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Coordinates: Sky map 14h 29m 42.9487s, −62° 40′ 46.141″
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==Pronunciation==
Proxima Centauri is sometimes pronounced to rhyme with [http://en.wiktionary.org/wiki/dowry dowry] but other people pronounce it to rhyme with [http://en.wiktionary.org/wiki/story story].


== See also ==
== See also ==

Revision as of 16:26, 30 April 2009

Proxima Centauri

Position of Proxima Centauri
Observation data
Epoch J2000.0      Equinox J2000.0 (ICRS)
Constellation Centaurus
Right ascension 14h 29m 42.9487s[1]
Declination −62° 40′ 46.141″[1]
Apparent magnitude (V) 11.05[1]
Characteristics
Spectral type M5.5 Ve[1]
U−B color index 1.43[1]
B−V color index 1.90[1]
Variable type Flare star
Astrometry
Radial velocity (Rv)−21.7 ± 1.8[2] km/s
Proper motion (μ) RA: −3775.40[1] mas/yr
Dec.: 769.33[1] mas/yr
Parallax (π)768.7 ± 0.3 mas[3]
Distance4.243 ± 0.002 ly
(1.3009 ± 0.0005 pc)
Absolute magnitude (MV)15.49[4]
Details
Mass0.123 ± 0.006[5] M☉
Radius0.145 ± 0.011[5] R☉
Luminosity (bolometric)0.0017[6] L☉
Surface gravity (log g)5.20 ± 0.23[5] cgs
Temperature3,042 ± 117[5] K
Rotation83.5 days[7]
Age4.85[8] years
Other designations
Alpha Centauri C, CCDM J14396-6050C, GCTP 3278.00, GJ 551, HIP 70890, LFT 1110, LHS 49, LPM 526, LTT 5721, NLTT 37460, V645 Centauri[1]
Database references
SIMBADdata

Proxima Centauri (Latin [proxima] Error: {{Lang}}: text has italic markup (help): meaning 'next to' or 'nearest to')[9] 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.[8]

Because of the proximity of this star, its angular diameter can be measured directly, yielding a diameter one-seventh that of the Sun.[8] Proxima Centauri's mass is about an eighth of the Sun's, and its average density is about 40 times that of the Sun.[note 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.[10] 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.[11] 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,[12] or nearly 300 times the current age of the universe.[13]

Searches for companions orbiting Proxima Centauri have been unsuccessful, ruling out the presence of brown dwarfs and supermassive planets.[14][15] Precision radial velocity surveys have also ruled out the presence of super-Earths within the star's habitable zone.[16][note 2] The detection of smaller objects will require the use of new instruments, such as the proposed Space Interferometry Mission.[17] Since Proxima Centauri is a red dwarf and a flare star, whether a planet orbiting this star could support life is disputed.[18][19] Because of the star's proximity, it has been proposed as a destination for interstellar travel.[20]

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.[21] He also suggested it be named Proxima Centauri.[22] 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.[23] 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.[24]

File:Proxima xray.jpg
X-ray image of Proxima Centauri. NASA/CXC/SAO credit.

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.[25] Proxima Centauri has since been the subject of study by most X-ray observatories, including XMM-Newton and Chandra.[26]

Because of Proxima Centauri's southern declination, it can only be viewed south of latitude 27° N.[note 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.[27][28] 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.[29]

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.[8] This star's absolute visual magnitude, or its visual magnitude as viewed from a distance of 10 parsecs, is 15.5.[4] Its total luminosity over all wavelengths is 0.17% that of the Sun,[6] 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.[30] More than 85% of its radiated power is at infrared wavelengths.[31]

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.[21] The star's estimated mass is only 12.3% of a solar mass, or 129 Jupiter masses.[8] The mean density of a main sequence star increases with decreasing mass,[32] 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.[note 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.[12]

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[26]—hot enough to radiate X-rays.[33] Indeed, the quiescent X-ray luminosity of this star, 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 1028 erg/s (1021 W.)[26]

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.[34] 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.[35] However, the overall activity level of this star is considered low compared to other M-class dwarfs,[11] 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.[36] The activity level also appears to vary with a period of roughly 442 days, which is shorter than the solar cycle of 11 years.[37]

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.[38]

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.[12]

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[3] 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°[39] from Alpha Centauri, or four times the angular diameter of the full Moon.[40] Proxima also has a relatively large proper motion—moving 3.85 arcseconds per year across the sky.[41] It has a radial velocity toward the Sun of 21.7 km/s.[1]

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.[42] Proxima will make its closest approach to the Sun, coming within 3.11 light years of the latter, in approximately 26,700 years.[2] 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.[43]

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]),[44] 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.[45] 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.[44]

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.[44]

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,[46] 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.[47]

Possible companions

See also: Habitability of red dwarf systems
RV-derived Upper Mass
Limits of Companion[16]
Orbital
period
(days) 		Separation
(AU) 		Maximum
Mass[note 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.[14][3] 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.[48]

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.[49] However a subsequent search using the Wide Field Planetary Camera 2 failed to locate any companions.[15] 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.[17]

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.[50] A planet orbiting within this zone will become tidally-locked to the star, completing a single rotation each orbit and maintaining the same face toward Proxima Centauri. However, the presence of an atmosphere could serve to redistribute the energy from the star-lit side to the far side of the planet.[18]

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.[51]

Other scientists, especially proponents of the Rare Earth hypothesis,[52] 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.[19]

Interstellar travel

The Sun as seen from the Alpha Centauri system

Proxima Centauri has been suggested as a possible first destination for interstellar travel,[20] although as a flare star it will not be particularly hospitable. In any case, at the fastest speed currently attained by a manned vehicle, a journey to Proxima Centauri would take about 110,000 years.[note 4] Project Longshot could theoretically reach the Alpha Centauri system in about 100 years by means of nuclear pulse propulsion.[53] From Proxima Centauri, the Sun would appear as a bright, 0.4 magnitude star in the constellation Cassiopeia.[note 5]

Pronunciation

Proxima Centauri is sometimes pronounced to rhyme with dowry but other people pronounce it to rhyme with story.

See also

Notes

  1. ^ a b 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 kg/m3
    = 40.3 · 1.41 kg/m3
    = 5.68 kg/m3

    where is the average solar density. See: "Sun: Facts & Figures". Solar System Exploration. NASA. 2008-06-11. Retrieved 2008-07-12. {{cite web}}: Unknown parameter |authors= ignored (help)

  2. ^ a b 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.
  3. ^ 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 site 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: Campbell, William Wallace (1899). The Elements of Practical Astronomy. London: Macmillan. Retrieved 2008-08-12.
  4. ^ The distance to Proxima Centauri is:
    (4.22 ly) × (9.46 km/ly) = 4.0 km
    The Apollo 10 achieved a record velocity of 24,791 mi/hr, or 11 km/s. (See: Orloff, Richard W. (2005-09-27). "APOLLO 10, The Fourth Mission: Testing the LM in Lunar Orbit, 18 May–26 May 1969". Apollo by the Numbers. NASA. Retrieved 2008-06-30.) Thus the journey would be completed in:
    time = distance/velocity = (4.0 km)/(11 km/s) = 3.6 s
    A year is about 3.2 seconds, so completing the journey would require 1.1 years.
  5. ^ The coordinates of the Sun would be diametrically opposite Proxima, at α=02h 29m 42.9487s, δ=+62° 40′ 46.141″. 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:
    See: Tayler, Roger John (1994). The Stars: Their Structure and Evolution. Cambridge University Press. p. 16. ISBN 0521458854.

References

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  14. ^ a b Kürster, M.; et al. (1999). "Precise radial velocities of Proxima Centauri. Strong constraints on a substellar companion". Astronomy & Astrophysics Letters. 344: L5–L8. Bibcode:1999A&A...344L...5K. Retrieved 2008-12-19. {{cite journal}}: Explicit use of et al. in: |author= (help)
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