Proxima Centauri is the nearest star to Earth after the Sun, located 4.25 light-years (1.3 parsecs) away in the southern constellation of Centaurus. Discovered in 1915 by Robert Innes, it is a small, low-mass star, too faint to be seen with the naked eye, with an apparent magnitude of 11.13. Proxima Centauri is a member of the Alpha Centauri star system, being identified as component Alpha Centauri C, and is 2.18° southwest of the Alpha Centauri AB pair. It is currently from AB, which it orbits with a period of about 550,000 years. Its Latin name means the 'nearest star of Centaurus'.
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 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 known exoplanets and one candidate exoplanet: Proxima Centauri b, Proxima Centauri d and the disputed 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 1.06 times that of Earth. with its hue shifted toward red-yellow by an effective temperature of . Its total luminosity over all wavelengths is only 0.16% that of the Sun, although when observed in the wavelengths of visible light to which the eye is most sensitive, 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 . The measured surface gravity of Proxima Centauri, given as the base-10 logarithm of the acceleration in units of cgs, is 5.20.
A 1998 study of photometric variations indicated that Proxima Centauri completes a full rotation once every 83.5 days. A subsequent time series analysis of chromospheric indicators in 2002 suggested a longer rotation period of days. Later observations of the star's magnetic field subsequently revealed that the star rotates with a period of days,
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 6 May 2019, a flare event bordering Solar M and X flare class, briefly became the brightest ever detected, with a far ultraviolet emission of .—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). 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. which is consistent with the star's estimated age of 4.85 years, The activity level appears to vary with a period of roughly 442 days, which is shorter than the Sun's 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 Sun's surface.
Life phases
left|thumb|upright=1.4|[[Alpha Centauri A and B are the bright apparent star to the left, which are in a triple star system with Proxima Centauri, circled in red. The bright star system to the right is the unrelated Beta Centauri.]]
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 any orbiting bodies for a period of several billion years. When the hydrogen fuel is exhausted, Proxima Centauri will then evolve into a helium white dwarf (without passing through the red giant phase) and steadily lose any remaining heat energy.
The Alpha Centauri system may have formed through a low-mass star being dynamically captured by a more massive binary of within their embedded star cluster before the cluster dispersed. 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 have disturbed 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. As the members of the Alpha Centauri pair continue to evolve and lose mass, Proxima Centauri is predicted to become unbound from the system in around 3.5 billion years from the present. Thereafter, the star will steadily diverge from the pair.
Motion and location
thumb|upright=1.2|Proxima Centauri (unlabeled) next to Alpha Centauri on a radar map of all known stellar and [[substellar objects within 9 light-years (ly), arranged clockwise in hours of right ascension, and marked by distance (▬) and position (◆)]]
Based on a parallax of , published in 2020 in Gaia Data Release 3, Proxima Centauri is from the Sun. , 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. or four times the angular diameter of the full Moon. Proxima Centauri has a relatively large proper motion—moving 3.85 arcseconds per year across the sky. It has a radial velocity towards the Sun of 22.2 km/s. The star is located in the G-Cloud.
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.
Alpha Centauri
Proxima Centauri has been suspected to be a companion of the Alpha Centauri binary star system since its discovery in 1915. For this reason, it is sometimes referred to as Alpha Centauri C. Data from the Hipparcos satellite, combined with ground-based observations, were consistent with the hypothesis that the three stars are a gravitationally bound system. 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.