Editing Arcturus (section)

==Physical characteristics==
[[File:Arcturus (optical).png|thumb|right|Optical image of Arcturus (DSS2 / MAST / STScI / NASA)]]
Based upon an annual [[stellar parallax|parallax]] shift of 88.83&nbsp;[[Minute and second of arc#Astronomy|milliarcseconds]], as measured by the [[Hipparcos]] satellite, Arcturus is {{Convert|11.26|pc|ly|abbr=off|lk=on|order=flip}} from Earth. The parallax [[margin of error]] is 0.54 milliarcseconds, translating to a distance margin of error of ±{{Convert|0.069|pc|ly|abbr=off|order=flip}}.<ref name="vanLeeuwen2007"/> Because of its proximity, Arcturus has a high [[proper motion]], two [[Minute and second of arc#Astronomy|arcseconds]] a year, greater than any [[first magnitude star]] other than α&nbsp;Centauri. It is [[list of nearest giant stars|the second-closest giant star]] to Earth, after [[Pollux (star)|Pollux]].

Arcturus is moving rapidly ({{cvt|122|km/s|mph|disp=or||}}) 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. (In antiquity, Arcturus was closer to the centre of the constellation.<ref name=rogers982/>) Arcturus is thought to be an [[old-disk star]],<ref name=ramirez_prieto_2011/> and appears to be moving with a group of 52 other such stars, known as the [[Arcturus stream]].<ref name=ramya/>

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 ([[Bolometric correction|bolometric]]) power output is about 180&nbsp;times that of the Sun. With a near-infrared [[J band (infrared)|J band]] [[Magnitude (astronomy)|magnitude]] of −2.2, only [[Betelgeuse]] (−2.9) and [[R Doradus]] (−2.6) are brighter. The lower output in visible light is due to a lower [[Luminous efficacy|efficacy]] as the star has a lower [[effective temperature|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.<ref name=ramirez_prieto_2011/>
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 {{Val|1.08|0.06|u=solar mass}},<ref name=ramirez_prieto_2011/> while the oxygen isotope ratio for a first [[dredge-up]] star gives a mass of {{solar mass|1.2}}.<ref name=abia/> The star, given its evolutionary state, is expected to have undergone significant mass loss in the past.<ref name=Lagarde_et_al_2015/> The star displays [[Stellar magnetic field|magnetic activity]] that is heating the [[stellar corona|coronal]] structures, and it undergoes a [[Solar cycle|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 (unit)|gauss]]. The magnetic activity appears to lie along four latitudes and is rotationally modulated.<ref name=Sennhauser2011/>

Arcturus is estimated to be around 6 to 8.5 billion years old,<ref name=ramirez_prieto_2011/> but there is some uncertainty about its evolutionary status.<ref name=Pavlenko2008/> Based upon the [[Color index|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 [[Stellar core|core]] to ignite the [[helium flash]].<ref name=ramirez_prieto_2011/> It has likely exhausted the [[hydrogen]] from its core and is now in its active [[red giant|hydrogen shell burning]] phase. However, Charbonnel et al. (1998) placed it slightly above the [[horizontal branch]], and suggested it has already completed the helium flash stage.<ref name=Pavlenko2008/>
[[File:Sun to Arcturus comparison.jpg|thumb|Size comparison between the [[Sun]], [[Beta Ursae Majoris]], [[Pollux (star)|Pollux]], and Arcturus.]]

===Spectrum===
Arcturus has [[stellar evolution|evolved]] off the main sequence to the [[red giant branch]], reaching an [[early-type star|early]] [[k-type star|K-type]] [[stellar classification]]. It is frequently assigned the spectral type of K0III,<ref name=gray/> but in 1989 was used as the spectral standard for type K1.5III Fe−0.5,<ref name=perkins/> with the suffix notation indicating a mild underabundance of iron compared to typical stars of its type. As the brightest K-type [[Giant star#Red giants|giant]] in the sky, it has been the subject of multiple [[spectral atlas|atlases]] with coverage from the [[ultraviolet]] to [[infrared]].<ref name=griffin1968/><ref name=hinckle2005/>

The spectrum shows a dramatic transition from [[emission line]]s in the ultraviolet to atomic [[absorption line]]s in the visible range and molecular absorption lines in the infrared. This is due to the optical depth of the atmosphere varying with wavelength.<ref name=hinckle2005/> The spectrum shows very strong absorption in some molecular lines that are not produced in the [[photosphere]] but in a surrounding shell.<ref name=tsuji2009/> 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 [[stellar wind|chromospheric wind]] steeply accelerating to 35–40&nbsp;km/s in this region.<ref name="Ohnaka_Morales_Marín_2018"/>

Astronomers term "metals" those elements with higher [[atomic number]]s 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]].<ref name=schaaf/>

===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 (Doppler shift of spectral lines) study of the star in April and May 1988, which showed variability with a frequency of the order of a few [[microhertz]] (μHz), the highest peak corresponding to 4.3&nbsp;μHz (2.7&nbsp;days) with an amplitude of 60 ms<sup>−1</sup>, with a [[frequency separation]] of c.&nbsp;5&nbsp;μHz. They suggested that the most plausible explanation for the variability of Arcturus is stellar oscillations.<ref name=belmonte1990/>

Asteroseismological measurements allow direct calculation of the mass and radius, giving values of {{Val|0.8|0.2|u=solar mass}} and {{Val|27.9|3.4|ul=solar radius}}. This form of modelling is still relatively inaccurate, but a useful check on other models.<ref name=kallinger/>

===Search for planets===
[[Hipparcos]] 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 very limits of humans' current ability to make it out. Recent results remain inconclusive, but do support the marginal ''Hipparcos'' detection of a binary companion.<ref name=verhoelst2005/>

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&nbsp;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&nbsp;[[astronomical unit]]s. 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.<ref name=Kgiants/>
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{{PlanetboxOrbit begin}}
{{PlanetboxOrbit hypothetical
| exoplanet = [[Arcturus b|b]]
| mass = 11.7
| period = 233
| semimajor = 1.1
| eccentricity = 0.147
}}
{{PlanetboxOrbit end}}
Put the starbox in once a planet has been confirmed, if it is a planet and not a brown dwarf.-->