Alpha Centauri (
Latinized from α Centauri and often abbreviated Alpha Cen or α Cen) is a
triple star system in the
constellation of
Centaurus
Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. I ...
. It consists of 3 stars: Alpha Centauri A (officially Rigil Kentaurus),
Alpha Centauri B (officially Toliman)
and Alpha Centauri C (officially Proxima Centauri).
Proxima Centauri is also the closest star to the
Sun at 4.2465
light-years
A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 101 ...
(1.3020
pc).
Alpha Centauri A and B are
Sun-like
Solar-type star, solar analogs (also analogues), and solar twins are stars that are particularly similar to the Sun. The stellar classification is a hierarchy with solar twin being most like the Sun followed by solar analog and then solar-typ ...
stars (
Class G and
K, respectively), and together they form the
binary star system Alpha Centauri AB. To the
naked eye, the two main components appear to be a single star with an
apparent magnitude
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
of −0.27. It is the brightest star in the constellation and the
third-brightest in the
night sky
The night sky is the nighttime appearance of celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon.
Natural light sources in a night sky inc ...
, outshone only by
Sirius and
Canopus.
Alpha Centauri A has 1.1 times the
mass
Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different element ...
and 1.5 times the
luminosity of the Sun, while Alpha Centauri B is smaller and cooler, at 0.9 times the
Sun's mass and less than 0.5 times its luminosity. The pair orbit around a
common centre with an orbital period of 79 years.
Their elliptical orbit is
eccentric, so that the distance between A and B varies from 35.6
astronomical unit
The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbi ...
s (AU), or about the distance between
Pluto
Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the Sun. It is the largest k ...
and the Sun, to 11.2 AU, or about the distance between
Saturn and the Sun.
Alpha Centauri C, or Proxima Centauri, is a small faint
red dwarf
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave (TV channel), Dave since 2009, gaining a ...
(
Class M). Though not visible to the naked eye, Proxima Centauri is the closest star to the Sun at a distance of , slightly closer than Alpha Centauri AB. Currently, the distance between Proxima Centauri and Alpha Centauri AB is about ,
equivalent to about 430 times the radius of
Neptune's orbit.
Proxima Centauri has three known planets:
Proxima b, an
Earth-sized planet in the
habitable zone discovered in 2016;
Proxima c, a
super-Earth 1.5 AU away, which is possibly surrounded by a huge
ring system, discovered in 2019; and
Proxima d, a candidate
sub-Earth which orbits very closely to the star, announced in 2022. Alpha Centauri A may have
a Neptune-sized habitable-zone planet, though it is not yet known to be planetary in nature and could be an artifact of the discovery mechanism. Alpha Centauri B has no known planets: planet
Bb, purportedly discovered in 2012, was subsequently found to likely not exist, and a separate transiting planet has yet to be confirmed.
Etymology and nomenclature
''α Centauri'' (
Latinised to ''Alpha Centauri'') is the system's
designation
Designation may refer to:
* Designation (law), the process of determining an incumbent's successor
* Professional certification
* Designation (landmarks), an official classification determined by a government agency or historical society
* Designa ...
given by
Johann Bayer in 1603. It bears the traditional name ''Rigil Kentaurus'', which is a Latinisation of the Arabic name ''Rijl al-Qinṭūrus,'' meaning 'the Foot of the
Centaur
A centaur ( ; grc, κένταυρος, kéntauros; ), or occasionally hippocentaur, is a creature from Greek mythology with the upper body of a human and the lower body and legs of a horse.
Centaurs are thought of in many Greek myths as bein ...
'.
The name is frequently abbreviated to ''Rigil Kent'' or even ''Rigil'', though the latter name is better known for
Rigel (Beta Orionis).
[R.H. Allen, ''Star Names and their Meanings'']
An alternative name found in European sources, ''Toliman'', is an approximation of the Arabic ''aẓ-Ẓalīmān'' (in older transcription, ''aṭ-Ṭhalīmān''), meaning 'the (two male) Ostriches', an appellation
Zakariya al-Qazwini had applied to
Lambda
Lambda (}, ''lám(b)da'') is the 11th letter of the Greek alphabet, representing the voiced alveolar lateral approximant . In the system of Greek numerals, lambda has a value of 30. Lambda is derived from the Phoenician Lamed . Lambda gave ris ...
and
Mu Sagittarii, also in the southern hemisphere.
A third name that has been applied is ''Bungula'' (), of obscure origin. Allen can only surmise it may have been coined from the Greek letter
beta
Beta (, ; uppercase , lowercase , or cursive ; grc, βῆτα, bē̂ta or ell, βήτα, víta) is the second letter of the Greek alphabet. In the system of Greek numerals, it has a value of 2. In Modern Greek, it represents the voiced labi ...
(β) and Latin 'hoof'.
[
Alpha Centauri C was discovered in 1915 by Robert T. A. Innes,] who suggested that it be named ''Proxima Centaurus'', . The name ''Proxima Centauri'' later became more widely used and is now listed 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, outreach ...
(IAU) as the approved proper name.
In 2016, the 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 List of proper names of stars, proper names for stars for the international astronomical community. It operates under ...
of the IAU, having decided to attribute proper names to individual component stars rather than to multiple systems, approved the name ''Rigil Kentaurus'' () as being restricted to ''Alpha Centauri A'' and the name ''Proxima Centauri'' () for ''Alpha Centauri C''. On 10 August 2018, the IAU approved the name ''Toliman'' () for ''Alpha Centauri B''.
Observation
To the naked eye, Alpha Centauri AB appears to be a single star, the brightest in the southern constellation of Centaurus
Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. I ...
. Their apparent angular separation varies over about 80 years between 2 and 22 arcsec (the naked eye has a resolution of 60 arcsec), but through much of the orbit, both are easily resolved in binoculars or small telescopes. At −0.27 apparent magnitude
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
(combined
Combined may refer to:
* Alpine combined (skiing), the combination of slalom and downhill skiing as a single event
** Super combined (skiing)
* Nordic combined (skiing), the combination of cross country skiing and ski jumping as a single event
* T ...
for A and B magnitudes), Alpha Centauri is a first-magnitude star and is fainter only than Sirius and Canopus. It is the outer star of ''The Pointers'' or ''The Southern Pointers'', so called because the line through Beta Centauri (Hadar/Agena), some 4.5° west, points to the constellation Crux
Crux () is a constellation of the southern sky that is centred on four bright stars in a cross-shaped asterism commonly known as the Southern Cross. It lies on the southern end of the Milky Way's visible band. The name ''Crux'' is Latin for ...
— the Southern Cross. The Pointers easily distinguish the true Southern Cross from the fainter asterism known as the False Cross.
South of about 29° South latitude, Alpha Centauri is circumpolar and never sets below the horizon.[This is calculated for a fixed latitude by knowing the star's declination (δ) using the formulae (90°+ δ). Alpha Centauri's declination is −60° 50′, so the observed ]latitude
In geography, latitude is a coordinate that specifies the north– south position of a point on the surface of the Earth or another celestial body. Latitude is given as an angle that ranges from –90° at the south pole to 90° at the north po ...
where the star is circumpolar will be south of −29° 10′ South or 29°. Similarly, the place where Alpha Centauri never rises for northern observers is north of the latitude (90°+ δ) N or +29° North. North of about 29° N latitude, Alpha Centauri never rises. Alpha Centauri lies close to the southern horizon when viewed from the 29° North latitude to the equator
The equator is a circle of latitude, about in circumference, that divides Earth into the Northern and Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the North and South poles. The term can al ...
(close to Hermosillo and Chihuahua City
The city of Chihuahua ''(La Ciudad de Chihuahua)'' () is the state capital of the Mexican state of Chihuahua. , the city of Chihuahua had a population of 925,762 inhabitants. while the metropolitan area had a population of 988,065 inhabitants.
A ...
in Mexico
Mexico ( Spanish: México), officially the United Mexican States, is a country in the southern portion of North America. It is bordered to the north by the United States; to the south and west by the Pacific Ocean; to the southeast by Guate ...
; Galveston, Texas
Galveston ( ) is a coastal resort city and port off the Southeast Texas coast on Galveston Island and Pelican Island in the U.S. state of Texas. The community of , with a population of 47,743 in 2010, is the county seat of surrounding Gal ...
; Ocala, Florida; and Lanzarote, the Canary Islands
The Canary Islands (; es, Canarias, ), also known informally as the Canaries, are a Spanish autonomous community and archipelago in the Atlantic Ocean, in Macaronesia. At their closest point to the African mainland, they are west of Mo ...
of Spain
, image_flag = Bandera de España.svg
, image_coat = Escudo de España (mazonado).svg
, national_motto = '' Plus ultra'' ( Latin)(English: "Further Beyond")
, national_anthem = (English: "Royal March")
, ...
), but only for a short time around its culmination. The star culminates each year at local midnight on 24 April and at local 9 p.m. on 8 June.
As seen from Earth, Proxima Centauri is 2.2° southwest from Alpha Centauri AB, about four times the angular diameter of the Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...
. Proxima Centauri appears as a deep-red star of a typical apparent magnitude of 11.1 in a sparsely populated star field, requiring moderately sized telescopes to be seen. Listed as V645 Cen in the '' General Catalogue of Variable Stars Version 4.2'', this UV Ceti-type flare star can unexpectedly brighten rapidly by as much as 0.6 magnitude at visual wavelengths, then fade after only a few minutes. Some amateur and professional astronomers regularly monitor for outbursts using either optical or radio telescopes. In August 2015, the largest recorded flares of the star occurred, with the star becoming 8.3 times brighter than normal on 13 August, in the B band (blue light region).
Alpha Centauri may be inside the G-cloud of the Local Bubble, and its nearest known system is the binary brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen (hydrogen-1, 1H) into helium in their cores, unlike a main sequence, main-sequence star. Instead, they have ...
system Luhman 16 at .
Observational history
Alpha Centauri is listed in the 2nd-century star catalog of Ptolemy. He gave its ecliptic coordinates, but texts differ as to whether the ecliptic latitude reads or . (Presently the ecliptic latitude is , but it has decreased by a fraction of a degree since Ptolemy
Claudius Ptolemy (; grc-gre, Πτολεμαῖος, ; la, Claudius Ptolemaeus; AD) was a mathematician, astronomer, astrologer, geographer, and music theorist, who wrote about a dozen scientific treatises, three of which were of import ...
's time due to proper motion.) In Ptolemy's time, Alpha Centauri was visible from Alexandria, Egypt, at but, due to precession
Precession is a change in the orientation of the rotational axis of a rotating body. In an appropriate reference frame it can be defined as a change in the first Euler angle, whereas the third Euler angle defines the rotation itself. In o ...
, its declination is now , and it can no longer be seen at that latitude. English explorer Robert Hues brought Alpha Centauri to the attention of European observers in his 1592 work ''Tractatus de Globis'', along with Canopus and Achernar, noting:
The binary nature of Alpha Centauri AB was recognized in December 1689 by Jean Richaud, while observing a passing comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or Coma (cometary), coma, and sometimes also a Comet ta ...
from his station in Puducherry. Alpha Centauri was only the second binary star to be discovered, preceded by Acrux.
The large proper motion of Alpha Centauri AB was discovered by Manuel John Johnson, observing from Saint Helena
Saint Helena () is a British overseas territory located in the South Atlantic Ocean. It is a remote volcanic tropical island west of the coast of south-western Africa, and east of Rio de Janeiro in South America. It is one of three consti ...
, who informed Thomas Henderson at the Royal Observatory, Cape of Good Hope of it. The parallax of Alpha Centauri was subsequently determined by Henderson from many exacting positional observations of the AB system between April 1832 and May 1833. He withheld his results, however, because he suspected they were too large to be true, but eventually published them in 1839 after Friedrich Wilhelm Bessel released his own accurately determined parallax for 61 Cygni in 1838. For this reason, Alpha Centauri is sometimes considered as the second star to have its distance measured because Henderson's work was not fully acknowledged at first. (The distance of Alpha Centauri from the Earth is now reckoned at .)
Later, John Herschel made the first micrometrical observations in 1834. Since the early 20th century, measures have been made with photographic plates.
By 1926, William Stephen Finsen calculated the approximate orbit elements close to those now accepted for this system. All future positions are now sufficiently accurate for visual observers to determine the relative places of the stars from a binary star ephemeris. Others, like D. Pourbaix (2002), have regularly refined the precision of new published orbital elements.
Robert T. A. Innes discovered Proxima Centauri in 1915 by blinking photographic plates taken at different times during a proper motion survey. These showed large proper motion and parallax similar in both size and direction to those of Alpha Centauri AB, which suggested that Proxima Centauri is part of the Alpha Centauri system and slightly closer to Earth than Alpha Centauri AB. As such, Innes concluded that Proxima Centauri was the closest star to Earth yet discovered.
Kinematics
All components of Alpha Centauri display significant proper motion against the background sky. Over centuries, this causes their apparent positions to slowly change. Proper motion was unknown to ancient astronomers. Most assumed that the stars were permanently fixed on the celestial sphere, as stated in the works of the philosopher Aristotle. In 1718, Edmond Halley found that some stars had significantly moved from their ancient astrometric
Astrometry is a branch of astronomy that involves precise measurements of the positions and movements of stars and other celestial bodies. It provides the kinematics and physical origin of the Solar System and this galaxy, the Milky Way.
Histor ...
positions.
In the 1830s, Thomas Henderson discovered the true distance to Alpha Centauri by analysing his many astrometric mural circle observations. He then realised this system also likely had a high proper motion. In this case, the apparent stellar motion was found using Nicolas Louis de Lacaille's astrometric observations of 1751–1752, by the observed differences between the two measured positions in different epochs.
Calculated proper motion of the centre of mass for Alpha Centauri AB is about 3620 mas/y (milliarcseconds per year) toward the west and 694 mas/y toward the north, giving an overall motion of 3686 mas/y in a direction 11° north of west.[Proper motions are expressed in smaller angular units than arcsec, being measured in milliarcsec (mas.) (thousandths of an arcsec). Negative values for proper motion in RA indicate the sky motion is from east to west, and in declination north to south.] The motion of the centre of mass is about 6.1 arcmin each century, or 1.02 ° each millennium. The speed in the western direction is and in the northerly direction . Using spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter ...
the mean radial velocity has been determined to be around towards the Solar System.[ This gives a speed with respect to the Sun of , very close to the peak in the distribution of speeds of nearby stars.
Since Alpha Centauri AB is almost exactly in the plane of the ]Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked ey ...
as viewed from Earth, many stars appear behind it. In early May 2028, Alpha Centauri A will pass between the Earth and a distant red star, when there is a 45% probability that an Einstein ring will be observed. Other conjunctions will also occur in the coming decades, allowing accurate measurement of proper motions and possibly giving information on planets.[
]
Predicted future changes
Based on the system's common proper motion and radial velocities, Alpha Centauri will continue to change its position in the sky significantly and will gradually brighten. For example, in about 6,200 AD, α Centauri's true motion will cause an extremely rare first-magnitude stellar conjunction with Beta Centauri, forming a brilliant optical double star in the southern sky. It will then pass just north of the Southern Cross or Crux
Crux () is a constellation of the southern sky that is centred on four bright stars in a cross-shaped asterism commonly known as the Southern Cross. It lies on the southern end of the Milky Way's visible band. The name ''Crux'' is Latin for ...
, before moving northwest and up towards the present celestial equator and away from the galactic plane. By about 26,700 AD, in the present-day constellation of Hydra
Hydra generally refers to:
* Lernaean Hydra, a many-headed serpent in Greek mythology
* ''Hydra'' (genus), a genus of simple freshwater animals belonging to the phylum Cnidaria
Hydra or The Hydra may also refer to:
Astronomy
* Hydra (constel ...
, Alpha Centauri will reach perihelion at away, though later calculations suggest that this will occur in 27,000 AD. At nearest approach, Alpha Centauri will attain a maximum apparent magnitude
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
of −0.86, comparable to present-day magnitude of Canopus, but it will still not surpass that of Sirius, which will brighten incrementally over the next 60,000 years, and will continue to be the brightest star as seen from Earth (other than the Sun) for the next 210,000 years.
Stellar system
Alpha Centauri is a triple star system, with its two main stars, Alpha Centauri A and Alpha Centauri B, together comprising a binary component. The ''AB'' designation, or older ''A×B'', denotes the mass centre of a main binary system relative to companion star(s) in a multiple star system. ''AB-C'' refers to the component of Proxima Centauri in relation to the central binary, being the distance between the centre of mass and the outlying companion. Because the distance between Proxima (C) and either of Alpha Centauri A or B is similar, the AB binary system is sometimes treated as a single gravitational object.
Orbital properties
The A and B components of Alpha Centauri have an orbital period of 79.762 years. Their orbit is moderately eccentric, as it has an eccentricity of almost 0.52; their closest approach or periastron is , or about the distance between the Sun and Saturn; and their furthest separation or apastron is , about the distance between the Sun and Pluto. The most recent periastron was in August 1955 and the next will occur in May 2035; the most recent apastron was in May 1995 and will next occur in 2075.
Viewed from Earth, the ''apparent orbit'' of A and B means that their separation and position angle (PA) are in continuous change throughout their projected orbit. Observed stellar positions in 2019 are separated by 4.92 arcsec through the PA of 337.1°, increasing to 5.49 arcsec through 345.3° in 2020. The closest recent approach was in February 2016, at 4.0 arcsec through the PA of 300°. The observed maximum separation of these stars is about 22 arcsec, while the minimum distance is 1.7 arcsec. The widest separation occurred during February 1976, and the next will be in January 2056.
Alpha Centauri C is about from Alpha Centauri AB, equivalent to about 5% of the distance between Alpha Centauri AB and the Sun. Until 2017, measurements of its small speed and its trajectory were of too little accuracy and duration in years to determine whether it is bound to Alpha Centauri AB or unrelated.
Radial velocity measurements made in 2017 were precise enough to show that Proxima Centauri and Alpha Centauri AB are gravitationally bound. The orbital period of Proxima Centauri is approximately years, with an eccentricity of 0.5, much more eccentric than Mercury's. Proxima Centauri comes within of AB at periastron, and its apastron occurs at .
Physical properties
Asteroseismic
Asteroseismology or astroseismology is the study of oscillations in stars. Stars have many resonant modes and frequencies, and the path of sound waves passing through a star depends on the speed of sound, which in turn depends on local temperatur ...
studies, chromospheric activity, and stellar rotation (gyrochronology Gyrochronology is a method for estimating the age of a low-mass (cool) Main sequence, main sequence star (spectral class F8 V or later) from its stellar rotation, rotation period. The term is derived from the Greek language, Greek words ''gyros, chr ...
) are all consistent with the Alpha Centauri system being similar in age to, or slightly older than, the Sun. Asteroseismic analyses that incorporate tight observational constraints on the stellar parameters for the Alpha Centauri stars have yielded age estimates of Gyr, Gyr, 5.2 ± 1.9 Gyr, 6.4 Gyr, and Gyr. Age estimates for the stars based on chromospheric activity (Calcium H & K emission) yield 4.4 ± 2.1 Gyr, whereas gyrochronology yields Gyr. Stellar evolution
Stellar evolution is the process by which a star changes over the course of time. Depending on the mass of the star, its lifetime can range from a few million years for the most massive to trillions of years for the least massive, which is con ...
theory implies both stars are slightly older than the Sun at 5 to 6 billion years, as derived by their mass and spectral characteristics.
From the orbital elements, the total mass of Alpha Centauri AB is about [, see formula] – or twice that of the Sun. The average individual stellar masses are about and , respectively, though slightly different masses have also been quoted in recent years, such as and , totalling . Alpha Centauri A and B have absolute magnitudes of +4.38 and +5.71, respectively.
Alpha Centauri AB System
= Alpha Centauri A
=
Alpha Centauri A, also known as Rigil Kentaurus, is the principal member, or primary, of the binary system. It is a solar-like main-sequence star with a similar yellowish colour, whose stellar classification is spectral type G2-V; it is about 10% more massive than the Sun, with a radius about 22% larger. When considered among the individual brightest stars
This is a list of stars arranged by their apparent magnitude – their brightness as observed from Earth. It includes all stars brighter than magnitude +2.50 in visible light, measured using a ''V''-band filter in the UBV photometric system. Stars ...
in the night sky, it is the fourth-brightest at an apparent magnitude of +0.01, being slightly fainter than Arcturus at an apparent magnitude
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
of −0.05.
The type of magnetic activity on Alpha Centauri A is comparable to that of the Sun, showing coronal variability due to star spots, as modulated by the rotation of the star. However, since 2005 the activity level has fallen into a deep minimum that might be similar to the Sun's historical Maunder Minimum. Alternatively, it may have a very long stellar activity cycle and is slowly recovering from a minimum phase.
= Alpha Centauri B
=
Alpha Centauri B, also known as Toliman, is the secondary star of the binary system. It is a main-sequence star of spectral type K1-V, making it more an orange colour than Alpha Centauri A; it has around 90% of the mass of the Sun and a 14% smaller diameter. Although it has a lower luminosity than A, Alpha Centauri B emits more energy in the X-ray
X-rays (or rarely, ''X-radiation'') are a form of high-energy electromagnetic radiation. In many languages, it is referred to as Röntgen radiation, after the German scientist Wilhelm Conrad Röntgen, who discovered it in 1895 and named it ' ...
band. Its light curve varies on a short time scale, and there has been at least one observed flare. It is more magnetically active than Alpha Centauri A, showing a cycle of compared to 11 years for the Sun, and has about half the minimum-to-peak variation in coronal luminosity of the Sun. Alpha Centauri B has an apparent magnitude of +1.35, slightly dimmer than Mimosa.
Alpha Centauri C (Proxima Centauri)
Alpha Centauri C, better known as Proxima Centauri, is a small main-sequence red dwarf
''Red Dwarf'' is a British science fiction comedy franchise created by Rob Grant and Doug Naylor, which primarily consists of a television sitcom that aired on BBC Two between 1988 and 1999, and on Dave (TV channel), Dave since 2009, gaining a ...
of spectral class M6-Ve. It has an absolute magnitude of +15.60, over 20,000 times fainter than the Sun. Its mass is calculated to be . It is the closest star to the Sun but is too faint to be visible to the naked eye.
Planetary system
The Alpha Centauri system as a whole has three confirmed planets, all of them around Proxima Centauri. While other planets have been claimed to exist around all of the stars, none of the discoveries have been confirmed.
Confirmed planets
Proxima Centauri
Proxima Centauri b is a terrestrial planet discovered in 2016 by astronomers at the European Southern Observatory
The European Organisation for Astronomical Research in the Southern Hemisphere, commonly referred to as the European Southern Observatory (ESO), is an intergovernmental organization, intergovernmental research organisation made up of 16 mem ...
(ESO). It has an estimated minimum mass of 1.17 ( Earth masses) and orbits approximately 0.049 AU from Proxima Centauri, placing it in the star's habitable zone.
Proxima Centauri c is a planet that was formally published in 2020 and could be a super-Earth or mini-Neptune. It has a mass of roughly 7 and orbits about 1.49 AU from Proxima Centauri with a period of . In June 2020, a possible direct imaging detection of the planet hinted at the potential presence of a large ring system. However, a 2022 study disputed the existence of this planet.
A 2020 paper refining Proxima b's mass excludes the presence of extra companions with masses above at periods shorter than 50 days, but the authors detected a radial-velocity curve with a periodicity of 5.15 days, suggesting the presence of a planet with a mass of about .[ This planet, Proxima Centauri d, was confirmed in 2022.]
Unconfirmed planets
Alpha Centauri A
In 2021, a candidate planet named Candidate 1
Candidate 1 (also known as C1 or Alpha Centauri Ab) is an unconfirmed exoplanet candidate directly imaged around Alpha Centauri A in February 2021. If confirmed as an exoplanet, it would orbit at approximately 1.1 AU away from Alpha Centauri A wi ...
(abbreviated as C1) was detected around Alpha Centauri A, thought to orbit at approximately 1.1 AU with a period of about one year, and to have a mass between that of Neptune and one-half that of Saturn, though it may be a dust disk or an artifact. The possibility of C1 being a background star has been ruled out.[Kevin Wagner's (lead author of paper?) video of discovery]
/ref> If this candidate is confirmed, the temporary name C1 will most likely be replaced with the scientific designation Alpha Centauri Ab in accordance with current naming conventions.
GO Cycle 1 observations are planned for the James Webb Space Telescope
The James Webb Space Telescope (JWST) is a space telescope which conducts infrared astronomy. As the largest optical telescope in space, its high resolution and sensitivity allow it to view objects too old, distant, or faint for the Hubble ...
(JWST) to search for planets around Alpha Centauri A. The observations are planned to occur at a date between July and August 2023. Pre-launch estimates predicted that JWST will be able to find planets with a radius of 5 at 1–3 au. Multiple observations every 3–6 months could push the limit down to 3 . Post-processing techniques could push the limit down to 0.5 to 0.7 . Post-launch estimates based on observations of HIP 65426 b
HIP 65426 b is a super Jupiter exoplanet orbiting the star HIP 65426. It was discovered on 6 July 2017 by the SPHERE consortium using the Spectro-Polarimetric High-Contrast Exoplanet Research (SPHERE) instrument belonging to the European Southe ...
find that JWST will be able to find planets even closer to Alpha Centauri A and could find a 5 planet at 0.5 to 2.5 au. Candidate 1 has an estimated radius between 3.3 and 11 and orbits at 1.1 au. It is therefore likely within the reach of JWST observations.
Alpha Centauri B
In 2012, a planet around Alpha Centauri B was reported, Alpha Centauri Bb, but in 2015 a new analysis concluded that that report was an artifact of the datum analysis.
A possible transit of a separate planet in 2013 has been observed. The transit event could correspond to a planetary body with a radius around . This planet would most likely orbit Alpha Centauri B with an orbital period of 20.4 days or less, with only a 5% chance of it having a longer orbit. The median of the likely orbits is 12.4 days. Its orbit would likely have an eccentricity of 0.24 or less. It likely has lakes of molten lava and would be far too close to Alpha Centauri B to harbour life
Life is a quality that distinguishes matter that has biological processes, such as Cell signaling, signaling and self-sustaining processes, from that which does not, and is defined by the capacity for Cell growth, growth, reaction to Stimu ...
.
Hypothetical planets
Additional planets may exist in the Alpha Centauri system, either orbiting Alpha Centauri A or Alpha Centauri B individually, or in large orbits around Alpha Centauri AB. Because both stars are fairly similar to the Sun (for example, in age and metallicity
In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as ...
), astronomers have been especially interested in making detailed searches for planets in the Alpha Centauri system. Several established planet-hunting teams have used various radial velocity or star transit
Transit may refer to:
Arts and entertainment Film
* ''Transit'' (1979 film), a 1979 Israeli film
* ''Transit'' (2005 film), a film produced by MTV and Staying-Alive about four people in countries in the world
* ''Transit'' (2006 film), a 2006 ...
methods in their searches around these two bright stars. All the observational studies have so far failed to find evidence for brown dwarf
Brown dwarfs (also called failed stars) are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen (hydrogen-1, 1H) into helium in their cores, unlike a main sequence, main-sequence star. Instead, they have ...
s or gas giants.
In 2009, computer simulations showed that a planet might have been able to form near the inner edge of Alpha Centauri B's habitable zone, which extends from 0.5 to 0.9 AU from the star. Certain special assumptions, such as considering that the Alpha Centauri pair may have initially formed with a wider separation and later moved closer to each other (as might be possible if they formed in a dense star cluster), would permit an accretion-friendly environment farther from the star. Bodies around Alpha Centauri A would be able to orbit at slightly farther distances due to its stronger gravity. In addition, the lack of any brown dwarfs or gas giants in close orbits around Alpha Centauri make the likelihood of terrestrial planets greater than otherwise. A theoretical study indicates that a radial velocity analysis might detect a hypothetical planet of in Alpha Centauri B's habitable zone.
Radial velocity measurements of Alpha Centauri B made with the High Accuracy Radial Velocity Planet Searcher spectrograph were sufficiently sensitive to detect a planet within the habitable zone of the star (i.e. with an orbital period P = 200 days), but no planets were detected.
Current estimates place the probability of finding an Earth-like planet around Alpha Centauri at roughly 75%. The observational thresholds for planet detection in the habitable zones by the radial velocity method are currently (2017) estimated to be about for Alpha Centauri A, for Alpha Centauri B, and for Proxima Centauri.
Early computer-generated models of planetary formation predicted the existence of terrestrial planets around both Alpha Centauri A and B,[See Lissauer and Quintana in references below] but most recent numerical investigations have shown that the gravitational pull of the companion star renders the accretion of planets difficult. Despite these difficulties, given the similarities to the Sun in spectral types, star type, age and probable stability of the orbits, it has been suggested that this stellar system could hold one of the best possibilities for harbouring extraterrestrial life on a potential planet.[
In the ]Solar System
The Solar System Capitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar ...
, it was once thought that Jupiter
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but slightly less than one-thousandt ...
and Saturn were probably crucial in perturbing comet
A comet is an icy, small Solar System body that, when passing close to the Sun, warms and begins to release gases, a process that is called outgassing. This produces a visible atmosphere or Coma (cometary), coma, and sometimes also a Comet ta ...
s into the inner Solar System, providing the inner planets with a source of water and various other ices. However, since isotope measurements of the deuterium
Deuterium (or hydrogen-2, symbol or deuterium, also known as heavy hydrogen) is one of two Stable isotope ratio, stable isotopes of hydrogen (the other being Hydrogen atom, protium, or hydrogen-1). The atomic nucleus, nucleus of a deuterium ato ...
to hydrogen
Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...
(D/H) ratio in comets Halley, Hyakutake, Hale–Bopp, 2002T7, and Tuttle yield values approximately twice that of Earth's oceanic water, more recent models and research predict that less than 10% of Earth's water was supplied from comets. In the Alpha Centauri system, Proxima Centauri may have influenced the planetary disk as the Alpha Centauri system was forming, enriching the area around Alpha Centauri with volatile materials. This would be discounted if, for example, Alpha Centauri B happened to have gas giants orbiting Alpha Centauri A (or vice versa), or if Alpha Centauri A and B themselves were able to perturb comets into each other's inner systems as Jupiter and Saturn presumably have done in the Solar System. Such icy bodies probably also reside in Oort clouds of other planetary systems. When they are influenced gravitationally by either the gas giants or disruptions by passing nearby stars, many of these icy bodies then travel star-wards. Such ideas also apply to the close approach of Alpha Centauri or other stars to the Solar System, when, in the distant future, the Oort Cloud might be disrupted enough to increase the number of active comets.
To be in the habitable zone, a planet around Alpha Centauri A would have an orbital radius of between about 1.2 and so as to have similar planetary temperatures and conditions for liquid water to exist. For the slightly less luminous and cooler Alpha Centauri B, the habitable zone is between about 0.7 and .
With the goal of finding evidence of such planets, both Proxima Centauri and Alpha Centauri-AB were among the listed "Tier-1" target stars for NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeedi ...
's Space Interferometry Mission (S.I.M.). Detecting planets as small as three Earth-masses or smaller within two AU of a "Tier-1" target would have been possible with this new instrument. The S.I.M. mission, however, was cancelled due to financial issues in 2010.
Circumstellar discs
Based on observations between 2007 and 2012, a study found a slight excess of emissions in the 24-µm (mid/far-infrared) band surrounding , which may be interpreted as evidence for a sparse circumstellar disc or dense interplanetary dust. The total mass was estimated to be between to the mass of the Moon
The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width ...
, or 10–100 times the mass of the Solar System's zodiacal cloud. If such a disc existed around both stars, disc would likely be stable to 2.8 AU, and would likely be stable to 2.5 AU This would put A's disc entirely within the frost line
The frost line—also known as frost depth or freezing depth—is most commonly the depth to which the groundwater in soil is expected to freeze. The frost depth depends on the climatic conditions of an area, the heat transfer properties of th ...
, and a small part of B's outer disc just outside.
View from this system
The sky from Alpha Centauri AB would appear much as it does from the Earth, except that Centaurus
Centaurus is a bright constellation in the southern sky. One of the largest constellations, Centaurus was included among the 48 constellations listed by the 2nd-century astronomer Ptolemy, and it remains one of the 88 modern constellations. I ...
would be missing its brightest star. The Sun would appear as a yellow star of apparent magnitude
Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...
+0.48, roughly the same as the average brightness of Betelgeuse from Earth. It would be at the antipodal point
In mathematics, antipodal points of a sphere are those diametrically opposite to each other (the specific qualities of such a definition are that a line drawn from the one to the other passes through the center of the sphere so forms a true ...
of Alpha Centauri AB's current right ascension
Right ascension (abbreviated RA; symbol ) is the angular distance of a particular point measured eastward along the celestial equator from the Sun at the March equinox to the ( hour circle of the) point in question above the earth.
When pai ...
and declination, at (2000), in eastern Cassiopeia, easily outshining all the rest of the stars in the constellation. With the placement of the Sun east of the magnitude 3.4 star Epsilon Cassiopeiae, nearly in front of the Heart Nebula, the "W" line of stars of Cassiopeia would have a "/W" shape.
A planet around either α Centauri A or B would see the other star as a very bright secondary. For example, an Earth-like planet at 1.25 AU from α Cen A (with a revolution period of 1.34 years) would get Sun-like illumination from its primary, and α Cen B would appear 5.7 to 8.6 magnitudes dimmer (−21.0 to −18.2), 190 to 2,700 times dimmer than α Cen A but still 150 to 2,100 times brighter than the full Moon. Conversely, an Earth-like planet at 0.71 AU from α Cen B (with a revolution period of 0.63 years) would get nearly Sun-like illumination from its primary, and α Cen A would appear 4.6 to 7.3 magnitudes dimmer (−22.1 to −19.4), 70 to 840 times dimmer than α Cen B but still 470 to 5,700 times brighter than the full Moon.
Other names
In modern literature, colloquial alternative names of Alpha Centauri include ''Rigil Kent'' (also ''Rigel Kent'' and variants;[Spellings include ''Rigjl Kentaurus'', Hyde T., "Ulugh Beighi Tabulae Stellarum Fixarum", ''Tabulae Long. ac Lat. Stellarum Fixarum ex Observatione Ulugh Beighi'' Oxford, 1665, p. 142, Hyde T., "In Ulugh Beighi Tabulae Stellarum Fixarum Commentarii", ''op. cit.'', p. 67, Portuguese ''Riguel Kentaurus'' da Silva Oliveira, R.]
"Crux Australis: o Cruzeiro do Sul"
, Artigos: Planetario Movel Inflavel AsterDomus. ) and ''Toliman'' (the latter of which became the proper name of Alpha Centauri B on 10 August 2018 by approval of 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, outreach ...
).
''Rigil Kent'' is short for ''Rigil Kentaurus'', which is sometimes further abbreviated to ''Rigil'' or ''Rigel'', though that is ambiguous with Beta Orionis, which is also called Rigel.
The name ''Toliman'' originates with Jacobus Golius' 1669 edition of Al-Farghani
Abū al-ʿAbbās Aḥmad ibn Muḥammad ibn Kathīr al-Farghānī ( ar, أبو العبّاس أحمد بن محمد بن كثير الفرغاني 798/800/805–870), also known as Alfraganus in the West, was an astronomer in the Abbasid court ...
's ''Compendium''. ''Tolimân'' is Golius' latinisation of the Arabic name "the ostriches", the name of an asterism of which Alpha Centauri formed the main star.
During the 19th century, the northern amateur popularist Elijah H. Burritt used the now-obscure name ''Bungula'', possibly coined from "β" and the Latin
Latin (, or , ) is a classical language belonging to the Italic branch of the Indo-European languages. Latin was originally a dialect spoken in the lower Tiber area (then known as Latium) around present-day Rome, but through the power ...
''ungula'' ("hoof").
Together, Alpha and Beta Centauri form the "Southern Pointers" or "The Pointers", as they point towards the Southern Cross, the asterism of the constellation of Crux
Crux () is a constellation of the southern sky that is centred on four bright stars in a cross-shaped asterism commonly known as the Southern Cross. It lies on the southern end of the Milky Way's visible band. The name ''Crux'' is Latin for ...
.
In Chinese astronomy
Astronomy in China has a long history stretching from the Shang dynasty, being refined over a period of more than 3,000 years. The ancient Chinese people have identified stars from 1300 BCE, as Chinese star names later categorized in the t ...
, ''Nán Mén'', meaning '' Southern Gate'', refers to an asterism consisting of Alpha Centauri and Epsilon Centauri. Consequently, the Chinese name
Chinese names or Chinese personal names are names used by individuals from Greater China and other parts of the Chinese-speaking world throughout East and Southeast Asia (ESEA). In addition, many names used in Japan, Korea and Vietnam are oft ...
for Alpha Centauri itself is ''Nán Mén Èr'', the Second Star of the Southern Gate.
To the Australian aboriginal Boorong people
The Wergaia or Werrigia people are an Aboriginal Australian group in the Mallee and Wimmera regions of north-Western Victoria, made up of a number of clans. The people were also known as the Maligundidj (in the Wotjobaluk language) which means ...
of northwestern Victoria, Alpha Centauri and Beta Centauri are ''Bermbermgle'', two brothers noted for their courage and destructiveness, who speared and killed ''Tchingal'' "The Emu" (the Coalsack Nebula). The form in Wotjobaluk is ''Bram-bram-bult''.[
]
Future exploration
Alpha Centauri is a likely first target for crewed or robotic interstellar exploration. Using current spacecraft technologies, crossing the distance between the Sun and Alpha Centauri would take several millennia, though the possibility of nuclear pulse propulsion or laser light sail technology, as considered in the Breakthrough Starshot program, could make the journey to Alpha Centauri in 20 years. An objective of such a mission would be to make a fly-by of, and possibly photograph, planets that might exist in the system. The existence of Proxima Centauri b, announced by the European Southern Observatory
The European Organisation for Astronomical Research in the Southern Hemisphere, commonly referred to as the European Southern Observatory (ESO), is an intergovernmental organization, intergovernmental research organisation made up of 16 mem ...
(ESO) in August 2016, would be a target for the Starshot program.
NASA
The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research.
NASA was established in 1958, succeedi ...
announced in 2017 that it plans to send a spacecraft to Alpha Centauri in 2069, scheduled to coincide with the 100th anniversary of the first manned lunar landing in 1969, Apollo 11
Apollo 11 (July 16–24, 1969) was the American spaceflight that first landed humans on the Moon. Commander Neil Armstrong and lunar module pilot Buzz Aldrin landed the Apollo Lunar Module ''Eagle'' on July 20, 1969, at 20:17 UTC, a ...
. Even at speed 10% of the speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit fo ...
(67 million mph), which NASA experts say may be possible, it would take a spacecraft 44 years to reach the constellation, by the year 2113, and will take another 4 years for a signal, by the year 2117 to reach Earth.
Historical distance estimates
See also
* Alpha Centauri in fiction
The planetary systems of stars other than the Sun and the Solar System are a staple element in many works of the science fiction genre.
Overview
The notion that there might be inhabited extrasolar planets can be traced at least as far back as G ...
* List of nearest stars and brown dwarfs
* Project Longshot
* Sagan Planet Walk
Notes
References
External links
SIMBAD observational data
Hypothetical planets or exploration
*
Alpha Centauri System
Alpha Centauri – Associação de Astronomia (Portuguese)
*
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G-type main-sequence stars
K-type main-sequence stars
M-type main-sequence stars
Centauri, Alpha
Maunder Minimum
Triple star systems
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Planetary systems with one confirmed planet
Centaurus (constellation)
Rigil Kentaurus
Centauri, Alpha
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Astronomical objects known since antiquity