Arcturus
Arcturus is a red giant star in the northern constellation of Boötes, and the brightest star in the constellation. It has the Bayer designation α Boötis, which is Latinized to Alpha Boötis and abbreviated Alf Boo or α Boo. With an apparent visual magnitude of −0.05, it is the fourth-brightest star in the night sky and the brightest in the northern celestial hemisphere. Arcturus forms one corner of the Spring Triangle asterism.
Located relatively close at 36.7 light-years from the Sun, Arcturus is a red giant of spectral type K1.5III—an aging star around 7.1 billion years old that has used up its core hydrogen and evolved off the main sequence. It is about the same mass as the Sun, but has expanded to 25 times its size and is around 170 times as luminous.
Nomenclature
The traditional name Arcturus is Latinised from the ancient Greek Ἀρκτοῦρος and means "Guardian of the Bear", ultimately from ἄρκτος, "bear" and οὖρος, "watcher, guardian". As ἄρκτος also came to mean "north", the name can also translate to "Guardian of the North".The designation of Arcturus as α Boötis was made by Johann Bayer in 1603. In 2016, the International Astronomical Union organized a Working Group on Star Names 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 Arcturus for α Boötis.
Observational history
Arcturus and its distinctive red color have been mentioned since antiquity and medieval times; Ptolemy described it as subrufa, and Geoffrey Chaucer referred to it as Alramih in A Treatise on the Astrolabe.In 1635, the French mathematician and astronomer Jean-Baptiste Morin observed Arcturus in the daytime with a telescope. This was the first recorded full daylight viewing for any star other than the Sun and supernovae.
Observation
With an apparent visual magnitude of −0.05, Arcturus is the brightest star in the northern celestial hemisphere and the fourth-brightest star in the night sky, after Sirius, Canopus and α Centauri. However, α Centauri AB is a binary star, whose components are each fainter than Arcturus. This makes Arcturus the third-brightest individual star, just ahead of α Centauri A, whose apparent magnitude. Arcturus has been seen at or just before sunset with the naked eye.Arcturus is visible from both of Earth's hemispheres as it is located 19° north of the celestial equator. The star culminates at midnight on April 27, and at 9 p.m. on June 10 being visible during the late northern spring or the southern autumn. From the Northern Hemisphere, an easy way to find Arcturus is to follow the arc of the handle of the Big Dipper. By continuing in this path, one can find Spica, "Arc to Arcturus, then spike to Spica". Together with the bright stars Spica and Regulus, Arcturus is part of the Spring Triangle asterism. With Cor Caroli, these four stars form the Great Diamond asterism.
Arcturus has a B-V color index of +1.23, roughly midway between Pollux and Aldebaran.
η Boötis, or Muphrid, is only 3.3 light-years distant from Arcturus, and would have a visual magnitude −2.5, about as bright as Jupiter at its brightest from Earth, whereas an observer on the former system would find Arcturus with a magnitude -5.0, slightly brighter than Venus as seen from Earth, but with an orangish color.
Physical characteristics
Based upon an annual parallax shift of 88.83 milliarcseconds, as measured by the Hipparcos satellite, Arcturus is from Earth. The parallax margin of error is 0.54 milliarcseconds, translating to a distance margin of error of ±. Because of its proximity, Arcturus has a high proper motion, two arcseconds a year, greater than any first magnitude star other than α Centauri. It is the second-closest giant star to Earth, after Pollux.Arcturus is moving rapidly relative to the Sun, and is now almost at its closest point to the Sun. Closest approach will happen in about 4,000 years, when the star will be a few hundredths of a light-year closer to Earth than it is today. Arcturus is thought to be an old-disk star, and appears to be moving with a group of 52 other such stars, known as the Arcturus stream.
With an absolute magnitude of −0.30, Arcturus is, together with Vega and Sirius, one of the most luminous stars in the Sun's neighborhood. It is about 110 times brighter than the Sun in visible light wavelengths, but this underestimates its strength as much of the light it gives off is in the infrared; total power output is about 180 times that of the Sun. With a near-infrared J band magnitude of −2.2, only Betelgeuse and R Doradus are brighter. The lower output in visible light is due to a lower efficacy as the star has a lower surface temperature than the Sun.
There have been suggestions that Arcturus might be a member of a binary system with a faint, cool companion, but no companion has been directly detected.
In the absence of a binary companion, the mass of Arcturus cannot be measured directly, but models suggest it is slightly greater than that of the Sun. Evolutionary matching to the observed physical parameters gives a mass of, while the oxygen isotope ratio for a first dredge-up star gives a mass of. The star, given its evolutionary state, is expected to have undergone significant mass loss in the past. The star displays magnetic activity that is heating the coronal structures, and it undergoes a solar-type magnetic cycle with a duration that is probably less than 14 years. A weak magnetic field has been detected in the photosphere with a strength of around half a gauss. The magnetic activity appears to lie along four latitudes and is rotationally modulated.
Arcturus is estimated to be around 6 to 8.5 billion years old, but there is some uncertainty about its evolutionary status. Based upon the color characteristics of Arcturus, it is currently ascending the red-giant branch and will continue to do so until it accumulates a large enough degenerate helium core to ignite the helium flash. It has likely exhausted the hydrogen from its core and is now in its active hydrogen shell burning phase. However, Charbonnel et al. placed it slightly above the horizontal branch, and suggested it has already completed the helium flash stage.
File:Sun to Arcturus comparison.jpg|thumb|Size comparison between the Sun, Beta Ursae Majoris, Pollux, and Arcturus.
Spectrum
Arcturus has evolved off the main sequence to the red giant branch, reaching an early K-type stellar classification. It is frequently assigned the spectral type of K0III, but in 1989 was used as the spectral standard for type K1.5III Fe−0.5, with the suffix notation indicating a mild underabundance of iron compared to typical stars of its type. As the brightest K-type giant in the sky, it has been the subject of multiple atlases with coverage from the ultraviolet to infrared.The spectrum shows a dramatic transition from emission lines in the ultraviolet to atomic absorption lines in the visible range and molecular absorption lines in the infrared. This is due to the optical depth of the atmosphere varying with wavelength. The spectrum shows very strong absorption in some molecular lines that are not produced in the photosphere but in a surrounding shell. Examination of carbon monoxide lines show the molecular component of the atmosphere extending outward to 2–3 times the radius of the star, with the chromospheric wind steeply accelerating to 35–40 km/s in this region.
Astronomers term "metals" those elements with higher atomic numbers than helium. The atmosphere of Arcturus has an enrichment of alpha elements relative to iron but only about a third of solar metallicity. Arcturus is possibly a Population II star.
Oscillations
As one of the brightest stars in the sky, Arcturus has been the subject of a number of studies in the emerging field of asteroseismology. Belmonte and colleagues carried out a radial velocity study of the star in April and May 1988, which showed variability with a frequency of the order of a few microhertz, the highest peak corresponding to 4.3 μHz with an amplitude of 60 ms−1, with a frequency separation of c. 5 μHz. They suggested that the most plausible explanation for the variability of Arcturus is stellar oscillations.Asteroseismological measurements allow direct calculation of the mass and radius, giving values of and. This form of modelling is still relatively inaccurate, but a useful check on other models.
Search for planets
satellite astrometry suggested that Arcturus is a binary star, with the companion about twenty times dimmer than the primary and orbiting close enough to be at the limits of resolution with current technology. Recent results remain inconclusive, but do support the marginal Hipparcos detection of a binary companion.In 1993, radial velocity measurements of Aldebaran, Arcturus and Pollux showed that Arcturus exhibited a long-period radial velocity oscillation, which could be interpreted as a substellar companion. This substellar object would be nearly 12 times the mass of Jupiter and be located roughly at the same orbital distance from Arcturus as the Earth is from the Sun, at 1.1 astronomical units. However, all three stars surveyed showed similar oscillations yielding similar companion masses, and the authors concluded that the variation was likely to be intrinsic to the star rather than due to the gravitational effect of a companion. So far, no substellar companion has been confirmed.
Mythology
One astronomical tradition associates Arcturus with the mythology around Arcas, who was about to shoot and kill his own mother Callisto who had been transformed into a bear. Zeus averted their imminent tragic fate by transforming the boy into the constellation Boötes, called Arctophylax "bear guardian" by the Greeks, and his mother into Ursa Major. The account is given in Hyginus's Astronomy.Aratus in his Phaenomena said that the star Arcturus lay below the belt of Arctophylax, and according to Ptolemy in the Almagest it lay between his thighs.
An alternative lore associates the name with the legend around Icarius, who gave the gift of wine to other men, but was murdered by them, because they had had no experience with intoxication and mistook the wine for poison. It is stated that Icarius became Arcturus while his dog, Maira, became Canicula, although "Arcturus" here may be used in the sense of the constellation rather than the star.