][66]

Stellar system

two large pale yellow circles and three small circles on black background. They denote the two giants, and Sun and two dwarfs of the Capella system.
Capella components compared with the Sun

There are several stars within a few

".[12]

American astronomer Robert 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.[69]

Capella A

Hertzsprung Russell diagram showing Capella Aa and Ab
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 yellow

semimajor axis of 111.11 ± 0.10 million km (0.74272 ± 0.00069 AU), roughly the distance between Venus 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 an orbital parallax with far better precision than the one measured directly. The stars are not near enough to each other for the Roche lobe of either star to have been filled and any significant mass transfer to have taken place, even during the red 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. The

absorption lines; the lines from the secondary are broadened and blurred by its rapid rotation.[45] 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,[13] although older values are still widely quoted such as G5IIIe + G0III from the Bright Star Catalogue[3] or G8III + G0III by Eggen.[66] Where the context is clear, these two components have been referred to as A and B.[71]

The individual apparent 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 being 2.5687 ± 0.0074 

bolometric luminosities are most accurately derived from their apparent magnitudes and bolometric 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,

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 overall metallicity is slightly less than the Sun's.[45]

The rotational period of each star can be measured by observing periodic variations in the

projected equatorial rotational velocities measured using doppler broadening of spectral lines are a standard measure and these are generally quoted.[45] Capella Aa has a projected rotational velocity of 4.1 ± 0.4 km per second, taking 104 ± 3 days to complete one rotation, while Capella Ab spins much more rapidly at 35.0 ± 0.5 km per second, 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 also tidally 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 than Rigel, Betelgeuse and Vega, which are also variable. The system has been classified as an RS Canum Venaticorum variable,[10] a class of binary stars with active chromospheres that cause huge starspots, but it is still only listed as a suspected variable in the General 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 its convection zone.[71]

The active atmospheres and closeness of these stars means that they are among the brightest X-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]

Capella HL

The seventh companion published for Capella, component H, is physically associated with the bright primary star. It is a

Gliese-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.[16]

The two stars are reported to have a 3.5-visual-magnitude difference (2.3 mag in the passband of the Gaia spacecraft) although the difference is much smaller at infrared 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]

Visual companions

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.[69]

Multiple/double star designation: WDS 05167+4600[27]
Component Primary
J2000.0
J2000.0
 Epoch of
observed
separation		 Angular
distance
from
primary		 Position
angle
(relative
to primary)		 Apparent
magnitude
(V)		 Database
reference
B A 05h 16m 42.7s+46° 00′ 55″ 1898 46.6
″
 23° 17.1
C A 05h 16m 35.9s+46° 01′ 12″ 1878 78.2
″
 318° 15.1
D A 05h 16m 40.1s+45° 58′ 07″ 1878 126.2
″
 183° 13.6
E A 05h 16.5m +46° 02′ 1908 154.1
″
 319° 12.1
F A 05h 16m 48.748s+45° 58′ 30.84″ 1999 112.0
″
 137° 10.21 SIMBAD
G A 05h 16m 31.852s+46° 08′ 27.42″ 2003 522.4
″
 349° 8.10 SIMBAD

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,

Guide Star Catalogue from Chandra 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.[27] 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 by speckle 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]

Etymology and culture

Capella traditionally marks the left shoulder of the constellation's eponymous charioteer, or, according to the 2nd-century astronomer Ptolemy's Almagest, the goat that the charioteer is carrying. In Bayer's 1603 work Uranometria, Capella marks the charioteer's back.[83] The three Haedi had been identified as a separate constellation by Pliny the Elder and Manilius, and were called Capra, Caper, or Hircus, all of which relate to its status as the "goat star".[40] Ptolemy merged the Charioteer and the Goats in the 2nd-century Almagest.[84]

In

Titans' defeat, after Zeus skinned the goat and wore it as his aegis.[85]

In medieval accounts, it bore the uncommon name Alhajoth (also spelled Alhaior, Althaiot, Alhaiset, Alhatod, Alhojet, Alanac, Alanat, Alioc), which (especially the last) may be a corruption of its Arabic name, العيوق, al-cayyūq.

Bedouin of the Negev and Sinai, Capella al-'Ayyūq ath-Thurayyā "Capella of the Pleiades", from its role as pointing out the position of that asterism.[88] Another name in Arabic was Al-Rākib "the driver", a translation of the Greek.[86]

To the ancient Balts, Capella was known as Perkūno Ožka "Thunder's Goat", or Tikutis.[89] Conversely in Slavic Macedonian folklore, Capella was Jastreb "the hawk", flying high above and ready to pounce on Mother Hen (the Pleiades) and the Rooster (Nath).[90]

kabbalistic sign with the name Hircus (Latin for goat).[91][92]

In

Chinese name 五車二 (Wŭ chē èr; English: Second of the Five Chariots).[95]

In

Gwich'in saw Capella and Menkalinan has forming shreets'ą įį vidzee, the right ear of the large circumpolar constellation Yahdii, which covered much of the night sky, and whose orientation facilitated navigation and timekeeping.[99]

In Australian Aboriginal mythology for the Boorong people of Victoria, Capella was Purra, the kangaroo, pursued and killed by the nearby Gemini twins, Yurree (Castor) and Wanjel (Pollux).[100] The Wardaman people of northern Australia knew the star as Yagalal, a ceremonial fish scale, related to Guwamba the barramundi (Aldebaran).[101]

Namesakes

See also

Notes

  1. ^
    CDS ID I/239 Archived 2016-03-03 at the Wayback Machine
    .)
  2. ^ the distance between the Earth and the Sun is one astronomical unit
  3. ^ Polaris is only second magnitude.
  4. ^ The lithium abundance, C12/C13 ratio and C/N ratio have all declined in Capella Aa but not in Capella Ab.

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