Proxima Centauri

From testwiki
Jump to navigation Jump to search

Template:Starbox begin Template:Starbox image Template:Starbox observe Template:Starbox character Template:Starbox astrometry Template:Starbox detail Template:Starbox catalog Template:Starbox reference Template:Starbox end

Proxima Centauri (Latin Template:Lang: meaning 'next to' or 'nearest to')[1] is a red dwarf star approximately 4.2 light-years distant in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa. The star may be part of the Alpha Centauri system, and it is the nearest star to the Sun.[2]

Because of the proximity of this star, its angular diameter can be measured directly, yielding a diameter one-seventh that of the Sun.[2] 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 1] Although it has a very low average luminosity, Proxima Centauri is a flare star that undergoes random increases in brightness because of magnetic activity.[3] 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.[4] The mixing of the fuel at Proxima Centauri's core through convection and the star's relatively low energy production rate means that it will be a main-sequence star for another four trillion years,[5] or nearly 300 times the current age of the universe.[6]

Searches for companions orbiting Proxima Centauri have been unsuccessful, ruling out the presence of brown dwarfs and supermassive planets.[7][8] Precision radial velocity surveys have also ruled out the presence of super-Earths within the star's habitable zone.[9][nb 2] The detection of smaller objects will require the use of new instruments, such as the proposed Space Interferometry Mission.[10] Since Proxima Centauri is a red dwarf and a flare star, whether a planet orbiting this star could support life is disputed.[11][12] Because of the star's proximity, it has been proposed as a destination for interstellar travel.[13]

Observation

Robert Innes, Director of the Union Observatory in Johannesburg, South Africa in 1915, discovered that Proxima Centauri had the same proper motion as Alpha Centauri.[14] He also suggested it be named Proxima Centauri.[15] In 1917, at the Royal Observatory at the Cape of Good Hope, the Dutch astronomer Joan Voûte measured the star's trigonometric parallax and determined that Proxima Centauri was indeed the same distance from the Sun as Alpha Centauri. It was also found to be the lowest-luminosity star known at the time.[16] In 1951, American astronomer Harlow Shapley announced that Proxima Centauri was 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.[17]

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.[18] Proxima Centauri has since been the subject of study by most X-ray observatories, including XMM-Newton and Chandra.[19]

Because of Proxima Centauri's southern declination, it can only be viewed south of latitude 27° N.[nb 3] 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.[20][21] 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.[22]

Characteristics

Proxima Centauri is classified as a red dwarf star because it belongs to the main sequence on the Hertzsprung-Russell diagram and it 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.[2] This star's absolute visual magnitude, or its visual magnitude as viewed from a distance of 10 parsecs, is 15.5.[23] Its total luminosity over all wavelengths is 0.17% that of the Sun,[24] 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.[25] More than 85% of its radiated power is at infrared wavelengths.[26]

This illustration shows the comparative sizes of (from left to right) the Sun, α Centauri A, α Centauri B and Proxima Centauri

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.[14] The star's estimated mass is only 12.3% of a solar mass, or 129 Jupiter masses.[2] The mean density of a main sequence star increases with decreasing mass,[27] and Proxima Centauri is no exception: it has a mean density of 56,800 kg/m3, compared with the Sun's mean density of 1,409 kg/m3.[nb 1]

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 a much higher proportion of its fuel before the fusion of hydrogen comes to an end.[5]

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[19]—hot enough to radiate X-rays.[28] Indeed, the quiescent X-ray luminosity of this star, approximately (4–16)Template:Esp erg/s ((4–16)Template:Esp 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.)[19]

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.[29] 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.[30] However, the overall activity level of this star is considered low compared to other M-class dwarfs,[4] which is consistent with the star's estimated age, since the activity level of a red dwarf is expected to steadily wane over billions of years as its stellar rotation rate decreases.[31] The activity level also appears to vary with a period of roughly 442 days, which is shorter than the solar cycle of 11 years.[32]

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.[33]

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.[5]

Distance and motion

Based on the parallax of 772.3 ± 2.4 milliarcseconds measured by Hipparcos (and the more precise parallax determined using the Fine Guidance Sensors on the Hubble Space Telescope of 768.7 ± 0.3[34] milliarcseconds), Proxima Centauri is roughly 4.2 light years from Earth, or 270,000 times more distant than the Sun. From Earth's vantage point, Proxima is separated by 2.18°[35] from Alpha Centauri, or four times the angular diameter of the full Moon.[36] Proxima also has a relatively large proper motion—moving 3.85 arcseconds per year across the sky.[37] It has a radial velocity toward the Sun of 21.7 km/s.[38]

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.[39] Proxima will make its closest approach to the Sun, coming within 3.11 light years of the latter, in approximately 26,700 years.[40] Proxima Centauri is orbiting through the Milky Way at a distance from the galactic core that varies from 8.3 to 9.5 kpc, and with an orbital eccentricity of 0.07.[41]

From the time of the discovery of Proxima, it was 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]),[42] Proxima Centauri may be in orbit about 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.[43] 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. More accurate measurement of the radial velocity is needed to confirm this hypothesis.[42]

If Proxima 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 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.[42]

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,[44] 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.[45]

Possible companions

Template:Seealso

RV-derived Upper Mass
Limits of Companion[9]
Orbital
period
(days) 		Separation
(AU) 		Maximum
Mass[nb 2]
(× Earth)
3.6–13.8 0.022–0.054 2–3
<100 <0.21 8.5
<1000 <1 16

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.[34][7] Unfortunately, the activity level of the star adds noise to the radial velocity measurements, limiting future prospects for detection of a companion using this method.[46]

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.[47] However a subsequent search using the Wide Field Planetary Camera 2 failed to locate any companions.[8] Proxima Centauri, along with Alpha Centauri A and B, are among the "Tier 1" target stars for NASA's proposed Space Interferometry Mission (SIM), which will theoretically be able to detect planets as small as three Earth-masses within two AU of a "Tier 1" target star.[10]

Artist's concept of a red dwarf star. NASA illustration.

The TV documentary Alien Worlds hypothesized that a life-sustaining planet could exist in orbit around Proxima Centauri or other red dwarf stars. 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.[48] 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.[11]

While Proxima Centauri being a flare star means that its flares could cause problems with the atmosphere of any planet in its habitable zone, 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 magnetic field, the field would deflect the particles from the atmosphere; even the slow rotation of a tidally locked M-dwarf planet—it 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.[49]

Other scientists, especially proponents of the Rare Earth hypothesis,[50] disagree that red dwarf stars 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.[12]

Interstellar travel

The Sun as seen from the Alpha Centauri system, using Celestia

Proxima Centauri has been suggested as a possible first destination for interstellar travel.[13] A flyby en route to Alpha Centauri is a similar alternative that does not require a deceleration phase. Nuclear pulse propulsion encompasses several technologies which 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.[51]

Although the Voyager program spacecraft are anticipated to become the first spacecraft to enter interstellar space, they move relatively slowly, at about 17 km/s, requiring well over 10,000 years to travel each light-year.[52] In comparison, Proxima is presently approaching at a rate of 21.7 km/s.[38] However, it will only come as close as 3.11 light-years, and then move further away after 26,700 years.[40] Thus, 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. The advent of ion thruster engines enables interstellar spacecraft faster than the Voyagers, such as the Innovative Interstellar Explorer proposal, but ion thrusters are believed to be too slow to get a working probe to another star.[51]

From Proxima Centauri, the Sun would appear as a bright, 0.4 magnitude star in the constellation Cassiopeia.[53] If current, non-nuclear propulsion were used, a voyage of thousands of years would likely require a ship large enough to carry a population of people that could be used for colonization of a planet.[54]

See also

Notes

Template:Reflist

References

Template:Reflist

Template:Featured article Template:Sky

Template:Link FA bg:Проксима Центавър ca:Pròxima Centauri cs:Proxima Centauri cy:Proxima Centauri da:Proxima Centauri de:Proxima Centauri es:Próxima Centauri eo:Proksima Centaŭro eu:Proxima Centauri fa:پروکسیما قنطورس fr:Proxima Centauri fy:Proksima Sentauri ko:센타우루스자리 프록시마 id:Proxima Centauri it:Proxima Centauri he:פרוקסימה קנטאורי lv:Centaura Proksima lt:Proxima Centauri hu:Proxima Centauri ml:പ്രോക്സിമ സെന്റോറി nl:Proxima Centauri ja:プロキシマ・ケンタウリ no:Proxima Centauri pl:Proxima Centauri pt:Proxima Centauri ro:Proxima Centauri ru:Проксима Центавра simple:Proxima Centauri sk:Proxima Centauri sr:Проксима Кентаури te:ప్రాక్సిమా సెంటారీ th:ดาวพร็อกซิมาคนครึ่งม้า tr:Proxima Centauri vo:Proxima Centauri zh:比邻星

  1. ↑ Template:Cite web
  2. ↑ 2.0 2.1 2.2 2.3 Cite error: Invalid <ref> tag; no text was provided for refs named ESO2003
  3. ↑ Template:Cite journal
  4. ↑ 4.0 4.1 Template:Cite journal
  5. ↑ 5.0 5.1 5.2 Template:Cite conference
  6. ↑ Template:Cite web
  7. ↑ 7.0 7.1 Template:Cite journal
  8. ↑ 8.0 8.1 Template:Cite journal
  9. ↑ 9.0 9.1 Template:Cite journal
  10. ↑ 10.0 10.1 Template:Cite web
  11. ↑ 11.0 11.1 Template:Cite journal
  12. ↑ 12.0 12.1 Template:Cite journal
  13. ↑ 13.0 13.1 Template:Cite book
  14. ↑ 14.0 14.1 Template:Cite web
  15. ↑ Template:Cite journal
  16. ↑ Template:Cite journal
  17. ↑ Template:Cite journal
  18. ↑ Template:Cite journal
  19. ↑ 19.0 19.1 19.2 Template:Cite journal
  20. ↑ Template:Cite web
  21. ↑ Template:Cite web
  22. ↑ Template:Cite book
  23. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named abs_mag
  24. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named lbol
  25. ↑ p. 8, Template:Cite book
  26. ↑ p. 357, Template:Cite journal
  27. ↑ Template:Cite book
  28. ↑ Template:Cite web
  29. ↑ Template:Cite journal
  30. ↑ Template:Cite journal
  31. ↑ Template:Cite journal
  32. ↑ Template:Cite journal
  33. ↑ Template:Cite journal
  34. ↑ 34.0 34.1 Template:Cite journal
  35. ↑ Template:Cite journal
  36. ↑ Template:Cite web
  37. ↑ Template:Cite conference
  38. ↑ 38.0 38.1 Cite error: Invalid <ref> tag; no text was provided for refs named SIMBAD
  39. ↑ Template:Cite journal
  40. ↑ 40.0 40.1 Template:Cite journal
  41. ↑ Template:Cite journal
  42. ↑ 42.0 42.1 42.2 Template:Cite journal
  43. ↑ Template:Cite journal
  44. ↑ Template:Cite journal
  45. ↑ Template:Cite journal
  46. ↑ Template:Cite journal
  47. ↑ Template:Cite journal
  48. ↑ Template:Cite conference
  49. ↑ Template:Cite web
  50. ↑ Template:Cite book
  51. ↑ 51.0 51.1 Template:Cite web
  52. ↑ Template:Cite book. Voyager 1 has an asymptotic velocity of 3.5 AU/yr, Voyager 2 an asymptotic velocity of 3.4 AU/yr.
  53. ↑ The coordinates of the Sun would be diametrically opposite Proxima, at α=Template:RA, δ=Template:DEC. The absolute magnitude Mv of the Sun is 4.83, so at a parallax π of 0.77199 the apparent magnitude m is given by 4.83 - 5(log10(0.77199) + 1) = 0.40. See: Template:Cite book
  54. ↑ Template:Cite journal


Cite error: <ref> tags exist for a group named "nb", but no corresponding <references group="nb"/> tag was found

Retrieved from "https://flaggedrevs-labs.wikimedia.beta.math.wmflabs.org/w/index.php?title=Proxima_Centauri&oldid=55"