Alpha Centauri

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For other uses, see Alpha Centauri (disambiguation).
Alpha Centauri A[1]/B[2]

The position of Alpha Centauri.
Observation data
Epoch J2000
Constellation
(pronunciation)		 Centaurus
Right ascension 14h 39m 36.4951/35.0803s
Declination -60° 50′ 02.308/13.761″
Apparent magnitude (V) -0.01/+1.33
Characteristics
Spectral type G2V/K2IV
U-B color index +0.23/+0.63
B-V color index +0.69/+0.90
Variable type None
Astrometry
Radial velocity (Rv) -21.6 km/s
Proper motion (μ) RA: -3678.19 mas/yr
Dec.: 481.84 mas/yr
Parallax (π) 747.23 ± 1.17 mas
Distance 4.365 ± 0.007 ly
(1.338 ± 0.002 pc)
Absolute magnitude (MV) 4.38/5.71
Details
Mass 1.100/0.907[3] M☉
Radius 1.227/0.865[3] R☉
Surface gravity (log g) 4.30/4.37[4]
Luminosity 1.519/0.500[3] L☉
Temperature 5,790/5,260[3] K
Metallicity 151/160%[3] Sun
Age 4.85×109[3] years
Visual binary orbit[5]
Companion Alpha Centauri B
Period (P) 79.9 yr
Semimajor axis (a) 17.59"
Eccentricity (e) 0.519
Inclination (i) 79.23°
Longitude of the node (Ω) 204.82°
Periastron epoch (T) 1955.59
Argument of periastron (ω) 231.80°
Database references
SIMBAD data
ARICNS data
Other designations
Rigil Kentaurus, Rigil Kent, Toliman, Bungula, FK5 538, CP(D)−60°5483, GC 19728, CCDM J14396-6050

α Cen A

Gl 559 A, HR 5459, HD 128620, GCTP 3309.00, LHS 50, SAO 252838, HIP 71683

α Cen B

Gl 559 B, HR 5460, HD 128621, LHS 51, HIP 71681

Proxima Cen

LHS 49, HIP 70890

Alpha Centauri / α Centauri / α Cen, also known as Rigil Kentaurus or just Rigel Kent, is the brightest star system in the southern constellation of Centaurus. It appears to the naked eye as the third brightest star in the entire night sky, being only outshone by Sirius and Canopus. By total visual magnitude Alpha Centauri AB (α Cen AB) is −0.27, which shines just fractionally brighter than the fourth brightest individual star in the night sky, Arcturus. In the Southern Hemisphere Rigel Kent is known as the outermost "pointer" to the Southern Cross, but it is too far south to be easily visible for most northern hemisphere observers. The other, inner "pointer" is Beta Centauri or Agena / Hadar. This star lies some 4.4o further west of Alpha Centauri, in between the Southern Cross and α Centauri itself.

Alpha Centauri has the distinction of being the closest of all the stars visible to the naked eye in the night sky. Its distance is about 4.37 or 4.4 light years or 1.3 parsecs .

Contents

[edit] Nature of Alpha Centauri

Alpha Centauri is a gravitational system consisting of three different stars. The brightest two stars make a close orbiting binary star, with an additional third distant and fainter component, called Proxima Centauri, Proxima or α Cen C. Comparing the two brightest individual stars, they rate, respectively, as the fourth and twenty-first brightest stars in the sky - if we exclude the Sun.

Observationally, α Centauri AB remains too close to be resolved by the naked-eye, but for more than two centuries, it has been very well observed and measured by telescopes as small as 5cm in aperture. This presently well established eighty-odd year orbit is therefore of very good accuracy.

Its duplicity, according to the renowned double star observer Robert Aitken (1961), and as now stated in the 6th Catalog of Binary Stars (2008), was originally discovered in December 1689 by Father Richaud from the city of Pondicherry in India while he was observing a comet. Proxima Centauri was later discovered by R.T.A. Innes in 1915 from South Africa, being detected by blinking two separate photographic plates taken at different times during one of his dedicated proper motion surveys. Proxima Centauri, like α Centauri AB, showed very large proper motions of both similar size and direction, and this immediately suggested they were likely associated as one system. Distance for Proxima was then later revealed by its tiny but highly significant annual circular movements against the background stars in the star's position - the so-called trigonometric parallax.

Alpha Centauri is now famously known the world over as the closest star system to our Solar System at about 4.37 light-years distant (about 41.5 trillion kilometres, 25.8 trillion miles or 277,600 AU). This original fact was discovered by Thomas Henderson who had made many stellar observations of both stars in the AB system. He measured accurately the trigonometric parallaxes of both stars between April 1832 and May 1833, but did not formally published them because he seriously doubted his own results - seemingly far too large to be true. Henderson eventually formally published these in 1839, only after Friedrich Wilhelm Bessel had released his own accurate parallax results for the star 61 Cygni in 1838. Hence, Alpha Centauri is now considered to be the second star to have a calculated distance, even though its present importance is now known as the nearest of all the night time stars.

Proxima Centauri is usually importantly regarded as part of the system, and presently in its orbit is just slightly closer to us than α Centauri AB. For this reason Proxima Centauri is celebrated as the closest star to the Sun, lying 4.22 light-years away. Most of the modern calculated distances for all three stars, as often expressed in the literature, have been derived from the trigonometric parallax placed in the Hipparcos catalog produced in 1991.

[edit] Alpha Centauri : The Gravitational System

Alpha Centauri is the collective name of a triple star system. It consisting of two main stars, α Cen A and α Cen B, together often labelled as α Cen AB, which is the established binary star system. A third component is the much smaller and dimmer red dwarf star named Proxima Centauri or α Cen C.

Size and color of the Sun compared to the stars in the Alpha Centauri system
Size and color of the Sun compared to the stars in the Alpha Centauri system


[edit] Alpha Centauri AB

Alpha Centauri A is the principal member or primary of the system and is slightly larger and more luminous than our Sun. Like the Sun, its similar yellowish-white colour main sequence star that has the stellar classification of a G2 V for its spectral type. From the mutual orbital parameters, α Cen A is about 10% more massive than our Sun and the radius has been measured as 23% bigger.[3]

Alpha Centauri B is the companion star or secondary to the primary star, and is slightly smaller and dimmer than the Sun. This also main sequence star's spectral type is K1 V, being an observed deeper orangish-yellow colour than the primary star. By mass, α Cen B is about 90% of the Sun, and it is 14.3% smaller in radius.[3] A solar-like rotation period of some 36.8 days has been determined.[6]

Both stars are gravitationally attached, whose orbit is moderately elliptical (e=0.5179), [7] unlike the planets and the majority of asteroids that orbit our own Sun. They can approach each other in the orbit as close as 11.2 astronomical units (1.669 billion kilometers or 1.04 billion miles: roughly the distance from the Sun to Saturn), or receding to 35.6 AU (5.9 billion km: being approximately the distance from the Sun to Pluto). The orbital period is 79.91 years. [1] From this we can easy calculate that the sum of the two masses is about double that of the Sun ([(11.2 + 35.6) / 2]3 / 79.912 = 2.0, see formula). These two stars using our knowledge of stellar evolution theory is currently calculated to be slightly older than the Sun.[3] The orbit makes the separation and position angle are continuously change, and according to the U.S.N.O.'s 6th Binary Star Catalogue : Ephemeris, the distance between the stars is 8.29 arcsec through P.A. 237o (2008) and 7.53 arcsec through P.A. 241 (2009). The next closest approach will be in February 2016, when the distance will reduce to 4.0 arcsec in PA 300o. (See External Reference. )

Apparent and real trajectory of B component relative to A component
Apparent and real trajectory of B component relative to A component


[edit] Proxima Centauri / Alpha Centauri C

Main article: Proxima Centauri

The much fainter red dwarf star named Proxima Centauri, "Proxima", Alpha Centauri C or even "α Cen C", is about 13,000 astronomical units (A.U.) away from Alpha Centauri AB (1.94 trillion kilometres, 13,000 A.U. or 0.21 ly – and about one-twentieth the distance between Alpha Centauri AB and the Sun). It may be in orbit around it, though the period must be in the order of 100,000 to 500,000 years or more. It is possible that the orbit might be hyperbolic, similar to the planetary sling-shot effect adopted by interplanetary spacecraft to change direction and velocity to a second planetary body, and so Proxima may leave the system after a few million years. Association with Alpha Centauri AB is unlikely to be entirely accidental, as it shares approximately the same motion through space as the inner binary star system. However, the true gravitational connect remains yet to be proven.

Seen from Earth, Proxima Centauri is separated by 2.2o south-west from Alpha Centauri AB. This is about four times the angular diameter of the Full Moon, and almost exact half the distance between Alpha Centauri and Beta Centauri. A moderate sized telescope is required to see Proxima.

Proxima usually appears as a 13.1 visual magnitude deep-red star in a poor star field of only several stars. The star is listed in the General Catalogue of Variable Stars (G.C.V.S. Version 4.2) as V 645 Cen, being a known UV Ceti-type flare star, which may suddenly and unexpectedly brighten by about two magnitudes or so. (11.0 visual magnitude is often quoted.) Both amateur and professional have, and continue, to monitor this star from time to time - both visually through optical telescopes and radio telescopes. Proxima is of spectral class M5Ve or M5VIIe, whose B-V colour index is +1.8. As such, the spectral class suggest this is either a small main sequence star (Type V) or sub-dwarf (VII). Its mass is about 0.4 solar masses.

The closest stars to the Alpha Centauri system are the Sun and Barnard's star (1.98 pc or 6.47 ly). From Earth the next nearest star from is 5.96 ly.

[edit] Appearance of the Sky from the Alpha Centauri

Looking toward Sol from Alpha Centauri in Celestia
Looking toward Sol from Alpha Centauri in Celestia

Viewed from near the Alpha Centauri system, the sky (other than the Alpha Centauri stars) would appear very much as it does to observers on Earth, with most of the constellations such as Ursa Major and Orion being almost unchanged. However, Centaurus would be missing its brightest star and our Sun would appear as a 0.5-magnitude star in Cassiopeia. Roughly speaking, the \/\/ of Cassiopeia would become a /\/\/, with the Sun at the end closest to ε Cassiopeiae. The position can easily be plotted as RA 02h39m35s, Dec. +60°50', or antipodal to Alpha Centauri's position as seen from Earth.

Nearby very bright stars such as Sirius and Procyon would appear to be in very different positions, as would Altair to a lesser extent. Sirius would become part of the constellation of Orion, appearing 2 degrees to the west of Betelgeuse, slightly dimmer than from here (-1.2). The stars Fomalhaut and Vega, although further away, would appear somewhat displaced as well. Proxima Centauri would be an inconspicuous 4.5 magnitude star, which considering it would only be a quarter of a light-year away shows just how faint Proxima really is.

From Proxima, the A–B pair of stars would appear as a very bright star with a combined magnitude of −6.80. Depending on the position of the binary in their orbit and the eyesight of the observer, the binary pair would appear noticeably double to the naked eye or as one unresolved star. Alpha Centauri A would have an apparent magnitude of −6.52 and Alpha Centauri B would have an apparent magnitude of −5.19.

[edit] Do Planets Orbit Around Alpha Centauri?

Discovery of additional planets orbiting both single stars and binary star systems, leaves the real possibility of finding either new planets in the Alpha Centauri AB system or planets revolving close to either α Cen A or α Cen B. With additional evidence, like both the principle stars being similar in nature to the Sun, I.e. high metallicity and similar ages, simply reinforces the astronomers view that it is very worthwhile to make detailed searches for planetary bodies around Alpha Centauri. Additionally, planets have been found in other similar binary systems, such as Gamma Cephei. Several established planet-hunting teams have used various radial velocity methods in their searches around these bright stars, but so far all have failed to find any suggestion of any giant gas planets, small terrestrial planets or brown dwarfs.

Other planetary astronomers consider, based on theoretical computer simulations, that any potential terrestrial planets that did once orbited near the stars' habitable zones are now likely no longer there. The loss of these small bodies may have happened during the system's formation several billion years ago, and all have since been ejected by significant disruptions caused by strong gravitational or perturbation effects generated between the two main stellar components.

In the not too distant future, and if our human technology advances enough to enable voyages for interstellar robotic probes, Alpha Centauri maybe first on the list for exo-planetary exploration. Needless to say, such lengthy trips to cross the huge empty gulfs between the stars would likely still take several centuries, and this still assumes that some interstellar spacecraft could obtain high enough velocities to get there. If present ground or orbit based observatories are unable to detect planets, future exploratory journeys may be the only means of obtaining direct evidence that such planets here do exist.

[edit] Alpha Centauri From Hypothetical Planets

A hypothetical planet around either α Centauri A or B would see the other star as a very bright secondary. For example, an Earth-like planet at 1.25 Astronomical Units from α Cen A (with an orbital period of 1.34 a) would get Sun-like illumination from its primary, and α Cen B would appear 5.7 to 8.6 magnitudes dimmer (−21.0 to −18.2), 190 to 2700 times dimmer than α Cen A but still 170 to 2300 times brighter than the full Moon. Conversely, an Earth-like planet at 0.71 AUs from α Cen B (with thee revolution period of 0.63 a) would get Sun-like illumination from its primary, and α Cen A would appear 4.6 to 7.3 magnitudes dimmer (−22.1 to −19.4), 70 to 840 times dimmer than α Cen B but still 520 to 6300 times brighter than the full Moon. In both cases the secondary sun would, in the course of the planet's year, appear to circle the sky. Either way, any hypothetical Earthlike planet around either star, the secondary sun would never be bright enough to adversely affect the climate nor significantly affect plant photosynthesis.

If we assume a low orbital inclination of the planet against the orbit of α Cen A and B, the secondary would start beside the primary at stellar conjunction. Half the period later, both stars would be placed opposite of each other in the sky- a 'stellar' opposition. If the distance in the orbit were as far away as Saturn, as similarly view from our Sun, this would mean that for about half the year, the night sky would appear dark blue, instead of being pitch black. At this time people could easily walk around and see the surrounding terrain with ease, and even reading a book would be possible without any artificial light. After another half period in the orbit, the stars would complete the cycle and return to conjunction. At this time the normal day and night cycle would return.

[edit] Possibilities In Planet Formation

Some computer models of planetary formation predict terrestrial planets around both Alpha Centauri A and B[8][9][10], but suggest that gas giant planets similar to our Jupiter and Saturn would not be able to form because of the binary stars' gravitational effects.[11] Given the similarities in star type, age and stability of the orbits it has been suggested that this stellar system may hold one of the best possibilities for extraterrestrial life.[12] However, some astronomers have speculated that any terrestrial planets in the Alpha Centauri system may be dry because it is believed that Jupiter and Saturn were crucial at directing comets into the inner solar system and providing the inner planets with a source of water. This would not be a problem, however, if Alpha Centauri B happened to play a similar role for Alpha Centauri A that the gas giants do for the Sun, and vice versa. Both stars are of the right spectral type to possibly harbour life on some potential planet. [13][14][15]

Any suspected planet around Alpha Centauri A would be about 1.25 AU away from the star if it were to have Earthlike temperatures, or about halfway between the distances of Earth's orbit and Mars' orbit in our own solar system. For dimmer, cooler Alpha Centauri B, the distance would be about 0.7 AU, or about the distance of Venus from the Sun.

Proxima Centauri, along with Alpha Centauri A and B, are among the "Tier 1" target stars for NASA's Space Interferometry Mission (SIM). SIM is designed to be able to detect planets as small as three Earth-masses or smaller within two Astronomical Units of a "Tier 1" target.[16]

[edit] Apparent Movement by Proper Motion

Apparent motion of Alpha Centauri relative to Beta Centauri.
Apparent motion of Alpha Centauri relative to Beta Centauri.

Around 5973 C.E, the significantly large proper motion of Alpha Centauri will lead to a situation where some earthbound observer would see a naked-eye visual double star adjoined with the slightly fainter 1st magnitude star Beta Centauri only 23 arcmin apart. This spectacular duo will form an example of a very rare stellar conjunction. In reality, Beta Centauri is just over 120 times more distant than Alpha Centauri, (530 to 4.3 light-years), passing each other like ships in the night.[citation needed] [17]

After this, Alpha Centauri will continue to slowly brighten, passing north of the Southern Cross or Crux, before moving in a northwest direction towards the celestial equator and away from the galactic plane. By 29,700 C.E, α Centauri will lie exactly 1.00 parsecs or 3.26 light-years, and will reach its maximum brightness of -0.86 magnitude - similar in brightness to present day Canopus. At this time it will be placed near the present-day constellation of Hydra. Soon after this relatively close solar approach, the system will then begin moving away from the Sun. In 43,300 AD, α Centauri will pass near 2nd magnitude Alpha Hydrae / Alphard. The visual magnitude at this time will be +1.03 and the distance will be 5.36 ly. [18]

The final fixed merge point will occur more than 100,000 years hence, where this once dominant bright star will finally disappear below naked-eye visibility somewhere in present day faint southern constellation of Telescopium. This unusual location is easily explained because α Centauri has a galactic orbital motion that happens to be highly tilted in respect of our Milky Way galaxy.

[edit] Origin of Name and Cultural Significance

The system bears the proper name Rigil Kentaurus[19] (often shortened to Rigil Kent.[20], former Rigjl Kentaurus[21][22], and Riguel Kentaurus[23] in Portuguese), derived from the Arabic phrase Rijl Qantūris[20] (meaning "Foot of the Centaur)," but is most often referred to by its Bayer designation Alpha Centauri. An alternative name is Toliman, whose etymology may be Arabic al-Zulmān (meaning "the Ostriches"), or Hebrew (meaning "The Heretofore and the Hereafter" and/or "Shoot of the Vine").[citation needed] (See Centaurus) Finally, it is sometimes called Bungula[24], possibly coined from "β" and the Latin ungula (meaning "hoof"). This latter name in modern times is, however, rarely used.

In Chinese, Alpha Centauri was called Nánmén'èr (南門二) "Second Star of the Southern Gate". As mentioned, Alpha and Beta Centauri together form the "Southern Pointers" to Crux, the Southern Cross.

[edit] Alpha Centauri in Modern Fiction

Main article: Alpha Centauri in fiction

Alpha Centauri's relative proximity makes it a in some ways likely the logical choice as "first port of call". Speculative fiction about interstellar travel predicts eventual human exploration, and even the discovery and colonization of planetary systems. These themes are common to many works of science fiction and video games. Because of its status as our nearest galactic neighbor, Alpha Centauri has frequently been referred to in science fiction stories involving interstellar travel.

[edit] References

  1. ^ LHS 50 -- High proper-motion Star. Centre de Données astronomiques de Strasbourg. Retrieved on 2008-06-06.
  2. ^ LHS 51 -- High proper-motion Star. Centre de Données astronomiques de Strasbourg. Retrieved on 2008-06-06.
  3. ^ a b c d e f g h i Kervella, Pierre; Thevenin, Frederic (March 15, 2003). A Family Portrait of the Alpha Centauri System. ESO. Retrieved on 2008-06-06.
  4. ^ Gilli, G.; Israelian, G.; Ecuvillon, A.; Santos, N. C.; Mayor, M. (2006). "Abundances of Refractory Elements in the Atmospheres of Stars with Extrasolar Planets". Astronomy and Astrophysics 449 (2): 723-736. doi:10.1051/0004-6361:20053850. 
  5. ^ Kervella, Pierre; Thévenin, Frédéric; Du Foresto, Vincent Coudé; Mignard, François (2007). "Deep imaging survey of the environment of Alpha Centauri - I. Adaptive optics imaging of Alpha Cen B with VLT-NACO". Astronomy and astrophysics 464 (1): 367-372. 
  6. ^ Guinan, E.; Messina, S.. "IAU Circular 6259, Alpha Centauri B", Central Bureau for Astronomical Telegrams. 
  7. ^ Hartkopf, W.; Mason, D. M.. "Sixth Catalog of Orbits of Visual Binaries", U.S.Naval Observatory, Washingto D.C.. 
  8. ^ Javiera Guedes, Terrestrial Planet Formation Around Alpha Cen B
  9. ^ see Lissauer and Quintana in references below
  10. ^ Javiera M. Guedes, Eugenio J. Rivera, Erica Davis, Gregory Laughlin, Elisa V. Quintana, Debra A. Fischer (to be published in 2008). Formation and Detectability of Terrestrial Planets Around Alpha Centauri B. Astrophysical Journal.
  11. ^ M. Barbier, F. Marzari, H. Scholl (2002). "Formation of terrestrial planets in close binary systems: The case of α Centauri A". Astronomy & Astrophysics 396: 219 – 224. doi:10.1051/0004-6361:20021357. 
  12. ^ P.A. Wiegert and M.J. Holman (1997). "The stability of planets in the Alpha Centauri system". The Astronomical Journal 113: 1445 – 1450. 
  13. ^ Lissauer, J. J., E. V. Quintana, J. E. Chambers, M. J. Duncan, and F. C. Adams. (2004). "Terrestrial Planet Formation in Binary Star Systems.". "Revista Mexicana de Astronomia y Astrofisica (Serie de Conferencias); First Astrophysics meeting of the Observatorio Astronomico Nacional: Gravitational Collapse: From Massive Stars to Planets"; 22: 99 – 103. 
  14. ^ Quintana, E. V.; Lissauer, J. J.; Chambers, J. E.; Duncan, M. J.; (2002). "Terrestrial Planet Formation in the Alpha Centauri System.". Astrophysical Journal 2: 982 - 996. 
  15. ^ Quintana, E. V.; Lissauer, J. J.;. "Terrestrial Planet Formation in Binary Star Systems.". "Planets in Binary Star Systems.". 
  16. ^ "Planet Hunting by Numbers," (Press Release), NASA, Stars and Galaxies, Jet Propulsion Laboratory, 18 October 2006. Retrieved 24 April 2007.
  17. ^ Hartung, E.J.; Frew, David Malin, David. "Astronomical Objects for Southern Telescopes", Cambridge University Press. 
  18. ^ Matthews, R.A.J. (1994). "The Close Approach of Stars in the Solar Neighbourhood". Quarterly Journal of the Royal Astronomical Society 35: 1 – 8. 
  19. ^ Bailey, F., "The Catalogues of Ptolemy, Ulugh Beigh, Tycho Brahe, Halley, and Hevelius," Memoirs of Royal Astronomical Society, vol. XIII, London, 1843.
  20. ^ a b Kunitzsch P., & Smart, T., A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations, Cambride, Sky Pub. Corp., 2006, p. 27
  21. ^ Hyde T., "Ulugh Beighi Tabulae Stellarum Fixarum", Tabulae Long. ac Lat. Stellarum Fixarum ex Observatione Ulugh Beighi, Oxford, 1665, p. 142.
  22. ^ Hyde T., "In Ulugh Beighi Tabulae Stellarum Fixarum Commentarii", op. cit., p. 67.
  23. ^ da Silva Oliveira, R., "Crux Australis: o Cruzeiro do Sul", Artigos: Planetario Movel Inflavel AsterDomus.
  24. ^ Burritt, E. H., Atlas, Designed to Illustrate the Geography of the Heavens, (New Edition), New York, F. J. Huntington and Co., 1835, pl. VII.

[edit] External links

[edit] References on Hypothetical Planets or Exploration

[edit] See Also