Capella

Nomenclature

α Aurigae (Latinised to Alpha Aurigae) is the star's Bayer designation. The traditional name Capella (Latin for small female goat) is a diminutive of capra (Latin for "female goat"). In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 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.

Observational history

Professor William Wallace Campbell of the Lick Observatory announced that Capella was binary in 1899, based on spectroscopic observations—he noted on photographic plates taken from August 1896 to February 1897 that a second spectrum appeared superimposed over the first, and that there was a doppler shift to 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). Almost simultaneously, British astronomer Hugh Newall had observed its composite spectrum with a four prism spectroscope attached to a 25-inch telescope at Cambridge in July 1899, concluding that it was a binary star system.

Many observers tried to discern the component stars without success. 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.

In 1914, Finnish astronomer Ragnar 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 February 1936, Carl L. Stearns observed that this companion appeared to be double itself; this was confirmed in September that year by Gerard Kuiper.

Visibility

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. 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 pole stars at the time.

In Bayer's 1603 work Uranometria, Capella marks the charioteer's back. A few degrees to the southwest lie the pair of stars known as "The Kids", or Haedi (Zeta and Eta Aurigae). Capella is closer to the north celestial pole than any other first magnitude star Its 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.

X-ray source

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 05^h 09^m 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. 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 (L_x) 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.

System

The Capella system consists of a bright binary of giant stars, orbiting at some distance from a fainter binary of red dwarfs. The two main stars are Capella Aa and Capella Ab, whereas their faint companions are Capella H and Capella L—the stars labelled Capella C through to G are actually unrelated stars in the same visual field. Based upon an annual parallax shift of 76.20 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-years (0.09 parsecs).

In a 1960 paper, American astronomer Olin J. Eggen concluded that Capella was a member of the Hyades moving group, a group of stars moving in the same direction as the 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. He adopted the spectral values of G8III and G0III, but noted the hotter star's properties were not consistent with its companion if of the same age, and suspected that it might have already been a red giant and be heating up once more.

Bright binary

Capella's bright binary consists of two type-G giant stars that have been calculated by Guillermo Torres and colleagues in 2009 to orbit each other every 104 days; the researchers reviewed all spectroscopic and interferometric measurements to date to come up with the properties of the component stars. They calculated that the first, primary, star (Capella Aa) had a surface temperature of around 4920 ± 70 K, a radius of around 11.87 ± 0.56 solar radii, a mass of around 2.466 ± 0.018 solar masses, and a luminosity, measured over all wavelengths, of 79.5 ± 4.8 times that of the Sun. The other, secondary, star (Capella Ab) had a surface temperature of approximately 5680 ± 70 K, a radius of 8.75 ± 0.32 solar radii, a mass of around 2.443 ± 0.013 solar masses, and a luminosity, again measured over all wavelengths, around 72.1 ± 3.6 times that of the Sun. In 2011, Weber and Strassmeier calculated masses for the pair of 2.573±0.009 and 2.488±0.008 solar masses for the primary and secondary respectively after reviewing 438 echelle spectrographs taken over three and a half years. Although Capella Aa is the brighter star when considering radiation at all wavelengths, it is the fainter when observed in visible light, with an apparent magnitude of 0.91, compared to the secondary's apparent magnitude of 0.76.

The system has been classified as a RS Canum Venaticorum variable, a class of binary stars with active chromospheres that cause huge starspots. Unusually for these systems, the hotter star, Capella Ab, has the more active atmosphere. It is likely to be located in the Hertzsprung gap—a stage where it is changing its angular momentum and deepening its convection zone.

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.

Companion binary

The 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.

Etymology and cultural significance

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 Greek mythology, the star represented the goat Amalthea that suckled Zeus. It was this goat whose horn, after accidentally being broken off by Zeus, was transformed into the Cornucopia, or "horn of plenty", which would be filled with whatever its owner desired. Capella forms an asterism with the stars Epsilon, Zeta, and Eta Aurigae, the latter two of which are known as the Haedi (the Kids). Though most often associated with Amalthea, Capella has sometimes been associated with Amalthea's owner, a nymph. The myth of the nymph says that the goat's hideous appearance, resembling a Gorgon, was partially responsible for the Titans' defeat, because Zeus skinned the goat and wore it as his aegis. The asterism containing the three goats had been a separate constellation; however, Ptolemy merged the Charioteer and the Goats in the 2nd century Almagest. Before that, Capella was sometimes seen as its own constellation—by Pliny the Elder and Manilius—called Capra, Caper, or Hircus, all of which relate to its status as the "goat star".

This symbolism dates back to Mesopotamia as a constellation called GAM, representing a scimitar or crook. It may have represented Capella alone or the modern constellation as a whole; this figure was alternatively called Gamlum or MUL.GAM in the MUL.APIN. The crook of Auriga stood for a goat-herd or shepherd. It was formed from most of the stars of the modern constellation; all of the bright stars were included except for Elnath, traditionally assigned to both Taurus and Auriga. 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. It is sometimes called the Shepherd's Star in English literature. Capella is thought to be mentioned in an Akkadian inscription dating to the 20th century BC.

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. ^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. 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. Another name in Arabic was Al-Rākib "the driver", a translation of the Greek.

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).

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 Hindu mythology, Capella was seen as the heart of Brahma, Brahma Hṛdaya. 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 Quechua it was known as Colca.

In Inuit astronomy, Capella, along with Menkalinan (Beta Aurigae), Pollux (Beta Geminorum) and Castor (Alpha Geminorum), formed a constellation Quturjuuk, "collar-bones", the two pairs of stars denoting a bone each. Used for navigation and time-keeping at night, the constellation was recognised from Alaska to western Greenland.

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.

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). The Wardaman people of northern Australia knew the star as Yagalal, a ceremonial fish scale, related to Guwamba the barramundi (Aldebaran).

Namesakes

Visual companions

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.