Proxima Centauri (Latin proxima, meaning "next to" or "nearest to" is a red dwarf, about 4.24 light-years from the Sun, inside the G-cloud, in the constellation of Centaurus. The star was discovered in 1915 by Scottish astronomer Robert Innes, the Director of the Union Observatory in South Africa. Proxima Centauri is the nearest known star to the Sun, but it is too faint to be seen with the naked eye, with an apparent magnitude of 11.05. Proxima Centauri forms a third component of the Alpha Centauri trinary star system, currently with a separation of about 12,950 AU (1.94 trillion km) and an orbital period of 550,000 years. At the present, Proxima is 2.18° to the southwest of Alpha Centauri.
Because of the proximity of this star, its distance from the Sun and angular diameter can be measured directly, from which it can be determined that its diameter is about one-seventh of that of the Sun. Proxima Centauri's mass is about an eight of the Sun's, and its average density is about 40 times that of the Sun. Although it has a very low average luminosity, Proxima is a flare star that undergoes random dramatic increases in brightness because of magnetic activity. The stars magnetic field is created by convection throghout the stellar body, and the resulting flare activity generates a total X-ray emission similar to that produced by the Sun. The mixing of the fuel at Proxima Centauri's core through convection and the star's relatively low energy-production rate suggest that it will be a main-sequence star for another four trillion years, or nearly 300 times the current age of the universe.
On August 24, 2016, the European Southern Observatory announced the discovery of the planet Proxima Centauri b, which orbits the star at a distance of 0.05 AU (7.5 million kilometers) with an orbital period of approximately 11.2 Earth days. Its estimated mass is around 1.3 times that of Earth. The equilibrium temperature of Proxima b is estimated to be within the range of where water could exist as liquid on its surface, thus placing it within the habitable zone of Proxima Centauri, although it is disputed whether or not life could exist on the planet due to its status as a flare star. Previous searches for orbiting companions have ruled out any presence of brown dwarfs and supermassive planets.
Scottish astronomer Robert Innes, the director of the Union Observatory in Johannesburg, South Africa, discovered a star that had the same proper motion as Alpha Centauri in 1915. He suggested that it should be named Proxima Centauri. In 1917, at the Royal Observatory at the Cape of Good Hope, the Dutch astronomer Joan Voûte measured the star's trignometric parallax at 0.755″±0.028″, and determined that Proxima Centauri was around 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 parllax of 0.783″±0.005″.
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 known at the time. 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 2002, optical interferometry with the Very Large Telescope (VLTI) found that the angular diameter of Proxima Centauri was 1.02±0.08 mas. 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 estimated mass is 12.2% M☉, or 129 Jupiter masses (MJ). The mean density of main-sequence stars increase with decreasing mass, The mean density of main-sequence stars increase with decreasing mass, and Proxima Centauri is no exception: it has a mean density of 47.1 x 103 kg/m3 (47.1 g/cm3), compared with the Sun's mean density of 1.411 x 103 kg/m3 (1.411 g/cm3).
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 116.6 ± 0.7 days. This was subsequently ruled out in favor of a rotation period of 82.6 ± 0.1 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.
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 increase the overall luminosity of the star. These flares can grow as large as the star and reach temperatures measured as high as 27 million K, which is hot enough to radiate X-rays. Proxima Centauri's quiescent X-ray luminosity, approximately (4-16) × 1026 erg/s ((4-16) × 1019 W), is roughly equal to that of the much larger Sun. The peak X-ray luminosity of the largest flares can reach 1028 erg/s (1021 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. 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 × 109 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 from red to blue. Near the end of this period it will become significantly more luminous, reaching 2.5% of the Sun's luminosity (L☉) 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.
Proxima Centauri is a red dwarf, because it belongs to the main sequence on the Hertzsprung–Russell diagram and is of spectral class M6. M6 means that it falls in the low-mass end of M-type stars. Its absolute visual magnitude, or its visual magnitude as viewed from a distance of 10 parsecs (33 ly), is 15.5. Its total luminosity over all wavelengths is 0.17% that of the Sun, 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. It has a regular activity cycle of starspots.
The first indications of the exoplanet were found in 2013 by Mikko Tuomi of the University of Hertfordshire from archival observation data. To confirm the possible discovery, the European Southern Observatory launched the Pale Red Dot 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 by 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.
Proxima Centauri b is a planet orbiting the star at a distance of roughly 0.05 AU (7.5 million km) with an orbital period of approximately 11.2 Earth days. Its estimated mass is at least 1.3 times that of the Earth. Moreover, the equilibrium temperature of Proxima 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.
A second signal in the range of 60 to 500 days was also detected, but its nature is still unclear due to stellar activity.
|Planet||Mass||Semimajor axis||Orbital period (days)||Eccentricity||Inclination||Radius|
|b||≥1.27 Earth mass||0.0485 AU||11.186 days||<0.35||N/A||0.8-1.5 Earth radius|
|c (unconfirmed)||≥6.0 Earth mass||1.48 AU||1894 days||N/A||N/A||N/A|