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. It may sometimes be used to refer to a single star. A large group of stars bound by gravitation is generally called a ''st ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

of Eridanus, abbreviated 40 Eri. It has the

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

Omicron² Eridani, which is Latinized from ο² Eridani and abbreviated Omicron² Eri or ο² Eri. Based on

parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

measurements taken by the

Gaia In Greek mythology, Gaia (; , a poetic form of ('), meaning 'land' or 'earth'),, , . also spelled Gaea (), is the personification of Earth. Gaia is the ancestral mother—sometimes parthenogenic—of all life. She is the mother of Uranus (S ...

mission, it is about 16.3

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s from the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

. 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 ( ; ; 15 November 1738 – 25 August 1822) was a German-British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel. Born in the Electorate of Hanover ...

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

Otto Struve Otto Lyudvigovich Struve (; 12 August 1897 – 6 April 1963) was a Russian-American astronomer of Baltic German origin. Otto was the descendant of famous astronomers of the Struve family; he was the son of Ludwig Struve, grandson of Otto Wilhel ...

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 Compact star, stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very density, dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place i ...

, 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 list of brightest stars, eighth-brightest star in the night sky, with an apparent visual magnitude of 0.34. It has the Bayer designation α Canis Min ...

and

Sirius Sirius is the brightest star in the night sky. Its name is derived from the Greek word (Latin script: ), meaning 'glowing' or 'scorching'. The star is designated Canis Majoris, Latinized to Alpha Canis Majoris, and abbr ...

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 88 modern constellations, modern constellations visible from southern England. They are named after John Flamsteed, au ...

and ο² Eridani ( Latinised to Omicron² Eridani) its

. 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. It may sometimes be used to refer to a single star. A large group of stars bound by gravitation is generally called a ''st ...

s, and adopted by the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

(IAU). also bears the

variable star designation In astronomy, a variable-star designation is a unique identifier given to variable stars. It extends the Bayer designation format, with an identifying label (as described below) preceding the Latin genitive of the name of the constellation in whic ...

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

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

word قيض () meaning "the

eggshell An eggshell is the outer covering of a hard-shelled egg (biology), egg and of some forms of eggs with soft outer coats. Worm eggs Nematode eggs present a two layered structure: an external vitellin layer made of chitin that confers mechanical ...

s", 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 classification of stars which appear on plots of stellar color versus brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or dwarf stars, and positions of star ...

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

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 A red dwarf is the smallest kind of star on the main sequence. Red dwarfs are by far the most common type of fusing star in the Milky Way, at least in the neighborhood of the Sun. However, due to their low luminosity, individual red dwarfs are ...

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 magnetic reconnection, the magne ...

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 at , but ejected much of its mass before it became a white dwarf. B and C

orbit In celestial mechanics, an orbit (also known as orbital revolution) 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 ...

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 In astrodynamics, the orbital eccentricity of an astronomical object is a dimensionless parameter that determines the amount by which its orbit around another body deviates from a perfect circle. A value of 0 is a circular orbit, values be ...

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 Gr ...

, near the border with

Serpens Caput Serpens () 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 IAU designated constellations, 88 modern constellations designated by the Inter ...

.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 mythology hero adapted from the Greek mythology, Greek hero Heracles. Hercules was one of the 48 constellations listed by the second-century astronomer Ptolemy, and it remains one of the ...

and

, the closest bright star being

Alpha Serpentis Alpha Serpentis or α Serpentis, formally named Unukalhai (), is a 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 constellation and ...

. 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 habitable zone (HZ), or more precisely the circumstellar habitable zone (CHZ), is the range of orbits around a star within which a planetary surface can support liquid water given sufficient atmospheric pressu ...

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 in 1609 (except the third law, which was fully published in 1619), describe the orbits of planets around the Sun. These laws replaced circular orbits and epicycles in ...

) 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 often called Earth's "twin" or "sister" planet for having almost the same size and mass, and the closest orbit to Earth's. While both are rocky planets, Venus has an atmosphere much thicker ...

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 (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

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 Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

-type life on planets near the flare star.

Search for planets

40 Eridani A shows periodic radial velocity variations, which were suggested to be caused by a planetary companion. The 42-day period is close to the stellar rotation period, which made the possible 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 remained inconclusive. Further studies in 2023 and 2024 concluded that the radial velocity signal very likely does originate from stellar activity, and not from a planet. The candidate planet would have had a

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

of , and lie 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.

In fiction

In the ''

Star Trek ''Star Trek'' is an American science fiction media franchise created by Gene Roddenberry, which began with the Star Trek: The Original Series, series of the same name and became a worldwide Popular culture, pop-culture Cultural influence of ...

'' franchise, the planet Vulcan orbits 40 Eridani A. Vulcan has been referenced in relation to the real-life search for exoplanets in this system. The hypothetical planet 40 Eridani A b is also mentioned in the book ''

Project Hail Mary ''Project Hail Mary'' is a 2021 science fiction novel by American writer Andy Weir. Set in the near future, it centers on school-teacher-turned-astronaut Ryland Grace, who wakes up from a coma afflicted with amnesia. He gradually remembers th ...

'' as the home of the eponymous Eridian species. In the Bobiverse series by Dennis E. Taylor, a double planet is found orbiting 40 Eridani A, and named Vulcan and Romulus by an uploaded intelligence in a von Neumann probe.

Notes

References

External links

Keid
at Jim Kaler's STARS.
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 Keid Eridanus (constellation) Eridani, Omicron2 1325 Durchmusterung objects Eridani, 40 0166 026965 019849 Flare stars Eridani, DY