40 Eridani is a

triple star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a ''star cluster'' or ''galaxy'', although, broadly speaking, ...

in the constellation of Eridanus, abbreviated 40 Eri. It has the Bayer designation Omicron² Eridani, which is Latinized from ο² Eridani and abbreviated Omicron² Eri or ο² Eri. Based on parallax measurements taken by the Gaia mission, it is about 16.3 light-years from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. The primary star of the system, designated 40 Eridani A and named Keid, is easily visible to the naked eye. It is orbited by a binary pair whose two components are designated 40 Eridani B and C, and which were discovered on January 31, 1783, by

William Herschel Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline ...

. It was again observed by Friedrich Struve in 1825 and by

Otto Struve Otto Struve (August 12, 1897 – April 6, 1963) was a Russian-American astronomer of Baltic German origins. In Russian, his name is sometimes given as Otto Lyudvigovich Struve (Отто Людвигович Струве); however, he spent most o ...

in 1851. In 1910, it was discovered that although component B was a faint star, it was white in color. This meant that it had to be a small star; in fact it was a

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes ...

, the first discovered.''White Dwarfs'', E. Schatzman, Amsterdam: North-Holland, 1958. ^{, p. 1} Although it is neither the closest white dwarf, nor the brightest in the night sky, it is by far the easiest to observe; it is nearly three magnitudes brighter than Van Maanen's Star, the nearest solitary white dwarf, and unlike the companions of

Procyon Procyon () is the brightest star in the constellation of Canis Minor and usually the eighth-brightest star in the night sky, with an apparent visual magnitude of 0.34. It has the Bayer designation α Canis Minoris, which is Latinized ...

and

Sirius Sirius is the brightest star in the night sky. Its name is derived from the Greek word , or , meaning 'glowing' or 'scorching'. The star is designated α Canis Majoris, Latinized to Alpha Canis Majoris, and abbreviated Alpha CM ...

it is not swamped in the glare of a much brighter primary.Keid
, Jim Kaler, STARS web page, accessed 15/5/2007, 10/12/2011.

Nomenclature

40 Eridani is the system's

Flamsteed designation A Flamsteed designation is a combination of a number and constellation name that uniquely identifies most naked eye stars in the modern constellations visible from southern England. They are named for John Flamsteed who first used them while co ...

and ο² Eridani ( Latinised to Omicron² Eridani) its Bayer designation. The designations of the sub-components – B and C – derive from the convention used by the ''Washington Multiplicity Catalog'' (WMC) for

multiple star system A star system or stellar system is a small number of stars that orbit each other, bound by gravitational attraction. A large group of stars bound by gravitation is generally called a '' star cluster'' or '' galaxy'', although, broadly speaki ...

s, and adopted by the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

(IAU). also bears the

variable star designation In astronomy, a variable star designation is a unique identifier given to variable stars. It uses a variation on the Bayer designation format, with an identifying label (as described below) preceding the Latin genitive of the name of the conste ...

DY Eridani. The system bore the traditional name ''Keid'' derived from the

Arabic Arabic (, ' ; , ' or ) is a Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C. E.Watson; Walter ...

word قيض () meaning "the eggshells," alluding to its neighbour Beid (Arabic "egg"). In 2016, the IAU organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(WGSN) to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems. It approved the name ''Keid'' for the component on 12 September 2016 and it is now so included in the List of IAU-approved Star Names.

Properties

40 Eridani A is a

main-sequence In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar He ...

dwarf Dwarf or dwarves may refer to: Common uses *Dwarf (folklore), a being from Germanic mythology and folklore * Dwarf, a person or animal with dwarfism Arts, entertainment, and media Fictional entities * Dwarf (''Dungeons & Dragons''), a humanoid ...

spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

K1, 40 Eridani B is a 9th magnitude

of spectral type DA4, and 40 Eridani C is an 11th magnitude red dwarf

flare star A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to the magnetic energy stored in th ...

of spectral type M4.5e. When component B was a main-sequence star, it is thought to have been the most massive member of the system, but ejected most of its mass before it became a white dwarf. B and C

orbit In celestial mechanics, an orbit is the curved trajectory of an object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an artificial satellite around an object or position in space such as ...

each other approximately 400 AU from the primary star, A. Their orbit has a semimajor axis of 35 AU and is rather elliptical with an orbital eccentricity of 0.410). As seen from the 40 Eridani system, the Sun is a 3.4-magnitude star in

Hercules Hercules (, ) is the Roman equivalent of the Greek divine hero Heracles, son of Jupiter and the mortal Alcmena. In classical mythology, Hercules is famous for his strength and for his numerous far-ranging adventures. The Romans adapted the ...

, near the border with

Serpens Caput Serpens ( grc, , , the Serpent) is a constellation in the northern celestial hemisphere. One of the 48 constellations listed by the 2nd-century astronomer Ptolemy, it remains one of the 88 modern constellations designated by the International ...

.From 40 Eridani the Sun would appear on the diametrically opposite side of the sky at the coordinates RA=, Dec=, which is located near the border of

Hercules (constellation) Hercules is a constellation named after Hercules, the Roman mythological hero adapted from the Greek hero Heracles. Hercules was one of the 48 constellations listed by the second-century astronomer Ptolemy, and it remains one of the 88 modern co ...

and

, the closest bright star being

Alpha Serpentis Alpha Serpentis or α Serpentis, formally named Unukalhai (), is a double star in the head (Serpens Caput) of the equatorial constellation of Serpens. With an apparent visual magnitude of 2.6, this star is the brightest in the constell ...

. The absolute magnitude of the Sun is 4.85, so, at a distance of 5.04 parsecs, the Sun would have an apparent magnitude

\beginm\ =\ M_v\ +\ 5\cdot((\log_\ 5.04)\ -\ 1)\ =\ 3.36\end

Potential for life

The

habitable zone In astronomy and astrobiology, the circumstellar habitable zone (CHZ), or simply the habitable zone, is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressure.J. F. Kast ...

of where a planet could exist with liquid water, is near 0.68 from A. At this distance a planet would complete a revolution in 223 Earth days (according to the third of

Kepler's laws In astronomy, Kepler's laws of planetary motion, published by Johannes Kepler between 1609 and 1619, describe the orbits of planets around the Sun. The laws modified the heliocentric theory of Nicolaus Copernicus, replacing its circular orbi ...

) and would appear nearly 20%From

\ h = \frac \,,

where is the apparent height, is the distance of the object, and is the actual size of the object. wider than the Sun does on Earth. An observer on a planet in the system would see the B-C pair as unusually bright white and reddish-orange stars in the night sky – magnitudes −8 and −6, slightly brighter than the appearance of

Venus Venus is the second planet from the Sun. It is sometimes called Earth's "sister" or "twin" planet as it is almost as large and has a similar composition. As an interior planet to Earth, Venus (like Mercury) appears in Earth's sky never f ...

seen from Earth as the evening star. It is unlikely that habitable planets exist around because they would have been sterilized by its evolution into a white dwarf. As for , it is prone to flares, which cause large momentary increases in the emission of

X-ray An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...

s as well as visible light. This would be lethal to

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

-type life on planets near the flare star.

Planetary system

40 Eridani A shows periodic radial velocity variations, which are likely to be caused by a planetary companion. The 42-day period is close to the stellar rotation period, making the planetary nature of the signal difficult to confirm. A 2018 study found that most evidence supports a planetary origin for the signal, but this has remained controversial, with a 2021 study characterizing the signal as a false positive. As of 2022, the cause of the radial velocity variations remains inconclusive. The likely planet has a

minimum mass In astronomy, minimum mass is the lower-bound calculated mass of observed objects such as planets, stars and binary systems, nebulae, and black holes. Minimum mass is a widely cited statistic for extrasolar planets detected by the radial veloci ...

of , and lies considerably interior to the habitable zone, receiving nine times more stellar flux than Earth, which is an even greater amount than Mercury, the innermost planet in the Solar System, on average receives from the Sun. It is one of the closest

super-Earth A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to ...

s known.

Notes

References

External links

Keid
at Jim Kaler's STARS.

at solstation.com.
Omicron(2) Eridani
entry at the Internet Stellar Database. {{DEFAULTSORT:40 Eridani Emission-line stars K-type main-sequence stars M-type main-sequence stars White dwarfs Eridani, 40 Triple star systems Hypothetical planetary systems Stars with proper names Eridanus (constellation) Eridani, Omicron2 1325 Durchmusterung objects Eridani, 40 0166 026965 019849 Eridani, DY