Proxima Centauri (center inset) as seen by Hubble
Epoch J2000.0 Equinox J2000.0 (ICRS)
|Pronunciation||/ /[nb 1]|
|Right ascension||14h 29m 42.9487s|
|Declination||−62° 40′ 46.141″|
|Apparent magnitude (V)||11.05|
|Spectral type||M5.5 Ve|
|Apparent magnitude (J)||5.35 ± 0.02|
|U−B color index||1.43|
|B−V color index||1.90|
|Variable type||Flare star|
|Radial velocity (Rv)||−21.7 ± 1.8 km/s|
|Proper motion (μ)||RA: −3775.40 mas/yr
Dec.: 769.33 mas/yr
|Parallax (π)||768.7 ± 0.3 mas|
|Distance||4.243 ± 0.002 ly
(1.3009 ± 0.0005 pc)
|Absolute magnitude (MV)||15.49|
|Mass||0.123 ± 0.006 M☉|
|Radius||0.141 ± 0.007 R☉|
|Luminosity (bolometric)||0.0017 L☉|
|Surface gravity (log g)||5.20 ± 0.23 cgs|
|Temperature||3,042 ± 117 K|
|Metallicity [Fe/H]||0.21 dex|
|Rotational velocity (v sin i)||2.7 ± 0.3 km/s|
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. It was discovered in 1915 by Scottish astronomer Robert Innes, the Director of the Union Observatory in South Africa, and is the nearest known star to the Sun, although it is too faint to be seen with the naked eye, with an apparent magnitude of 11.05. Its distance to the second- and third-nearest stars, which form the bright binary Alpha Centauri, is 0.237 ± 0.011 ly (15,000 ± 700 AU). Proxima Centauri is very likely part of a triple star system with Alpha Centauri A and B, but its orbital period may be greater than 500,000 years.
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 eighth of the Sun's, and its average density is about 40 times that of the Sun.[nb 2] 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 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 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.
Searches for companions orbiting Proxima Centauri have been unsuccessful, ruling out the presence of brown dwarfs and supermassive planets. Precision radial velocity surveys have also ruled out the presence of super-Earths within the star's habitable zone.[nb 3] The detection of smaller objects will require the use of new instruments, such as the James Webb Space Telescope, which is scheduled for deployment in 2018. Because Proxima Centauri is a red dwarf and a flare star, whether a planet orbiting this star could support life is disputed. Nevertheless, because of the star's proximity to Earth, it has been proposed as a destination for interstellar travel.
In 1915, 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. He suggested 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 0.755 ± 0.028″ 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 this star is closer, with a parallax 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 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.
Because of Proxima Centauri's southern declination, it can only be viewed south of latitude 27° N.[nb 4] Red dwarfs such as Proxima Centauri are far 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 an apparent visual magnitude of 11, so a telescope with an aperture of at least 8 cm (3.1 in.) is needed to observe this star even under ideal viewing conditions—under clear, dark skies with Proxima Centauri well above the horizon.
Proxima Centauri is classified as a red dwarf because it belongs to the main sequence on the Hertzsprung–Russell diagram and is of spectral class M5.5. It is further classified as a "late M-dwarf star", meaning that at M5.5, it falls to the low-mass extreme of M-type stars. This star's absolute visual magnitude, or its visual magnitude as viewed from a distance of 10 parsecs, 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.
In 2002, optical interferometry with the Very Large Telescope (VLTI) found that the angular diameter of Proxima Centauri was 1.02 ± 0.08 milliarcsec. 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 only 12.3% of a solar mass, or 129 Jupiter masses. The mean density of a main-sequence star increases with decreasing mass, and Proxima Centauri is no exception: it has a mean density of 56.8 × 103 kg/m3 (56.8 g/cm3), compared with the Sun's mean density of 1.411 × 103 kg/m3 (1.411 g/cm3).[nb 2]
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—hot enough to radiate X-rays. Indeed, the quiescent X-ray luminosity of this star, 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.)
The chromosphere of this star 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. However, the overall activity level of this star is considered low compared to other M-class 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, resulting in no more than 20% of the Sun's mass loss rate from the solar wind. Because the star is much smaller than the Sun, however, 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 and warming up any orbiting bodies for a period of several billion years. Once 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 the parallax of 768.7 ± 0.3 milliarcseconds, measured using the Hipparcos astrometry satellite, and more precisely with the Fine Guidance Sensors on the Hubble Space Telescope, Proxima Centauri is about 4.24 light years from the Sun, or 270,000 times more distant than the Earth is from the Sun. From Earth's vantage point, Proxima is separated by 2.18° from Alpha Centauri, or four times the angular diameter of the full Moon. Proxima 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 21.7 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 33,000 years, after which the closest star to the Sun will be Ross 248. In 2001, J. García-Sánchez et al. predicted that Proxima will make its closest approach to the Sun, coming within 3.11 light years of the latter, in approximately 26,700 years. A 2010 study by V. V. Bobylev predicted a closest approach distance of 2.90 ly in about 27,400 years. Proxima Centauri is orbiting through the Milky Way at a distance from the Galactic Center that varies from 8.3 to 9.5 kpc, with an orbital eccentricity of 0.07.
Ever since the discovery of Proxima it has been suspected to be a true companion of the Alpha Centauri binary star system. At a distance to Alpha Centauri of just 0.21 ly (15,000 ± 700 astronomical units [AU]), Proxima Centauri may be in orbit around Alpha Centauri, with an orbital period of the order of 500,000 years or more. For this reason, Proxima is sometimes referred to as Alpha Centauri C. Modern estimates, taking into account the small separation between and relative velocity of the stars, suggest that the chance of the observed alignment being a coincidence is roughly one in a million. Data from the Hipparcos satellite, combined with ground-based observations, is consistent with the hypothesis that the three stars are truly a bound system. If so, Proxima would currently be near apastron, the farthest point in its orbit from the Alpha Centauri system. Such a triple Proxima Cen-alpha Cen A/B system can form naturally through a low-mass star being dynamically captured by a more massive binary of 1.5–2 solar masses within their embedded star cluster before the cluster disperses. More accurate measurement of the radial velocity is needed to confirm this hypothesis.
If Proxima was bound to the Alpha Centauri system during its formation, the stars would be likely to share the same elemental composition. The gravitational influence of Proxima may 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. Any terrestrial planets in the system may have been enriched by this material.
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. If it is determined that Proxima Centauri is not gravitationally bound to Alpha Centauri, then such a moving group would help explain their relatively close proximity.
Proxima Centauri distance estimates
|Source||Parallax, mas||Distance, pc||Distance, ly||Ref.|
|Voûte (1917)||755 ± 28||1.32 ± 0.05||4.32+0.17
|Innes (1917)||~ 784||~ 1.3||~ 4.2|||
|Alden (1928)||783 ± 5||1.277 ± 0.008||4.165+0.027
|Woolley et al. (1970)||761 ± 5||1.314 ± 0.009||4.286 ± 0.028|||
|Gliese & Jahreiß (1991)||771.8 ± 4.1||1.296 ± 0.007||4.226+0.023
|van Altena et al. (1995)||769.8 ± 6.1||1.299 ± 0.01||4.24 ± 0.03|||
|Perryman et al. (1997)
|772.33 ± 2.42||1.295 ± 0.004||4.223 ± 0.013|||
|Perryman et al. (1997)
|Benedict et al. (1999)||768.7 ± 0.3||1.3009 ± 0.0005||4.243 ± 0.0017|||
|Henry et al. (2004)||1.17 ± 0.19||3.82 ± 0.62|||
|Jao et al. (2005)||774.25 ± 2.08||1.292 ± 0.003||4.213 ± 0.011|||
|van Leeuwen (2007)||771.64 ± 2.60||1.296 ± 0.004||4.227 ± 0.014|||
|RECONS TOP100 (2012)||768.85 ± 0.29[nb 5]||1.3006 ± 0.0005||4.2421 ± 0.0016|||
Non-trigonometric distance estimates are marked in italic. The best estimate is marked in bold.
If a massive planet is orbiting Proxima Centauri, some displacement of the star would occur over the course of each orbit. If the orbital plane of the planet is not perpendicular to the line of sight from the Earth, then this displacement would cause periodic changes in the radial velocity of Proxima Centauri. The fact that multiple measurements of the star's radial velocity have detected no such shifts has lowered the maximum mass that a possible companion to Proxima Centauri could possess. The activity level of the star adds noise to the radial velocity measurements, limiting future prospects for 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. However, a subsequent search using the Wide Field Planetary Camera 2 failed to locate any companions. Proxima Centauri, along with Alpha Centauri A and B, was among the "Tier 1" target stars for NASA's now-canceled Space Interferometry Mission (SIM), which would theoretically have been able to detect planets as small as three Earth-masses within two AU of a "Tier 1" target star.
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 0.023–0.054 AU from the star, and would have an orbital period of 3.6–14 days. A planet orbiting within this zone will experience tidal locking to the star, so that Proxima Centauri moves little in the planet's sky, and most of the surface experiences either day or night perpetually. However, 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 (see continued theories). Gibor Basri of the University of California, Berkeley, even 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. However, 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 dwarf 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. The tide-locked rotation may result in a relatively weak planetary magnetic moment, leading to strong atmospheric erosion by coronal mass ejections from Proxima Centauri.
Proxima Centauri has been suggested as a possible first destination for interstellar travel. The star is in motion toward Earth at a rate of 21.7 km/s. Ηowever, after 26,700 years, when it will come as close as 3.11 light-years, it will begin to move farther away. If non-nuclear propulsion were used, a voyage of a spacecraft to a planet orbiting Proxima Centauri would probably require thousands of years. For example, Voyager 1, which is now travelling 17.043 km/s (38,120 mph) relative to the Sun, would reach Proxima in 73,775 years, were the spacecraft traveling 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, beginning within the next century, inspiring several studies such as Project Orion, Project Daedalus, and Project Longshot.
- Proxima is pronounced //. Centauri may be pronounced /sɛnˈtɔriː/ or /sɛnˈtɔraɪ/.
- The density (ρ) is given by the mass divided by the volume. Relative to the Sun, therefore, the density is:
= = 0.123 · 0.145−3 · 1.41 × 103 kg/m3 = 40.3 · 1.41 × 103 kg/m3 = 5.68 × 104 kg/m3
where is the average solar density. See:
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- This is actually an upper limit on the quantity m sin i, where i is the angle between the orbit normal and the line of sight. If the planetary orbits are close to face-on as observed from Earth, more massive planets could have evaded detection by the radial velocity method.
- 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°.
- Weighted parallax based on parallaxes from van Altena et al. (1995), Benedict et al. (1999), Jao et al. (2005) and van Leeuwen (2007).
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- James, Andrew (2008-03-11). "A Voyage to Alpha Centauri". The Imperial Star - Alpha Centauri. Southern Astronomical Delights. Retrieved 2008-08-05.
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