Capella
Capella is the brightest star in the northern constellation of Auriga. It has the Bayer designation α Aurigae, which is Latinised to Alpha Aurigae and abbreviated Alpha Aur or α Aur. Capella is the sixth-brightest star in the night sky, and the third-brightest in the northern celestial hemisphere after Arcturus and Vega. A prominent object in the northern sky, it is circumpolar to observers north of 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. 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 systems organized in two binary pairs stars: the primary pair Capella Aa and Capella Ab; and the secondary pair Capella H and Capella L. The primaries are two bright-yellow giant stars, both of which are around 2.5 Solar masses. The secondary pair, are around 10,000 astronomical units from the first. They are faint, small and relatively cool red dwarfs.
Capella Aa and Capella Ab have exhausted their core hydrogen, and cooled and expanded, moving off the main 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 with spectral class G8III; it is 78.7 times the Sun's luminosity and 11.98 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 G0III; it is 72.7 times as luminous as the Sun and 8.83 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
α Aurigae is the star system's Bayer designation. It also has the Flamsteed designation 13 Aurigae. It is listed in several multiple star catalogues as ADS 3841, CCDM J05168+4559, and WDS J05167+4600. As a relatively nearby star system, Capella is listed in the 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 Latin for female goat; the alternative name Capra was more commonly used in classical times. It is the translation of the Greek star name Aἴξ meaning "the Goat". As the sound of the Greek term for the goat 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. In 2016, the International Astronomical Union organized a Working Group on Star Names to catalogue 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. The catalogue of star names lists Capella as applying to the star α Aurigae Aa.
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.Capella is thought to be mentioned in an Akkadian inscription dating to the 20th century BC. 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. It is sometimes called the Shepherd's Star in English literature. Capella was seen as a portent of rain in classical times.
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.
Multiple status
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. Almost simultaneously, British astronomer Hugh Newall had observed its composite spectrum with a four prism spectroscope attached to a 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 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. This pair are designated Capella H and L.
X-ray source
Two Aerobee-Hi rocket flights on September 20, 1962, and March 15, 1963, detected and confirmed an X-ray source in Auriga at RA Dec, identified as Capella. A major milestone in stellar X-ray astronomy happened on April 5, 1974, with the detection of the strongest emission of X-rays up to that time from Capella, measured at more than 10,000 times the x-ray luminosity of the Sun. 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 of is four orders of magnitude above the Sun's X-ray luminosity. Capella's X-rays are thought to be primarily from the corona of the most 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 would require magnetic confinement, unless it is a free-flowing coronal wind.
Observation
With an average apparent magnitude of +0.08, Capella is the brightest object in the constellation Auriga, the sixth-brightest star in the night sky, the third-brightest in the northern celestial hemisphere, and the fourth-brightest visible to the naked eye from the latitude 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.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°N: for the whole of the United Kingdom and Canada, 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. 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.
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.
Distance
Based on an annual parallax shift of 76.20 milliarcseconds as measured by the Hipparcos satellite, this system is estimated to be from Earth, with a margin of error of 0.3 light-year. An alternative method to determine the distance is via the orbital parallax, which gives a distance of with a margin of error of only 0.1%. 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. At this range, it would have shone at apparent magnitude −0.82, comparable to Canopus today.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.