Epoch J2000.0 Equinox J2000.0 (ICRS)
|Right ascension||05h 16m 41.3591s[note 1]|
|Declination||+45° 59′ 52.768″[note 1]|
|Apparent magnitude (V)||0.91[note 2]||0.76[note 2]|
(0.03 to 0.16)
|U-B color index||+0.44|
|B-V color index||+0.80|
|V-R color index||−0.3|
|R-I color index||+0.44|
|Variable type||RS CVn|
|Radial velocity (Rv)||29.19 ± 0.074[note 3] km/s|
|RA (μα cos δ)||75.52[note 1] mas/yr|
|Dec. (μδ)||−427.11[note 1] mas/yr|
|Parallax (π)||77.29 ± 0.89 mas|
|Distance||42.2 ± 0.5 ly
(12.9 ± 0.1 pc)
|Absolute magnitude (MV)||0.35[note 4]||0.20[note 4]|
|Mass||2.69 ± 0.06 M☉||2.56 ± 0.04 M☉|
|Radius||12.2 ± 0.2 R☉||9.2 ± 0.4 R☉|
|Luminosity (bolometric)||78.5 ± 1.2 L☉||77.6 ± 2.6 L☉|
|Temperature||4940 ± 50 K||5700 ± 100 K|
|Metallicity||40% Sun[note 5]|
|Rotation||106 ± 3 d ||8.64 ± 0.09 d |
|Rotational velocity (v sin i)||3km/s||36km/s|
|Age||5.2 × 108  years|
|Period (P)||104.022 ± 0.002 d|
|Semimajor axis (a)||56.47 ± 0.05 mas|
|Eccentricity (e)||0.0000 ± 0.0002|
|Inclination (i)||137.18 ± 0.05°|
|Longitude of node (Ω)||40.8 ± 0.1°|
|Periastron epoch (T)||2447528.45 ± 0.02 JD|
Capella (α Aurigae, α Aur, Alpha Aurigae, Alpha Aur) is the brightest star in the constellation Auriga, the sixth brightest star in the night sky and the third brightest star in the northern celestial hemisphere, after Arcturus and Vega. Although it appears to be a single star to the naked eye, it is actually a star system of four stars in two binary pairs. The first pair consists of two bright, large type-G giant stars, both with a radius around 10 times the Sun's, in close orbit around each other. These two stars are thought to be cooling and expanding on their way to becoming red giants. The second pair, around 10,000 astronomical units from the first, consists of two faint, small and relatively cool red dwarfs. The Capella system is relatively close, at only 42.2 light-years (12.9 pc) from Earth.
The Capella system consists of a bright binary pair of giant stars, orbiting at some distance from a fainter binary pair of red dwarves. The system is a member of the Hyades moving group, a group of stars moving in the same direction as the Hyades cluster.
Bright binary pair
Capella was first announced to be binary in 1899, based on spectroscopic observations. 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. This was the first interferometric measurement of any object outside the Solar System. A high-precision orbit was published in 1994 based on observations by the Mark III Stellar Interferometer, again at Mount Wilson Observatory. 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.
The bright binary pair of Capella consists of two type-G giant stars. The first, primary, star has a surface temperature of approximately 4900 K, a radius of approximately 12 solar radii, a mass of approximately 2.7 solar masses, and a luminosity, measured over all wavelengths, approximately 79 times that of the Sun. The other, secondary, star has a surface temperature of approximately 5700 K, a radius of approximately 9 solar radii, a mass of approximately 2.6 solar masses, and a luminosity, again measured over all wavelengths, approximately 78 times that of the Sun. Although the primary is the brighter star when considering radiation at all wavelengths, it is the fainter when observed in visible light, with an apparent visual magnitude of approximately 0.91, compared to the secondary's apparent visual magnitude of 0.76.
The pair is a non-eclipsing binary—that is, as seen from Earth, neither star passes in front of the other. The two components orbit each other at a distance of around 100 million km and an orbital period of approximately 104 days. The stars were probably of spectral class A during their main-sequence lifetime, similar to Vega; they are now expanding, cooling, and brightening to become red giants, a process that will take a few million years. It is thought that the more massive star of the pair has begun fusing helium to carbon and oxygen at its center, a process that has not yet begun for the less massive star.
Two Aerobee-Hi rocket flights on September 20, 1962, and March 15, 1963, apparently detected and confirmed an X-ray source in Auriga at RA 05h 09m Dec +45°. It was identified as Capella which is in the error box. Capella was much more readily detected on the second rocket flight.
Stellar X-ray astronomy started on April 5, 1974, with the detection of X-rays from Capella. A rocket flight on that date briefly calibrated its attitude control system when a star sensor pointed the payload axis at Capella (α Aur). 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. The X-ray luminosity (Lx) of ~1024 W (1031 erg s−1) is four orders of magnitude above the Sun's X-ray luminosity.
Capella is a source of X-rays, thought to be primarily from the corona of the more massive star. Capella is ROSAT 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 using HEAO 1 required magnetic confinement unless it was a free-flowing coronal wind.
In 1914, R. Furuhjelm observed that the spectroscopic binary mentioned above had a faint companion star, which, as its proper motion was similar to that of the spectroscopic binary, was probably physically bound to it. In 1936, Carl L. Stearns observed that this companion appeared to be double itself; this was later confirmed by G. P. Kuiper. This double companion star is a binary system of red dwarfs, thought to be separated from the pair of G-type giants by a distance of around 10,000 AU. Although this pair has only been observed to cover approximately 30° of its orbit, a rough, preliminary orbit has been computed, giving an orbital period of approximately 400 years.
Capella appears to be a rich yellowish white color. However during daylight observation with a telescope, the yellow color is more apparent due to the contrast against the blue sky. It is the brightest star in the constellation Auriga, the sixth brightest star in the night sky, the third brightest star in the northern celestial hemisphere (after Arcturus and Vega), and the fourth brightest star visible to the naked eye from the latitude 40° N. It is closer to the north celestial pole than any other first magnitude star (Polaris is only second magnitude). It lies a few degrees to the northeast from the triangle of stars known as "The Kids" (ε, ζ, and η Aurigae).
Capella's northern declination is such that it is actually invisible south of latitude 44°S – this includes southernmost New Zealand, Argentina and Chile as well as the Falkland Islands. Conversely it is circumpolar north of 44°north: for the whole of the United Kingdom and Scandinavia, most of France, Canada and the northernmost 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.
Capella was the brightest star in the night sky from 210,000 years ago to 160,000 years ago, at about -1.8 in magnitude. At -1.1, Aldebaran was brightest before this period, and it and Capella were situated rather close to each other and served as boreal polestars at the time.
Etymology and cultural significance
The name Capella (English: small female goat) is from Latin, and is a diminutive of the Latin Capra (English: female goat). 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 Roman mythology, the star represented the goat Amalthea that suckled Jupiter. It was this goat whose horn, after accidentally being broken off by Jupiter, was transformed into the Cornucopia, or "horn of plenty", which would be filled with whatever its owner desired. Astrologically, Capella portends civic and military honors and wealth. In the Middle Ages, it was considered a Behenian fixed star, with the stone sapphire and the plants horehound, mint, mugwort, and mandrake as attributes. Cornelius Agrippa listed its kabbalistic sign with the name Hircus (Latin for goat).
In medieval accounts, it also has 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. cAyyūq has no clear significance in Arabic, but may be an Arabized form of the Greek αίξ aiks "goat"; cf. the modern Greek Αίγα Aiga, the feminine of goat.
Capella is thought to be mentioned in an Akkadian inscription dating to the 20th century BC. It is sometimes called the Shepherd's Star in English literature. Other names used by other cultures include: in Arabic, Al-Rākib "the driver", a translation of the Greek; and in Quechua, Colca;
To the 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. Conversely in Macedonian folklore, Capella was Jastreb "the hawk", flying high above and ready to pounce on Mother Hen (the Pleiades) and the Rooster (Nath).
In Hindu mythology, Capella was seen as the heart of Brahma, Brahma Hridaya. In traditional Chinese astronomy, Capella was part of the asterism 五車 (Simplified Chinese: 五车; Wŭ chē; English: Five Chariots), which consisted of Capella together with β, ι, and θ Aurigae, as well as β Tauri. Since it was the second star in this asterism, it has the name 五車二 (Simplified Chinese: 五车二; Wŭ chē èr; English: Second of the Five Chariots). In Australian Aboriginal mythology for the Booroung people of Victoria, Capella was Purra, the kangaroo, pursued and killed by the nearby Gemini twins, Yurree (Castor) and Wanjel (Pollux).
The Hawaiians saw Capella as part of an asterism Ke ka o Makali'i ("The canoe bailer of Makali'i") that helped them navigate at sea. Called Hoku-lei "star wreath", it formed this asterism with Procyon, Sirius, Castor and Pollux. In Tahitian folklore, Capella was Tahi-ari'i, the wife of '"Fa'a-nui (Auriga) and mother of prince Ta'urua (Venus) who sails his canoe across the sky.
- USS Capella (AK-13) and USNS Capella (T-AKR-293), both of United States Navy ships.
- Mazda Capella, a model of automobile manufactured by Mazda
In addition to the stars mentioned above, Capella has six additional visual companions—that is, stars that appear to be close to Capella in the sky. However, they are not thought to be physically close to Capella. They are shown in the table below.
|Multiple/double star designation: WDS 05167+4600|
|B||A||05h 16m 42.7s||+46° 00′ 55″||1898||46.6″||23°||17.1||Simbad|
|C||A||05h 16m 35.9s||+46° 01′ 12″||1878||78.2″||318°||15.1||Simbad|
|D||A||05h 16m 40.1s||+45° 58′ 07″||1878||126.2″||183°||13.6||Simbad|
|E||A||05h 16.5m||+46° 02′||1908||154.1″||319°||12.1||Simbad|
|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|
- Astrometric data, mirrored by SIMBAD from the Hipparcos catalogue, pertains to the center of mass of the Capella Aa/Ab binary system. See Volume 1, The Hipparcos and Tycho Catalogues, European Space Agency, 1997, §2.3.4, and the entry in the Hipparcos catalogue (CDS ID I/239.)
- The cooler and more massive star, the spectroscopic primary, is the visually fainter star. See Hummel et al. 1994, §1.
- Radial velocity figure is for the center of mass of the Capella Aa/Ab binary system. See Pourbaix 2000, Table 2.
- From apparent magnitude and parallax.
- From Z=0.02 for the Sun and Hummel et al. 1994, §6.3, which gives Z=0.008 for Capella.
- NAME CAPELLA -- Variable of RS CVn type, database entry, SIMBAD. Accessed on line December 23, 2008.
- Capella, Stars, Jim Kaler. Accessed on line December 23, 2008.
- NSV 1897, database entry, New Catalogue of Suspected Variable Stars, the improved version, Sternberg Astronomical Institute, Moscow, Russia. Accessed on line December 23, 2008.
- Very high precision orbit of Capella by long baseline interferometry, C. A. Hummel et al., The Astronomical Journal 107, #5 (May 1994), pp. 1859–1867, doi:10.1086/116995, Bibcode: 1994AJ....107.1859H. See §1 for spectral types, Table 1 for orbit, Table 5 for stellar parameters, and §6.3 for the age of the system.
- HR 1708, database entry, The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version), D. Hoffleit and W. H. Warren, Jr., CDS ID V/50. Accessed on line December 23, 2008.
- Resolved double-lined spectroscopic binaries: A neglected source of hypothesis-free parallaxes and stellar masses, D. Pourbaix, Astronomy and Astrophysics Supplement 145 (August 2000), pp. 215–222, doi:10.1051/aas:2000237, Bibcode: 2000A&AS..145..215P; see Table 2.
- On the rotation period of Capella, K. G. Strassmeier et al., Astronomische Nachrichten 322, #2 (March 2001), pp. 115–124, doi:10.1002/1521-3994(200106)322:2, Bibcode: 2001AN....322..115S.
- CHANDRA-LETGS X-ray observations of Capella. Temperature, density and abundance diagnostics, R. Mewe et al., Astronomy and Astrophysics 368 (March 2001), pp. 888–900, Bibcode: 2001A&A...368..888M, doi:10.1051/0004-6361:20010026; see §1.
- Sirius Matters, Noah Brosch, Springer: 2008, ISBN 1-4020-8318-1, p. 46.
- Entry 05167+4600, The Washington Double Star Catalog, United States Naval Observatory. Accessed on line December 24, 2008.
- Capella, in The Hundred Greatest Stars, James B. Kaler, Springer, 2002, ISBN 0-387-95436-8 (see §18); also doi:10.1007/0-387-21625-1_19.
- Capella HL, T. R. Ayres, pp. 202–204, in Cool Stars, Stellar Systems, and the Sun: Proceedings of the Third Cambridge Workshop on Cool Stars, Stellar Systems, and the Sun, Held in Cambridge, Massachusetts, October 5–7, 1983, edited by Sallie L. Baliunas and Lee Hartmann, Berlin/Heidelberg, Springer-Verlag, 1984, Lecture Notes in Physics, vol. 193, ISBN 978-3-540-12907-3; doi:10.1007/3-540-12907-3_204, Bibcode: 1984LNP...193..202A
- The Hyades stream: an evaporated cluster or an intrusion from the inner disk?, B. Famaey, F. Pont, X. Luri, S. Udry, M. Mayor, and A. Jorissen, Astronomy and Astrophysics 461, #3 (January 2007), pp. 957–962, doi:10.1051/0004-6361:20065706, Bibcode: 2007A&A...461..957F.
- The spectroscopic binary Capella, W. W. Campbell, Astrophysical Journal 10 (October 1899), p. 177, doi:10.1086/140625, Bibcode: 1899ApJ....10..177C.
- Variable velocities of stars in the line of sight, H. F. Newall, The Observatory 22 (December 1899), pp. 436–437, Bibcode: 1899Obs....22..436N.
- Classical Observations of Visual Binary and Multiple Stars, B. Mason, pp. 88–96, in Binary Stars as Critical Tools and Tests in Contemporary Astrophysics, Proceedings of the 240th Symposium of the International Astronomical Union, Held in Prague, Czech Republic, August 22–25, 2006, William I. Hartkopf, Edward F. Guinan, and Petr Harmanec, eds., pub. Cambridge University Press, 2006, ISBN 0-521-86348-1; doi:10.1017/S1743921307003857; see p. 94.
- Application of Michelson's interferometer method to the measurement of close double stars, J. A. Anderson, Astrophysical Journal 51 (June 1920), pp. 263–275, doi:10.1086/142551, Bibcode: 1920ApJ....51..263A.
- Modern Optical Interferometry, Astronomical Optical Interferometry: A Literature Review, Bob Tubbs, St. John's College, Cambridge, April 1997. Accessed on line December 30, 2008.
- The first images from an optical aperture synthesis array: mapping of Capella with COAST at two epochs, J. E. Baldwin et al., Astronomy and Astrophysics 306 (February 1996), pp. L13–L16, Bibcode: 1996A&A...306L..13B.
- The Brightest Stars: Discovering the Universe through the Sky's Most Brilliant Stars, Fred Schaaf, Hoboken, New Jersey: John Wiley & Sons, 2008, ISBN 978-0-471-70410-2; see pp. 153–155.
- Fisher PC, Meyerott AJ (Jan 1964). "Stellar X-Ray Emission". Ap J. 139 (1): 123–42. Bibcode:1964ApJ...139..123F. doi:10.1086/147742.
- Catura RC, Acton LW, Johnson HM (March 1975). "Evidence for X-ray emission from Capella". Ap J 196 (pt.2): L47–9. Bibcode:1975ApJ...196L..47C. doi:10.1086/181741.
- Chandra/HETGS Observations of the Capella System: The Primary as a Dominating X-Ray Source, Kazunori Ishibashi et al., The Astrophysical Journal 644, #2 (June 2006), pp. L117–L120, doi:10.1086/505702, Bibcode: 2006ApJ...644L.117I.
- Güdel M (2004). "X-ray astronomy of stellar coronae". The Astronomy and Astrophysics Review 12 (2–3): 71–237. arXiv:astro-ph/0406661. Bibcode:2004A&ARv..12...71G. doi:10.1007/s00159-004-0023-2.
- G 96-29 -- High proper-motion Star, database entry, SIMBAD. Accessed on line December 23, 2008.
- NAME CAPELLA L -- Star in double system, database entry, SIMBAD. Accessed on line December 23, 2008.
- Chromospheric activity, kinematics, and metallicities of nearby M dwarfs, J. R. Stauffer and L. W. Hartmann, Astrophysical Journal Supplement Series 61 (July 1986), pp. 531–568, Bibcode: 1986ApJS...61..531S, doi:10.1086/191123; see Table 1.
- Infrared Spectra of Low-Mass Stars: Toward a Temperature Scale for Red Dwarfs, by S. K. Leggett, F. Allard, Graham Berriman, Conard C. Dahn, and Peter H. Hauschildt, Astrophysical Journal Supplement 104 (May 1996), pp. 117–143, Bibcode: 1996ApJS..104..117L, doi:10.1086/192295; see Tables 3, 6 and 7.
- Multiplicity among M dwarfs, Debra A. Fischer and Geoffrey W. Marcy, The Astrophysical Journal 396, #1 (September 1, 1992), pp. 178–194, Bibcode: 1992ApJ...396..178F, doi:10.1086/171708; see Table 1.
- Parallax and motions of the Capella system, W. D. Heintz, Astrophysical Journal 195 (January 1975), pp. 411–412, doi:10.1086/153340, Bibcode: 1975ApJ...195..411H
- (German) Ein schwacher Begleiter zu Capella, R. Furuhjelm, Astronomische Nachrichten 197, #4715 (April 1914), p. 181, Bibcode: 1914AN....197..181F.
- Note on duplicity of Capella H, Carl L. Stearns, Astronomical Journal 45, #1048 (July 1936), p. 120, doi:10.1086/105349, Bibcode: 1936AJ.....45..120S.
- Confirmation of the Duplicity of Capella H, G. P. Kuiper, Astrophysical Journal 84 (October 1936), p. 359, doi:10.1086/143788, Bibcode: 1936ApJ....84Q.359K.
- Schaaf 2008, p. 146.
- Burnham 1978, p. 261.
- Schaaf 2008, p. 151.
- Arnold, H.P. (1999). The Photographic Atlas of the Stars. Bristol, UK: IOP Publishing Ltd. p. 68. ISBN 0750306548.
- Schaaf 2008, p. 155.
- Star-names and Their Meanings, Richard Hinckley Allen, New York: G. E. Stechert, 1899; see p. 86.
- Schaaf 2008, p. 152.
- Allen 1899, p. 88.
- The Philosophy of Natural Magic, Heinrich Cornelius Agrippa, Forgotten Books, 2008. ISBN 1-60680-260-7; see p. 85.
- (Latin) De occulta philosophia, Henricus Cornelius Agrippa ab Nettesheym; edited and with commentary by Karl Anton Nowotny, Graz: Akademische Druck-u. Verlagsanstalt, 1967; see pp. 49, 209, 447.
- Allen 1899, p. 85.
- Edward William Lane's Arabic-English Lexicon: cwq
- Allen 1899, p. 87.
- Bedouin Star-Lore in Sinai and the Negev, Clinton Bailey, Bulletin of the School of Oriental and African Studies, University of London 37, #3 (1974), pp. 580–596; see p. 595.
- Cenev, Gjore. "Macedonian Folk Constellations". Publications of the Astronomical Observatory of Belgrade 85: 97–109.
- (Chinese) AEEA 天文教育資訊網, Activities of Exhibition and Education in Astronomy, National Museum of Natural Science, Taiwan. Accessed on line December 31, 2008.
- Exploring Ancient Skies: An Encyclopedic Survey of Archaeoastronomy, David H. Kelley, E. F. Milone, and Anthony F. Aveni, Birkhäuser, 2005, ISBN 0-387-95310-8; see p. 322.
- (Chinese) 香港太空館 - 研究資源 - 亮星中英對照表, Hong Kong Space Museum. Accessed on line December 31, 2008.
- On the astronomy and mythology of the Aborigines of Victoria, W. E. Stanbridge, Proc. of the Philosophical Instituite of Victoria, Transactions 2 (1857), pp. 137–140; see p. 140.
- Brosch, Noah (2008). Sirius Matters. Springer. p. 46. ISBN 1-4020-8318-1.
- Henry, Teuira (1907). "Tahitian Astronomy: Birth of Heavenly Bodies". The Journal of the Polynesian Society 16 (2): 101–04. JSTOR 20700813.
- Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System, Robert Burnham, Courier Dover Publications, 1978, ISBN 0-486-23567-X; see vol. 1, p. 264.
- BD+45 1077B -- Star in double system, database entry, SIMBAD. Accessed on line December 24, 2008.
- BD+45 1077C -- Star in double system, database entry, SIMBAD. Accessed on line December 24, 2008.
- BD+45 1077D -- Star in double system, database entry, SIMBAD. Accessed on line December 24, 2008.
- BD+45 1077E -- Star in double system, database entry, SIMBAD. Accessed on line December 24, 2008.
- BD+45 1077F -- Star in double system, database entry, SIMBAD. Accessed on line December 24, 2008.
- BD+45 1077G -- Star in double system, database entry, SIMBAD. Accessed on line December 24, 2008.
- HR 1708, catalog entry, Bright Star Catalogue.
- GJ 194, catalog entry, Preliminary Version of the Third Catalogue of Nearby Stars, W. Gliese and H. Jahreiss, 1991, CDS ID V/70A.
- Capella, image at Aladin.
- Images of the bright binary pair from 13 September 1995 and 28 September 1995 (note fainter blobs are just noise)