thumb|right|The location of Proxima Centauri (circled in red) Proxima Centauri is a small, low-mass star located  away from the Sun in the southern constellation of Centaurus.
Its Latin name means the 'nearest [star] of Centaurus'.
It was discovered in 1915 by Robert Innes and is the nearest-known star to the Sun.
With a quiescent apparent magnitude 11.13, it is too faint to be seen with the unaided eye.
Proxima Centauri is a member of the Alpha Centauri star system, being identified as component Alpha Centauri C, and is 2.18° to the southwest of the Alpha Centauri AB pair.
It is currently  from AB, which it orbits with a period of about 550,000 years.
Proxima Centauri is a red dwarf star with a mass about 12.5% of the Sun's mass (), and average density about 33 times that of the Sun.
Because of Proxima Centauri's proximity to Earth, its angular diameter can be measured directly.
Its actual diameter is about one-seventh (14%) the diameter of the Sun.
Although it has a very low average luminosity, Proxima Centauri is a flare star that randomly undergoes dramatic increases in brightness because of magnetic activity.
The star's magnetic field is created by convection throughout the stellar body, and the resulting flare activity generates a total X-ray emission similar to that produced by the Sun.
The thorough internal mixing of its fuel by convection through its core, and Proxima's relatively low energy-production rate, mean that it will be a main-sequence star for another four trillion years.
Proxima Centauri has two confirmed exoplanets: Proxima Centauri b and Proxima Centauri c. Proxima Centauri b orbits the star at a distance of roughly  with an orbital period of approximately 11.2 Earth days.
Its estimated mass is at least 1.17 times that of Earth.
Proxima b is orbiting within Proxima Centauri's habitable zone—the range where temperatures are right for liquid water to exist on its surface—but because Proxima Centauri is a red dwarf and a flare star, its habitability is disputed.
A super-Earth, Proxima Centauri c, orbits roughly  away every .
A faint additional signal was detected in a 2019 exoplanet search using radial velocity data, with a period of 5.15 days.
Possible explanations for the signal include undiscovered exoplanets or statistical noise.
Observation
In 1915, the Scottish astronomer Robert Innes, director of the Union Observatory in Johannesburg, South Africa, discovered a star that had the same proper motion as Alpha Centauri.
This is the original Proxima Centauri discovery paper.
He suggested that it be named Proxima Centauri (actually Proxima Centaurus).
In 1917, at the Royal Observatory at the Cape of Good Hope, the Dutch astronomer Joan Voûte measured the star's trigonometric parallax at  and determined that Proxima Centauri was approximately the same distance from the Sun as Alpha Centauri.
It was also found to be the lowest-luminosity star known at the time.
An equally accurate parallax determination of Proxima Centauri was made by American astronomer Harold L. Alden in 1928, who confirmed Innes's view that it is closer, with a parallax of .
In 1951, American astronomer Harlow Shapley announced that Proxima Centauri is a flare star.
Examination of past photographic records showed that the star displayed a measurable increase in magnitude on about 8% of the images, making it the most active flare star then known.
The proximity of the star allows for detailed observation of its flare activity.
In 1980, the Einstein Observatory produced a detailed X-ray energy curve of a stellar flare on Proxima Centauri.
Further observations of flare activity were made with the EXOSAT and ROSAT satellites, and the X-ray emissions of smaller, solar-like flares were observed by the Japanese ASCA satellite in 1995.
Proxima Centauri has since been the subject of study by most X-ray observatories, including XMM-Newton and Chandra.
In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) to catalogue and standardize proper names for stars.
The WGSN approved the name Proxima Centauri for this star on August 21, 2016, and it is now so included in the List of IAU approved Star Names.
Because of Proxima Centauri's southern declination, it can only be viewed south of latitude 27° N.For a star south of the zenith, the angle to the zenith is equal to the Latitude minus the Declination.
The star is hidden from sight when the zenith angle is 90° or more, i.e., below the horizon.
Thus, for Proxima Centauri: :Highest latitude = 90° + (−62.68°) = 27.32°.
See:  Red dwarfs such as Proxima Centauri are too faint to be seen with the naked eye.
Even from Alpha Centauri A or B, Proxima would only be seen as a fifth magnitude star.
It has apparent visual magnitude 11, so a telescope with an aperture of at least  is needed to observe it, even under ideal viewing conditions—under clear, dark skies with Proxima Centauri well above the horizon.
In 2016, a superflare was observed from Proxima Centauri, the strongest flare ever seen.
The optical brightness increased by a factor of 68× to approximately magnitude 6.8.
It is estimated that similar flares occur around five times every year but are of such short duration, just a few minutes, that they have never been observed before.
On 2020 April 22 and 23, the New Horizons spacecraft took images of two of the nearest stars, Proxima Centauri and Wolf 359.
When compared with Earth-based images, a very large parallax effect was easily visible.
However, this was mostly useful for illustrative purposes and did not improve on previous distance measurements.
Physical properties
Proxima Centauri is a red dwarf, because it belongs to the main sequence on the Hertzsprung–Russell diagram and is of spectral class M5.5.
M5.5 means that it falls in the low-mass end of M-type dwarf stars.
Its absolute visual magnitude, or its visual magnitude as viewed from a distance of , is 15.5.
Its total luminosity over all wavelengths is 0.17% that of the Sun,See Table 1,  and p. 57,  although when observed in the wavelengths of visible light the eye is most sensitive to, it is only 0.0056% as luminous as the Sun.
More than 85% of its radiated power is at infrared wavelengths.
In 2002, optical interferometry with the Very Large Telescope (VLTI) found that the angular diameter of Proxima Centauri is .
Because its distance is known, the actual diameter of Proxima Centauri can be calculated to be about 1/7 that of the Sun, or 1.5 times that of Jupiter.
The star's mass, estimated from stellar theory, is , or 129 Jupiter masses ().
The mass has been calculated directly, although with less precision, from observations of microlensing events to be .
Lower mass main-sequence stars have higher mean density than higher mass ones, and Proxima Centauri is no exception: it has a mean density of , compared with the Sun's mean density of .
<ref group="nb" name="density">The density (ρ) is given by the mass divided by the volume.
Relative to the Sun, therefore, the density is:
:
where \begin{smallmatrix}\rho_{\odot}\end{smallmatrix} is the average solar density.
See: * *</ref> The measured surface gravity of Proxima Centauri, given as the base-10 logarithm of the acceleration in units of cgs, is 5.20.
This is 162 times the surface gravity on Earth.The standard surface gravity on the Earth is , for a 'log g' value of 2.992.
The difference in logarithms is 5.20 − 2.99 = 2.21, yielding a multiplier of 10<sup>2.21</sup> = 162.
For the Earth's gravity, see: *
A 1998 study of photometric variations indicates that Proxima Centauri rotates once every 83.5 days.
A subsequent time series analysis of chromospheric indicators in 2002 suggests a longer rotation period of  days.
This was subsequently ruled out in favor of a rotation period of  days.
Because of its low mass, the interior of the star is completely convective, causing energy to be transferred to the exterior by the physical movement of plasma rather than through radiative processes.
This convection means that the helium ash left over from the thermonuclear fusion of hydrogen does not accumulate at the core but is instead circulated throughout the star.
Unlike the Sun, which will only burn through about 10% of its total hydrogen supply before leaving the main sequence, Proxima Centauri will consume nearly all of its fuel before the fusion of hydrogen comes to an end after about 4 trillion years.
Convection is associated with the generation and persistence of a magnetic field.
The magnetic energy from this field is released at the surface through stellar flares that briefly (as short as per ten seconds) increase the overall luminosity of the star.
On May 1, 2019, an extreme flare event briefly became the brightest ever detected, with a far ultraviolet emission of .
These flares can grow as large as the star and reach temperatures measured as high as 27 million K—hot enough to radiate X-rays.
Proxima Centauri's quiescent X-ray luminosity, approximately (4–16) erg/s ((4–16) W), is roughly equal to that of the much larger Sun. The peak X-ray luminosity of the largest flares can reach  erg/s ( W).
Proxima Centauri's chromosphere is active, and its spectrum displays a strong emission line of singly ionized magnesium at a wavelength of 280 nm.
About 88% of the surface of Proxima Centauri may be active, a percentage that is much higher than that of the Sun even at the peak of the solar cycle.
Even during quiescent periods with few or no flares, this activity increases the corona temperature of Proxima Centauri to 3.5 million K, compared to the 2 million K of the Sun's corona, and its total X-ray emission is comparable to the sun's.
Proxima Centauri's overall activity level is considered low compared to other red dwarfs, which is consistent with the star's estimated age of 4.85 years, since the activity level of a red dwarf is expected to steadily wane over billions of years as its stellar rotation rate decreases.
The activity level also appears to vary with a period of roughly 442 days, which is shorter than the solar cycle of 11 years.
Proxima Centauri has a relatively weak stellar wind, no more than 20% of the mass loss rate of the solar wind.
Because the star is much smaller than the Sun, the mass loss per unit surface area from Proxima Centauri may be eight times that from the solar surface.
A red dwarf with the mass of Proxima Centauri will remain on the main sequence for about four trillion years.
As the proportion of helium increases because of hydrogen fusion, the star will become smaller and hotter, gradually transforming into a so-called  "blue dwarf".
Near the end of this period it will become significantly more luminous, reaching 2.5% of the Sun's luminosity () and warming up any orbiting bodies for a period of several billion years.
When the hydrogen fuel is exhausted, Proxima Centauri will then evolve into a white dwarf (without passing through the red giant phase) and steadily lose any remaining heat energy.
Distance and motion
Based on a parallax of , published in 2020 in Gaia Data Release 3, Proxima Centauri is  from the Sun.
Previously published parallaxes include:  in 2018 by Gaia DR2, , in 2014 by the Research Consortium On Nearby Stars; , in the original Hipparcos Catalogue, in 1997;  in the Hipparcos New Reduction, in 2007; and  using the Hubble Space Telescope fine guidance sensors, in 1999.
From Earth's vantage point, Proxima Centauri is separated from Alpha Centauri by 2.18 degrees, or four times the angular diameter of the full Moon.
Proxima Centauri also has a relatively large proper motion—moving 3.85 arcseconds per year across the sky.
It has a radial velocity toward the Sun of 22.2 km/s.
Among the known stars, Proxima Centauri has been the closest star to the Sun for about 32,000 years and will be so for about another 25,000 years, after which Alpha Centauri A and Alpha Centauri B will alternate approximately every 79.91 years as the closest star to the Sun.
In 2001, J. García-Sánchez et al. predicted that Proxima Centauri will make its closest approach to the Sun in approximately 26,700 years, coming within .
A 2010 study by V. V. Bobylev predicted a closest approach distance of  in about 27,400 years, followed by a 2014 study by C. A. L. Bailer-Jones predicting a perihelion approach of  in roughly 26,710 years.
Proxima Centauri is orbiting through the Milky Way at a distance from the Galactic Centre that varies from , with an orbital eccentricity of 0.07.
Ever since the discovery of Proxima Centauri, it has been suspected to be a true companion of the Alpha Centauri binary star system.
Data from the Hipparcos satellite, combined with ground-based observations, were consistent with the hypothesis that the three stars are a bound system.
For this reason, Proxima Centauri is sometimes referred to as Alpha Centauri C. Kervella et al. (2017) used high-precision radial velocity measurements to determine with a high degree of confidence that Proxima and Alpha Centauri are gravitationally bound.
Proxima Centauri's orbital period around the Alpha Centauri AB barycenter is  years with an eccentricity of ; it approaches Alpha Centauri to  at periastron and retreats to  at apastron.
At present, Proxima Centauri is  from the Alpha Centauri AB barycenter, nearly to the farthest point in its orbit.
Such a triple system can form naturally through a low-mass star being dynamically captured by a more massive binary of  within their embedded star cluster before the cluster disperses.
However, more accurate measurements of the radial velocity are needed to confirm this hypothesis.
If Proxima Centauri was bound to the Alpha Centauri system during its formation, the stars are likely to share the same elemental composition.
The gravitational influence of Proxima might also have stirred up the Alpha Centauri protoplanetary disks.
This would have increased the delivery of volatiles such as water to the dry inner regions, so possibly enriching any terrestrial planets in the system with this material.
Alternatively, Proxima Centauri may have been captured at a later date during an encounter, resulting in a highly eccentric orbit that was then stabilized by the galactic tide and additional stellar encounters.
Such a scenario may mean that Proxima Centauri's planetary companions have had a much lower chance for orbital disruption by Alpha Centauri.
Six single stars, two binary star systems, and a triple star share a common motion through space with Proxima Centauri and the Alpha Centauri system.
The space velocities of these stars are all within 10 km/s of Alpha Centauri's peculiar motion.
Thus, they may form a moving group of stars, which would indicate a common point of origin, such as in a star cluster.
Planetary system
RV-derived upper mass limits of potential companions<ref name="aaa488">{{cite journal |title=Toward detection of terrestrial planets in the habitable zone of our closest neighbor: Proxima Centauri |last1=Endl |first1=M. |last2=Kürster |first2=M. |name-list-style=amp |date=2008 |journal=[[Astronomy and Astrophysics]] |volume=488 |issue=3 |pages=1149–1153 |doi=10.1051/0004-6361:200810058 |bibcode=2008A&A...488.1149E |arxiv=0807.1452|s2cid=55095650 }}</ref>
So far, as of 2021, two planets have been confirmed to orbit around Proxima Centauri, with one being close to Earth's size and within the habitable zone ("b") and another which may be a gas dwarf that orbits much farther out ("c").
There are signs a third, even smaller planet could be orbiting closer than both of the planets.
However, this has not been confirmed.
Ever since the first exoplanets were discovered, there has been a hunt for exoplanets in the Alpha Centauri system.
Multiple measurements of the star's radial velocity constrained the maximum mass that a detectable companion to Proxima Centauri could possess.
The activity level of the star adds noise to the radial velocity measurements, complicating detection of a companion using this method.
In 1998, an examination of Proxima Centauri using the Faint Object Spectrograph on board the Hubble Space Telescope appeared to show evidence of a companion orbiting at a distance of about 0.5 AU.
A subsequent search using the Wide Field Planetary Camera 2 failed to locate any companions.
Astrometric measurements at the Cerro Tololo Inter-American Observatory appear to rule out a Jupiter-sized planet with an orbital period of 2−12 years.
Planet b
Proxima Centauri b, or Alpha Centauri Cb, orbits the star at a distance of roughly  with an orbital period of approximately 11.2 Earth days.
Its estimated mass is at least 1.17 times that of the Earth.
Moreover, the equilibrium temperature of Proxima Centauri b is estimated to be within the range where water could exist as liquid on its surface; thus, placing it within the habitable zone of Proxima Centauri.
The first indications of the exoplanet Proxima Centauri b were found in 2013 by Mikko Tuomi of the University of Hertfordshire from archival observation data.Aron, Jacob.
August 24, 2016.
Proxima b: Closest Earth-like planet discovered right next door.
New Scientist.
Retrieved August 24, 2016.
To confirm the possible discovery, a team of astronomers launched the Pale Red DotPale Red Dot is a reference to Pale Blue Dot, a distant photo of Earth taken by Voyager 1.
project in January 2016.
On August 24, 2016, the team of 31 scientists from all around the world, led by Guillem Anglada-Escudé of Queen Mary University of London, confirmed the existence of Proxima Centauri b through a peer-reviewed article published in Nature.
The measurements were performed using two spectrographs: HARPS on the ESO 3.6 m Telescope at La Silla Observatory and UVES on the 8 m Very Large Telescope at Paranal Observatory.
Several attempts to detect a transit of this planet across the face of Proxima Centauri have been made.
A transit-like signal appearing on September 8, 2016, was tentatively identified, using the Bright Star Survey Telescope at the Zhongshan Station in Antarctica.
Planet c
thumb|right|Schematic of the orbit of Proxima c around its host star.
Proxima Centauri c is a super-Earth or gas dwarf about 7 Earth masses orbiting at roughly  every .
If Proxima Centauri b were the star's Earth, Proxima Centauri c would be equivalent to Neptune.
Due to its large distance from Proxima Centauri, it is unlikely to be habitable, with a low equilibrium temperature of around 39 K.
The planet was first reported by Italian astrophysicist Mario Damasso and his colleagues in April 2019.
Damasso's team had noticed minor movements of Proxima Centauri in the radial velocity data from the ESO's HARPS instrument, indicating a possible additional planet orbiting Proxima Centauri.
In 2020, the planet's existence was confirmed by Hubble astrometry data from c. 1995.
A possible direct imaging counterpart was detected in the infrared with the SPHERE, but the authors admit that they "did not obtain a clear detection."
If their candidate source is in fact Proxima Centauri c, it is too bright for a planet of its mass and age, implying that the planet may have a ring system with a radius of around 5 .
If this direct imaging detection is confirmed, Proxima Centauri c will be the closest exoplanet ever directly imaged.
Other discoveries
In 2016, in a paper that helped to confirm Proxima Centauri b's existence, a second signal in the range of 60 to 500 days was also detected.
However, its nature is still unclear due to stellar activity and inadequate sampling.
In 2017, a team of astronomers using the Atacama Large Millimeter/submillimeter Array reported detecting a belt of cold dust orbiting Proxima Centauri at a range of 1−4 AU from the star.
This dust has a temperature of around 40 K and has a total estimated mass of 1% of the planet Earth.
They also tentatively detected two additional features: a cold belt with a temperature of 10 K orbiting around 30 AU and a compact emission source about 1.2 arcseconds from the star.
There was also a hint at an additional warm dust belt at a distance of 0.4 AU from the star.
However, upon further analysis, these emissions were determined to be most likely the result of a large flare emitted by the star in March 2017.
The presence of dust is not needed to model the observations.
In 2019, a team of astronomers revisited the data from ESPRESSO about Proxima Centauri b to refine its mass.
While doing so, the team found another radial velocity spike with a periodicity of 5.15 days.
They estimated that if it were a planetary companion, it would be no less than 0.29 Earth masses.
The discovery was released in 2020.
Habitability
Prior to the discovery of Proxima Centauri b, the TV documentary Alien Worlds hypothesized that a life-sustaining planet could exist in orbit around Proxima Centauri or other red dwarfs.
Such a planet would lie within the habitable zone of Proxima Centauri, about  from the star, and would have an orbital period of 3.6–14 days.
A planet orbiting within this zone may experience tidal locking to the star.
If the orbital eccentricity of this hypothetical planet is low, Proxima Centauri would move little in the planet's sky, and most of the surface would experience either day or night perpetually.
The presence of an atmosphere could serve to redistribute the energy from the star-lit side to the far side of the planet.
Proxima Centauri's flare outbursts could erode the atmosphere of any planet in its habitable zone, but the documentary's scientists thought that this obstacle could be overcome.
Gibor Basri of the University of California, Berkeley, mentioned that "no one [has] found any showstoppers to habitability."
For example, one concern was that the torrents of charged particles from the star's flares could strip the atmosphere off any nearby planet.
If the planet had a strong magnetic field, the field would deflect the particles from the atmosphere; even the slow rotation of a tidally locked planet that spins once for every time it orbits its star would be enough to generate a magnetic field, as long as part of the planet's interior remained molten.
Other scientists, especially proponents of the rare-Earth hypothesis, disagree that red dwarfs can sustain life.
Any exoplanet in this star's habitable zone would likely be tidally locked, resulting in a relatively weak planetary magnetic moment, leading to strong atmospheric erosion by coronal mass ejections from Proxima Centauri.
Future exploration
Because of the star's proximity to Earth, Proxima Centauri has been proposed as a flyby destination for interstellar travel.
Proxima Centauri currently moves toward Earth at a rate of 22.2 km/s.
(Barycenter of system moving closer, while rotation around AB is away from Sun, i.e. prograde).
After 26,700 years, when it will come within 3.11 light-years, it will begin to move farther away.
If non-nuclear, conventional propulsion technologies are used, the flight of a spacecraft to Proxima Centauri and its planets would probably require thousands of years.
For example, Voyager 1, which is now travelling  relative to the Sun, would reach Proxima Centauri in 73,775 years, were the spacecraft travelling in the direction of that star.
A slow-moving probe would have only several tens of thousands of years to catch Proxima Centauri near its closest approach, and could end up watching it recede into the distance.
Nuclear pulse propulsion might enable such interstellar travel with a trip timescale of a century, inspiring several studies such as Project Orion, Project Daedalus, and Project Longshot.
Project Breakthrough Starshot aims to reach the Alpha Centauri system within the first half of the 21st century, with microprobes travelling at 20% of the speed of light propelled by around 100 gigawatts of Earth-based lasers.
The probes would perform a fly-by of Proxima Centauri to take photos and collect data of its planets' atmospheric compositions.
It would take 4.25 years for the information collected to be sent back to Earth.
From Proxima Centauri, the Sun would appear as a bright 0.4-magnitude star in the constellation Cassiopeia, similar to that of Achernar or Procyon from Earth.The coordinates of the Sun would be diametrically opposite Proxima Centauri, at α=, δ=.
The absolute magnitude M<sub>v</sub> of the Sun is 4.83, so at a parallax π of 0.77199 the apparent magnitude m is given by 4.83 − 5(log<sub>10</sub>(0.77199) + 1) = 0.40.
See:
In December 2020, a candidate SETI radio signal BLC-1 was announced as potentially coming from the star.
The signal was later determined to be human-made radio interference.
Notes
References
External links
Category:M-type main-sequence stars Category:Flare stars Category:Emission-line stars Category:Centaurus (constellation) 191510??
*C 0551 070890 Centauri, V645 Category:Planetary systems with two confirmed planets Category:Stars with proper names Category:TIC objects
