Proxima Centauri

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Proxima Centauri

Position of Proxima Centauri
Observation data
Epoch J2000.0      Equinox J2000.0
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 1.49[1]
B−V color index 1.90[1]
Variable type Flare star
Astrometry
Radial velocity (Rv)−21.7[2] km/s
Proper motion (μ) RA: −3775.64[1] mas/yr
Dec.: 768.16[1] mas/yr
Parallax (π)771.99 ± 2.25 mas[1]
Distance4.22 ± 0.01 ly
(1.295 ± 0.004 pc)
Absolute magnitude (MV)15.49[3]
Details
Mass0.123[4] M☉
Radius0.145[4] R☉
Luminosity1.38 × 10−4[4] L☉
Surface gravity (log g)5.20[5] cgs
Temperature3,040[4] K
Rotation83.5 days[6]
Age4.85 × 109[4] years
Other designations
Database references
SIMBADdata

Proxima Centauri (Latin proximus, proxima, proximum: meaning 'next to' or 'nearest to')[7] is a red dwarf star approximately 4.22 light-years away in the constellation of Centaurus. It was discovered in 1915 by Robert Innes, the Director of the Union Observatory in South Africa. The star is thought to be part of the Alpha Centauri system and is the nearest star to the Sun.[4] Because of the star's proximity, Proxima Centauri has been proposed as a destination for interstellar travel.[8]

Proxima Centauri is categorized as a flare star, as it undergoes random increases in luminosity because of magnetic activity.[9] The magnetic field is created by convection of the entire star, and the resulting flare activity produces a total X-ray emission similar to that from the Sun.[10] Proxima Centauri only has about an eighth of the Sun's mass, and consequently it has a very low luminosity. Because of its proximity, the size of this star can be measured directly, giving a diameter only one-seventh the size of the Sun.[4] The relatively low energy production rate and convective stirring of the fuel at the core means that Proxima Centauri will remain on the main sequence—burning hydrogen fuel—for another four trillion years.[11]

Searches for companions orbiting Proxima Centauri have thus far proven unsuccessful.[12][13] These attempts have constrained the maximum possible mass of a companion, and the detection of smaller objects will require the use of new instruments such as the proposed Space Interferometry Mission.[14] Because Proxima Centauri is a red dwarf and a flare star, it continues to be disputed whether a planet orbiting this star could support life.[15][16]

Observation

Proxima Centauri was discovered to share the same proper motion as Alpha Centauri in 1915 by Robert Innes while he was Director of the Union Observatory in Johannesburg, South Africa.[17] Innes also suggested the name Proxima Centauri for the star.[18] In 1917 at the Royal Observatory at the Cape of Good Hope, the Dutch astronomer Joan Voûte measured the 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.[19] 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 measureable increase in magnitude about 8% of the time, making it the most active flare star then discovered.[20]

Because of Proxima Centauri's southern declination, it can only be viewed south of latitude 27° N, which is located just to the north of Miami, Florida.[21] Red dwarfs such as Proxima Centauri are, in general, far too faint to be observable with the naked eye. It has an apparent 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] Even from Alpha Centauri A or B, Proxima would only be seen as a 5th magnitude star.[23] If the Sun were to become as dim as Proxima, all of the planets except Venus would be too faint to be seen with the naked eye, and even Venus at its brightest would be a barely visible 6th magnitude.[24]

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 spectral class M. 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.[4] This star has an absolute magnitude of 15.5,[3] which is the magnitude as viewed from a distance of 10 parsecs.

This illustration shows the relative 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) was used to measure an angular diameter of 1.02 ± 0.08 milliarcsec for Proxima Centauri. 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.[17] The estimated mass is only 12.3% of a solar mass, or 129 Jupiter masses.[4] The chromosphere of this star is active and it displays a strong emission of singly-ionized magnesium at 280 nm.[25]

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.[11]

Convection is associated with the generation and storage 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 of 2 million K[9]—hot enough to radiate X-rays.[26] Indeed the quiescent X-ray luminosity of this star is roughly equal to that of the much larger Sun. However, the overall activity level of this star is considered relatively low compared to other M-class dwarfs.[10] A lower activity level on Proxima Centauri is consistent with the star's estimated age, as the activity level of a red dwarf is expected to steadily wane over billions of years as the stellar rotation rate decreases.[27] The activity level appears to vary with a period of roughly 442 days, much as the Sun undergoes an 11-year solar cycle.[28]

A red dwarf with the mass of Proxima Centauri will remain on the main sequence for about four trillion years. As proportion of helium increase 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 a red giant phase) and steadily lose any remaining heat energy.[11]

Distance and kinematics

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[29] milliarcseconds), Proxima Centauri is roughly 4.2 light years from Earth, or 270,000 times more distant than the Sun. Its closest neighbors are Alpha Centauri A and B (at 0.21 light years), the Sun, and Barnard's Star (at 6.6 light years).[30][31] From Earth's vantage point, Proxima is separated by 2.18°[32] from Alpha Centauri, or four times the angular diameter of the full Moon.

At least among the known stars, Proxima Centauri has been the closest star to the Sun for about the last 32,000 years and will be so for about another 9,000 years, when it will be replaced by Barnard's Star.[33] Proxima Centauri has a relatively large proper motion—moving 3.85 arcseconds per year across the sky.[34] In approximately 26,700 years, Proxima Centauri will make its closest approach to the Sun, coming within 3.11 light years.[2] Proxima Centauri is orbiting through the Milky Way at a distance from the galactic core that varies from 8.3–9.5 kpc and with an orbital eccentricity of 0.07.[35]

From the time of the discovery of Proxima, it was suggested that it was likely to be a true companion of the Alpha Centauri double star system. At a distance to Alpha Centauri of just 0.21 ly (15,000 ± 700 AU),[36] Proxima Centauri may be in orbit about Alpha, with an orbital period on 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.[37]

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 furthest point in its orbit from the Alpha Centauri system). More accurate measurement of the radial velocity is needed to confirm this conclusion.[36]

If Proxima was bound to the Alpha Centauri system during its formation, this likely means that the stars share the same elementary composition. In addition, the gravitational influence of Proxima may 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.[36]

Possible companions

See also: Habitability of red dwarf systems
RV-derived Upper Mass
Limits of Companion[12]
Orbital
period
(days) 		Separation
(A.U.) 		Maximum
Mass
(× Jupiter)
50 0.13 3.7
600 0.69 8.3
3000 1.00 22

Should a massive planet orbit Proxima Centauri, some displacement of the star would be expected to occur over the course of each orbit. If this orbital plane is inclined toward the line of sight from the Earth then this displacement would cause changes in the radial velocity of Proxima Centauri. However no such shifts have yet been observed despite multiple radial velocity measurements. This puts significant constraints on the maximum mass that such a companion could possess.[12][29]

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.[38] However, a subsequent search using the Wide Field Planetary Camera 2 failed to locate any companions.[13] Proxima Centauri, along with Alpha Centauri A and B, are among the "Tier 1" target stars for NASA's proposed Space Interferometry Mission (SIM). Theoretically, SIM will be able to detect planets as small as three Earth-masses within two Astronomical Units of a "Tier 1" target.[14]

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 with a corresponding orbital period of 2.8–14 days.[39] 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.[15]

Since Proxima Centauri is a flare star, these flares could cause problems with the atmosphere of any planet in the habitable zone. However, the documentary's scientists thought these obstacles could be overcome (see Continued theories).

"No one found any showstoppers to habitability," says Gibor Basri of the University of California, Berkeley. One concern was that because M dwarfs frequently produce flares, the resulting torrents of charged particles could strip the atmosphere off any nearby planet. If the planet had a magnetic field, though, it would deflect the particles from the atmosphere. And 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.[40]

Other scientists, especially proponents of the Rare Earth hypothesis,[41] 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.[16]

Interstellar travel

The Sun as seen from the Alpha Centauri system

Proxima Centauri has been suggested as a possible first destination for interstellar travel,[8] although as a flare star it would not be particularly hospitable. However, even at the fastest speed currently attained by a manned vehicle the journey to Proxima Centauri would take about 110,000 years.[42] Project Longshot could theoretically reach the Alpha Centauri system in about 100 years by means of nuclear pulse propulsion.[43] From Proxima Centauri, the Sun would appear as a bright, 0.4 magnitude star in the constellation Cassiopeia.[44]

See also

Notes and references

  1. ^ a b c d e f g h i j "SIMBAD query result: V* V645 Cen -- Flare Star". Centre de Données astronomiques de Strasbourg. Retrieved 2007-07-09.—some of the data is located under "Measurements".
  2. ^ a b García-Sánchez, J. (2001). "Stellar encounters with the solar system". Astronomy and Astrophysics. 379: 634–659. doi:10.1051/0004-6361:20011330. Retrieved 2008-06-12. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  3. ^ a b For apparent magnitude m and parallax π, the absolute magnitude Mv is given by:
  4. ^ a b c d e f g h i Kervella, Pierre; Thevenin, Frederic (2003-03-15). "A Family Portrait of the Alpha Centauri System: VLT Interferometer Studies the Nearest Stars". ESO. Retrieved 2007-07-09.{{cite news}}: CS1 maint: multiple names: authors list (link)
  5. ^ Ségransan, D. (2003). "First radius measurements of very low mass stars with the VLTI". Astronomy and Astrophysics. 397: L5–L8. doi:10.1051/0004-6361:20021714. Retrieved 2008-06-13. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  6. ^ Benedict, G. Fritz; et al. (1998). "Photometry of Proxima Centauri and Barnard's Star Using Hubble Space Telescope Fine Guidance Sensor 3: A Search for Periodic Variations". The Astronomical Journal. 116 (1): 429–439. doi:10.1086/300420. Retrieved 2007-07-09. {{cite journal}}: Explicit use of et al. in: |author= (help)
  7. ^ "Latin Resources". Joint Association of Classical Teachers. Retrieved 2007-07-15.
  8. ^ a b Gilster, Paul (2004). Centauri Dreams: Imagining and Planning. Springer. ISBN 038700436X.
  9. ^ a b Christian, D. J. (2004). "A Detailed Study of Opacity in the Upper Atmosphere of Proxima Centauri". The Astrophysical Journal. 612 (2): 1140–1146. doi:10.1086/422803. Retrieved 2008-06-13. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  10. ^ a b Wood, B. E.; Linsky, J. L.; Müller, H.-R.; Zank, G. P. (2001). "Observational Estimates for the Mass-Loss Rates of α Centauri and Proxima Centauri Using Hubble Space Telescope Lyα Spectra". The Astrophysical Journal. 547 (1): L49–L52. doi:10.1086/318888. Retrieved 2007-07-09.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  11. ^ a b c Adams, Fred C. "Red Dwarfs and the End of the Main Sequence". Gravitational Collapse: From Massive Stars to Planets. Revista Mexicana de Astronomía y Astrofísica. pp. 46–49. Retrieved 2008-06-24. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)
  12. ^ a b c Kürster, M.; et al. (1999). "Precise radial velocities of Proxima Centauri". Astronomy & Astrophysics Letters. 344: L5–L8. Retrieved 2007-07-11. {{cite journal}}: Explicit use of et al. in: |author= (help)
  13. ^ a b Schroeder, Daniel J. (2000). "A Search for Faint Companions to Nearby Stars Using the Wide Field Planetary Camera 2". The Astronomical Journal. 119 (2): 906–922. doi:10.1086/301227. Retrieved 2008-06-25. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  14. ^ a b Watanabe, Susan (2006-10-18). "Planet-Finding by Numbers". NASA JPL. Retrieved 2007-07-09.
  15. ^ a b Tarter, Jill C.; et al. (2007). "A Reappraisal of The Habitability of Planets around M Dwarf Stars". Astrobiology. 7 (1): 30–65. doi:10.1089/ast.2006.0124. {{cite journal}}: Explicit use of et al. in: |author= (help)
  16. ^ a b Khodachenko, Maxim L.; et al. (2007). "Coronal Mass Ejection (CME) Activity of Low Mass M Stars as An Important Factor for The Habitability of Terrestrial Exoplanets. I. CME Impact on Expected Magnetospheres of Earth-Like Exoplanets in Close-In Habitable Zones". Astrobiology. 7 (1): 167–184. doi:10.1089/ast.2006.0127. {{cite journal}}: Explicit use of et al. in: |author= (help)
  17. ^ a b Queloz, Didier (2002-11-29). "How Small are Small Stars Really? VLT Interferometer Measures the Size of Proxima Centauri and Other Nearby Stars". European Southern Observatory. Retrieved 2007-07-09.
  18. ^ Alden, Harold L. (1928). "Alpha and Proxima Centauri". Astronomical Journal. 39 (913): 20–23. doi:10.1086/104871. Retrieved 2008-06-28.
  19. ^ Voûte, J. (1917). "A 13th magnitude star in Centaurus with the same parallax as α Centauri". Monthly Notices of the Royal Astronomical Society. 77: 650–651. Retrieved 2007-07-09.
  20. ^ Shapley, Harlow (1951). "Proxima Centauri as a Flare Star". Proceedings of the National Academy of Sciences of the United States of America. 37 (1): 15–18. doi:10.1073/pnas.37.1.15. Retrieved 2007-07-11.
  21. ^ Campbell, William Wallace (1899). The Elements of Practical Astronomy. London: Macmillan.—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°.
  22. ^ Sherrod, P. Clay (2003). A Complete Manual of Amateur Astronomy: Tools and Techniques for Astronomical Observations. Courier Dover Publications. ISBN 0486428206. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  23. ^ "Proxima Centauri UV Flux Distribution". ESA/Laboratory for Space Astrophysics and Theoretical Physics. Retrieved 2007-07-11.
  24. ^ The difference in absolute magnitude between Proxima Centauri and the Sun is 15.49 – 4.83 = 10.66. Venus reaches a maximum apparent magnitude of −4.6, so the corresponding magnitude of Venus in the same orbit around Proxima Centauri would be −4.6 + 10.66 = 6.06.
  25. ^ E. F., Guinan (1996). "Proxima Centauri: Rotation, Chromosperic Activity, and Flares". Bulletin of the American Astronomical Society. 28: 942. Retrieved 2008-06-14. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  26. ^ Staff (2006-08-30). "Proxima Centauri: The Nearest Star to the Sun". Harvard-Smithsonian Center for Astrophysics. Retrieved 2007-07-09.
  27. ^ Stauffer, J. R. (1986). "Chromospheric activity, kinematics, and metallicities of nearby M dwarfs". Astrophysical Journal Supplement Series. 61 (2): 531–568. Retrieved 2008-06-29. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  28. ^ Cincunegui, C.; Díaz, R. F.; Mauas, P. J. D. (2007). "A possible activity cycle in Proxima Centauri". Astronomy and Astrophysics. 461 (3): 1107–1113. doi:10.1051/0004-6361:20066027. Retrieved 2007-07-11.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  29. ^ a b Benedict, G. Fritz; et al. (1999). "Interferometric Astrometry of Proxima Centauri and Barnard's Star Using HUBBLE SPACE TELESCOPE Fine Guidance Sensor 3: Detection Limits for Substellar Companions". The Astronomical Journal. 118 (2): 1086–1100. doi:10.1086/300975. Retrieved 2007-07-21. {{cite journal}}: Explicit use of et al. in: |author= (help)
  30. ^ "Barnard's Star". SolStation. Retrieved 2007-08-06.
  31. ^ "Alpha Centauri 3". SolStation. Retrieved 2007-07-21.
  32. ^ Kirkpatrick, J. Davy; et al. (1999). "Brown Dwarf Companions to G-type Stars. I: Gliese 417B and Gliese 584C". The Astronomical Journal. 121: 3235–3253. doi:10.1086/321085. Retrieved 2008-06-23. {{cite journal}}: Explicit use of et al. in: |author= (help)
  33. ^ Bell, George H. (2001). "The Search for the Extrasolar Planets: A Brief History of the Search, the Findings and the Future Implications, Section 2". Arizona State University. Retrieved 2007-07-09. — Full description of the Van de Kamp planet controversy.
  34. ^ Benedict, G. F.; et al. "Astrometric Stability and Precision of Fine Guidance Sensor #3: The Parallax and Proper Motion of Proxima Centauri" (PDF). Proceedings of the HST Calibration Workshop. pp. 380–384. Retrieved 2007-07-11. {{cite conference}}: Explicit use of et al. in: |author= (help); Unknown parameter |booktitle= ignored (|book-title= suggested) (help)
  35. ^ Allen, C. (1998). "The galactic orbits of nearby UV Ceti stars". Revista Mexicana de Astronomia y Astrofisica. 34: 37–46. Retrieved 2008-06-13. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  36. ^ a b c Wertheimer, Jeremy G.; Laughlin, Gregory (2006). "Are Proxima and α Centauri Gravitationally Bound?". The Astronomical Journal. 132 (5): 1995–1997. doi:10.1086/507771. Retrieved 2007-07-09.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  37. ^ Matthews, Robert; Gilmore, Gerard (1993). "Is Proxima really in orbit about Alpha CEN A/B?". MNRAS. 261: L5.{{cite journal}}: CS1 maint: multiple names: authors list (link)
  38. ^ Schultz, A. B. (1998). "A possible companion to Proxima Centauri". Astronomical Journal. 115: 345–350. doi:10.1086/300176. Retrieved 2008-06-25. {{cite journal}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  39. ^ Endl, M. (June 18–21, 2002). "Extrasolar Terrestrial Planets: Can We Detect Them Already?". In Drake Deming (ed.). Conference Proceedings, Scientific Frontiers in Research on Extrasolar Planets. Washington DC. pp. 75–79. Retrieved 2008-06-23. {{cite conference}}: Unknown parameter |booktitle= ignored (|book-title= suggested) (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)CS1 maint: date format (link)
  40. ^ Alpert, Mark (November 2005). "Red Star Rising". Scientific American. Retrieved 2008-05-19.
  41. ^ Ward, Peter D. (2000). Rare Earth: Why Complex Life is Uncommon in the Universe. Springer. ISBN 0387987010. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  42. ^ The distance to Proxima Centauri is:
    (4.22 ly) × (9.46×1012 km/ly) = 4.0×1013 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 = (4.0×1013 km)/(11 km/s) = 3.6×1012 s
    A year is about 3.2×107 seconds, so completing the journey would require 1.1×105 years.
  43. ^ Beals, K. A. (1988). "Project Longshot, an Unmanned Probe to Alpha Centauri" (PDF). NASA-CR-184718. U. S. Naval Academy. Retrieved 2008-06-13. {{cite web}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  44. ^ The coordinates of the Sun would be diametrically opposite Proxima, at α=02h 29m 42.9487s, δ=+62° 40′ 46.141″. The absolute magnitude of the Sun is 4.83, so at a distance of 1.295 pc the apparent magnitude is 4.83 - 5((log10 0.77199 + 1) = 0.40.

External links

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