Editing Epsilon Eridani (section)

== Properties ==
[[File:Size epsilon eridani.png|left|thumb|Illustration of the relative sizes of Epsilon Eridani (left) and the Sun (right)|alt=A glowing orange orb on the left half and a slightly larger glowing yellow orb on the right against a black background]]

At a distance of {{convert|10.50|ly|pc|abbr=in}}, Epsilon Eridani is the 13th-nearest known star (and ninth nearest solitary star or [[stellar system]]) to the Sun as of 2014.<ref name=RECONS /> Its proximity makes it one of the most studied stars of its [[stellar classification|spectral type]].<ref name=mnras398_3_1495 /> Epsilon Eridani is located in the northern part of the constellation Eridanus, about 3° east of the slightly brighter star [[Delta Eridani]]. With a declination of −9.46°, Epsilon Eridani can be viewed from much of Earth's surface, at suitable times of year. Only to the north of [[80th parallel north|latitude 80°&nbsp;N]] is it permanently hidden below the horizon.<ref name=campbell1899 /> The [[apparent magnitude]] of 3.73 can make it difficult to observe from an urban area with the unaided eye, because the night skies over cities are obscured by [[light pollution]].<ref name=narisada2004 />

Epsilon Eridani has an estimated mass of 0.82 [[solar mass]]es<ref name=mnras403_3_1368 /><ref name=Baines /> and a radius of 0.738 [[solar radius|solar radii]].<ref name=Rains2020/> It shines with a luminosity of only 0.34 [[solar luminosity|solar luminosities]].<ref name=apj460_993 /> The estimated [[effective temperature]] is 5,084&nbsp;K.<ref name=aaa411_3_559 /> With a stellar classification of K2&nbsp;V, it is the second-nearest [[K-type main-sequence star]] (after [[Alpha Centauri]] B).<ref name=RECONS /> Since 1943 the [[stellar spectrum|spectrum]] of Epsilon Eridani has served as one of the stable anchor points by which other stars are classified.<ref name=baas25_1319 /> Its [[metallicity]], the fraction of elements heavier than [[helium]], is slightly lower than the Sun's.<ref name=aaa415 /> In Epsilon Eridani's [[chromosphere]], a region of the outer atmosphere just above the light emitting [[photosphere]], the abundance of iron is estimated at 74% of the Sun's value.<ref name=aaa415 /> The proportion of [[lithium]] in the atmosphere is five times less than that in the Sun.<ref name=Metcalfe2013 />

Epsilon Eridani's K-type classification indicates that the spectrum has relatively weak [[absorption line]]s from absorption by hydrogen ([[Balmer lines]]) but strong lines of neutral atoms and singly [[ion]]ized [[Calcium#H and K lines|calcium]] (Ca II). The [[luminosity class]] V (dwarf) is assigned to stars that are undergoing [[thermonuclear fusion]] of hydrogen in their core. For a K-type main-sequence star, this fusion is dominated by the [[proton–proton chain reaction]], in which a series of reactions effectively combines four hydrogen nuclei to form a helium nucleus. The energy released by fusion is transported outward from the core through [[radiative transfer|radiation]], which results in no net motion of the surrounding plasma. Outside of this region, in the envelope, energy is carried to the photosphere by [[Convection zone|plasma convection]], where it then radiates into space.<ref name=karttunen_oja2007 />

=== Magnetic activity ===

Epsilon Eridani has a higher level of [[Stellar magnetic field|magnetic activity]] than the Sun, and thus the outer parts of its atmosphere (the [[chromosphere]] and [[solar corona|corona]]) are more dynamic. The average magnetic field strength of Epsilon Eridani across the entire surface is {{val|1.65|0.30|e=-2|u=[[Tesla (unit)|tesla]]}},<ref name=aaa318_429 /> which is more than forty times greater than the {{nowrap|(5–40) × 10<sup>−5</sup> T}} magnetic-field strength in the Sun's photosphere.<ref name=apj591_2_1248 /> The magnetic properties can be modelled by assuming that regions with a [[magnetic flux]] of about 0.14&nbsp;T randomly cover approximately 9% of the photosphere, whereas the remainder of the surface is free of magnetic fields.<ref name=apj1_439_2_939 /> The overall magnetic activity of Epsilon Eridani shows co-existing {{Val|2.95|0.03}} and {{Val|12.7|0.3}} year activity cycles.<ref name=Metcalfe2013 /> Assuming that its radius does not change over these intervals, the long-term variation in activity level appears to produce a temperature variation of 15 K, which corresponds to a variation in [[UBV photometric system|visual magnitude]] (V) of 0.014.<ref name=apj441 />

The magnetic field on the surface of Epsilon Eridani causes variations in the [[hydrodynamic]] behaviour of the photosphere. This results in greater [[jitter]] during [[Doppler spectroscopy|measurements of its radial velocity]]. Variations of {{nowrap|15 m s}}<sup>−1</sup> were measured over a 20&nbsp;year period, which is much higher than the [[measurement uncertainty]] of {{nowrap|3 m s}}<sup>−1</sup>. This makes interpretation of periodicities in the radial velocity of Epsilon Eridani, such as those caused by an orbiting planet, more difficult.<ref name=iau202 />

[[File:EpsEriLightCurve.png|thumb|upright=1.5|left|A [[light curve]] for Epsilon Eridani, showing averages of the [[Photometric system#Photometric letters|b and y band]] magnitudes between 2014 and 2021.<ref name="Roettenbacher"/> The inset shows the periodic variation over a 12.3-day rotational period.<ref name=apj102_5_1813/>]]
Epsilon Eridani is classified as a [[BY Draconis variable]] because it has regions of higher magnetic activity that move into and out of the line of sight as it rotates.<ref name=gcvs /> Measurement of this [[rotational modulation]] suggests that its equatorial region rotates with an average period of 11.2&nbsp;days,<ref name=an328_10 /> which is less than half of the rotation period of the Sun. Observations have shown that Epsilon Eridani varies as much as 0.050 in V magnitude due to [[starspot]]s and other short-term magnetic activity.<ref name=apj102_5_1813 /> [[Photometry (astronomy)|Photometry]] has also shown that the surface of Epsilon Eridani, like the Sun, is undergoing [[differential rotation]] i.e. the rotation period at equator differs from that at high [[latitude]]. The measured periods range from 10.8 to 12.3 days.<ref name=apj441 /><ref name=rotation group=note /> The [[axial tilt]] of Epsilon Eridani toward the line of sight from Earth is highly uncertain: estimates range from 24° to 72°.<ref name=an328_10 />

The high levels of chromospheric activity, strong magnetic field, and relatively fast rotation rate of Epsilon Eridani are characteristic of a young star.<ref name=apj1_412_2_797 /> Most estimates of the age of Epsilon Eridani place it in the range from 200 million to 800 million years.<ref name=aaa488_2_771 /> The low abundance of heavy elements in the chromosphere of Epsilon Eridani usually indicates an older star, because the [[interstellar medium]] (out of which stars form) is steadily enriched by heavier elements produced by older generations of stars.<ref name=aaa358_850 /> This anomaly might be caused by a [[diffusion]] process that has transported some of the heavier elements out of the photosphere and into a region below Epsilon Eridani's [[convection zone]].<ref name=cjaa8_5_591 />

The [[X-ray]] luminosity of Epsilon Eridani is about {{val|2|e=28|u=[[erg]]·s<sup>–1</sup>}} ({{val|2|e=21|u=[[Watt|W]]}}). It is more luminous in X-rays than the Sun at [[Solar cycle|peak activity]]. The source for this strong X-ray emission is Epsilon Eridani's hot corona.<ref name=apj243_234 /><ref name=apj457_882 /> Epsilon Eridani's corona appears larger and hotter than the Sun's, with a temperature of {{val|3.4|e=6|u=K}}, measured from observation of the corona's ultraviolet and X-ray emission.<ref name=mnras385_4_1691 /> It displays a cyclical variation in X-ray emission that is consistent with the magnetic activity cycle.<ref name=Coffaro_et_al_2020/>

The [[stellar wind]] emitted by Epsilon Eridani expands until it collides with the surrounding [[interstellar medium]] of diffuse gas and dust, resulting in a bubble of heated hydrogen gas (an [[astrosphere]], the equivalent of the [[heliosphere]] that surrounds the Sun). The [[absorption spectrum]] from this gas has been measured with the [[Hubble Space Telescope]], allowing the properties of the stellar wind to be estimated.<ref name=mnras385_4_1691 /> Epsilon Eridani's hot corona results in a mass loss rate in Epsilon Eridani's stellar wind that is 30 times higher than the Sun's. This stellar wind generates the astrosphere that spans about {{convert|8000|au|pc|abbr=on}} and contains a [[bow shock]] that lies {{convert|1600|au|pc|abbr=on}} from Epsilon Eridani. At its estimated distance from Earth, this astrosphere spans 42&nbsp;arcminutes, which is wider than the apparent size of the full Moon.<ref name=apj574_1 />

=== Kinematics ===
Epsilon Eridani has a high [[proper motion]], moving −0.976&nbsp;arcseconds per year in [[right ascension]] (the celestial equivalent of longitude) and 0.018&nbsp;arcseconds per year in [[declination]] (celestial latitude), for a combined total of 0.962 arcseconds per year.<ref name=aaa474_2_653 />{{refn|1=The total proper motion μ can be computed from:
: μ<sup>2</sup> = (μ<sub>α</sub> cos δ)<sup>2</sup> + μ<sub>δ</sub><sup>2</sup>
where μ<sub>α</sub> is the proper motion in right ascension, μ<sub>δ</sub> is the proper motion in declination, and δ is the declination.<ref name="birney" /> This yields:
: μ<sup>2</sup> = (−975.17 · cos(−9.458°))<sup>2</sup> + 19.49<sup>2</sup> = 925658.1
or μ equals 962.11.|group="note"|name=total_pm}} The star has a radial velocity of {{convert|+15.5|km/s|mph|abbr=on}} (away from the Sun).<ref name=rgcrv66 /> The [[space velocity (astronomy)|space velocity]] components of Epsilon Eridani in the [[galactic coordinate system|galactic co-ordinate system]] are {{nowrap|(U, V, W)}} = {{nowrap|(−3, +7, −20) km/s}}, which means that it is travelling within the [[Milky Way]] at a mean [[galactocentric distance]] of 28.7&nbsp;kly (8.79&nbsp;kiloparsecs) from the core along an orbit that has an [[orbital eccentricity|eccentricity]] of 0.09.<ref name=ab6_2_308 /> The [[Stellar kinematics|position and velocity]] of Epsilon Eridani indicate that it may be a member of the [[Ursa Major Moving Group]], whose members share a common motion through space. This behaviour suggests that the moving group originated in an [[open cluster]] that has since diffused.<ref name=an325_1_3 /> The estimated age of this group is {{nowrap|500±100 million}} years,<ref name=apj125_4_1980 /> which lies within the range of the age estimates for Epsilon Eridani.

During the past million years, three stars are believed to have come within {{convert|7|ly|pc|abbr=on}} of Epsilon Eridani. The most recent and closest of these encounters was with [[Kapteyn's Star]], which approached to a distance of about {{convert|3|ly|pc|abbr=on}} roughly 12,500 years ago. Two more distant encounters were with [[Sirius]] and [[Ross 614]]. None of these encounters are thought to have been close enough to affect the circumstellar disk orbiting Epsilon Eridani.<ref name=asp2001_227 />

Epsilon Eridani made its closest approach to the Sun about 105,000&nbsp;years ago, when they were separated by {{convert|7|ly|pc|abbr=on}}.<ref name=aaa379 /> Based upon a simulation of close encounters with nearby stars, the binary star system [[Luyten 726-8]], which includes the [[variable star]] [[UV Ceti]], will encounter Epsilon Eridani in approximately 31,500 years at a minimum distance of about 0.9&nbsp;ly (0.29&nbsp;parsecs). They will be less than 1&nbsp;ly (0.3&nbsp;parsecs) apart for about 4,600 years. If Epsilon Eridani has an [[Oort cloud]], Luyten 726-8 could gravitationally [[Perturbation (astronomy)|perturb]] some of its [[Exocomet|comets]] with long [[orbital period]]s.<ref name=potemine10 />{{unreliable source?|date=September 2021}}