A journey through time and space: Alpha Centauri

From the depths of ancient Greek mythology we learn that the half-man, half-horse centaur Chiron was the immortal offspring of the Titan King Cronus, sired through a forbidden tryst with the sea nymph Philyra. Roman poet Ovid further tells us in his Fasti that Chiron eventually settled into a cave at Mount Pelion in central Greece, and it was from there that he became a renowned teacher of medicine, music and hunting, but had a tragic death. Accidentally shot in the foot by Heracles, with an arrow that had been dipped in Hydra's blood, Chiron was condemned to suffer a wound that never healed and continuous pain. Taking pity on Chiron, however, Zeus benevolently allowed him to die and placed his body among the stars (Ridpath 1989). Writing in his Syntaxis Mathematica, the great Claudius Ptolemy included Centaurus in his list of 48 original constellations. It is the brightest star in the constellation of Centaurus, a Centauri, that symbolically (although not officially) depicts the wounded left hoof of Chiron. Indeed, the ancient Arabic astronomers knew a Centauri, the third brightest naked-eye star in the entire sky, as Al Riji al Kentauris, which translates to “the Centaur's foot”, and this gives us the Latinized name Rigel Kentaurus. The mythological description of Centaurus takes us to a time perhaps four to five thousand years before the present day (Chapman 2002). The presence of a Centauri (figure 1) was known to our ancestors at a time well before recorded history began, but the discovery of its true nature only started about 300 years ago. The modern narrative opens in December of 1689, with Jesuit missionary and astronomer Jean Richaud observing comet C/1698 ×1 through a small telescope from Pondicherry in India (Rao et al. 1984). The comet cut a trail through the constellation of Centaurus and, while recording its progress, Richaud chanced to notice that a Centauri could be resolved into a double star: “the two stars seemed to be practically touching each other”. Writing nearly 150 years later, in 1847, John F W Herschel described a Centauri as being “truly a noble object”, and noted that it consisted of “two individuals, both of a high ruddy or orange colour, though that of the smaller is of a somewhat more sombre and brownish cast”. α Centauri is the bright yellowish star seen at the middle left. It is one of the “pointers” to the star at the top of the Southern Cross, which is seen on the right of this wide-field image, next to the dark area known as the coalsack. (ESO, Claus Madsen) While observations of a Centauri were occasionally reported during the 18th century, its real astronomical significance was only realized in 1839, when reluctant Cape Astronomer Thomas Henderson reported upon measurements of its parallax: a Centauri, with a parallax of 747.1 ± 1.2 mas, is located at a distance of 1.339 ± 0.002 pc away, and is the closest stellar system to the Sun. Henderson's result has stood the test of time and, even in the light of more sensitive stellar surveys, a Centauri remains our closest neighbour, lifting it, thereby, beyond the realm of the ordinary. Yet even with Henderson's result in place a Centauri only slowly became an object of scientific interest. Some measure of its attraction to astronomers is illustrated in figure 2, where the data relating to publications listed in the NASA Astrophysics Data System (ADS) are shown in five-year totals starting in 1830. The first paper on a Centauri in the ADS is actually ascribed to the wrong author, being attributed to the Reverend William Pearson (one of the founding members of the RAS) rather than Thomas Henderson, and concerns the 1839 announcement that it “appears probable, that these two stars have a sensible parallax of about one second of space” (Henderson 1839). The literary history of a and Proxima Centauri from 1830 to 2012. Top: Five-year counts of scientific publications as derived from the NASA Astrophysics Data System. Orange bars correspond to publications relating to a Centauri, while the inset red bars relate to Proxima. Bottom: Google Ngram Viewer plots for references (literary as well as scientific) relating to a Centauri (orange line) and Proxima (red). The data is derived from word correlations found in the 5.2 million books that have been digitized (up to 2008) by Google Inc. For the next century astronomers set about refining the parallax measurement and establishing the system's orbital parameters. It was only in the early 1970s that the current interest in a Centauri really began, and it was at this time that the first papers on atmospheric composition, physical properties and evolutionary status appeared. Most recently, much research has been directed towards radial velocity measurements and the search for companion planets. Also shown in figure 2 is a Google Ngram Viewer plot for the keyword “Alpha Centauri”. This more expansive representation indicates how the star has been referenced in the general literature as well as in scientific publications, journals, newspapers and magazines. The Ngram Viewer results suggest an intriguing ∼30-year interval between peaks of interest, which we suggest might be a measure or reflection of a book's publishing lifetime and the career of a typical astronomer – each generation of astronomers and publishers apparently rediscovering an interest in the a Centauri system. Not only does figure 2 illustrate the unfolding story of a Centauri, it also portrays the subtext relating to Proxima Centauri. Discovered by Robert Innes in 1915, the main point of initial interest concerning Proxima was its similar proper motion to that of a Centauri (Innes 1915) – and this in spite of a 2° separation upon the sky (figure 3). Proxima became an object of greater interest, however, once Joan Gijsbertus Voûte announced its first parallax determination (Voûte 1917). Not only did Proxima have the same proper motion as a Centauri, it was also located at the same distance from us – indeed, it was slightly closer to the Sun than a Cen AB. Remarkably, writing upon their closeness in distance and commonality of proper motion, Voûte questioned in his 1917 paper, “are they physically connected or members of the same drift?” In spite of nearly 100 additional years of observations, astronomers are still asking this very same question. With respect to its publication history, figure 2 indicates that Proxima has paralleled that, all be it at a lower level, of a Centauri. A family portrait of the a Centauri system. (Left): a Cen AB is highly overexposed in this image so that its faint, apparent magnitude +11, companion Proxima Centauri (arrowed) is revealed. Proxima is presently located about 2° away from a Cen AB as seen on the sky. (Upper right): The region surrounding a Cen AB. (Lower right): Star field surrounding Proxima (arrowed). (ESO) So much for the human written record: published accounts take us back to the late 16th century AD. Greek mythology and, to some extent, reliable history take us back to perhaps the 12th century BC (the time of the putative Trojan Wars). Earlier than these times we enter the realm of unrecorded and decidedly unreliable history, and must turn to numerical computation to learn more about the a Centauri system. The observed parallax, radial velocity and proper motion data reveal that a Centauri became our nearest stellar neighbour some 50 000 years ago (Bobylev 2010). In addition, looking briefly to the future, uncertain as it may also be, further computations indicate that a Centauri will remain our nearest stellar companion for another 72 000 years (Matthews 1994, Bobylev 2010). Indeed, it will not reach its closest solar approach point for a further 27 000 years. 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In of composition, a Cen A and and Proxima have with of and et al. and 2010). Indeed, it is the commonality of proper motion, distance and that for a Cen AB and Proxima being in rather than their being at the present the and the of Proxima are with it an of years figure of and in with the stellar derived by et al. In of there is or even about the stars that the a Cen AB Proxima is a star properties it might be at first that the system is only of – there being more in the to It however, the very of the a Centauri system that it and a for being the of one or more planets. 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In of known – a star with a similar and as Proxima – indicates that in about stars located at the very of the main et al. Indeed, as of the has with and orbital of and – it that the in stellar at may the In of for Proxima Centauri has been et al. for an search for with the and that with a greater than with orbital between and is et al. that the with orbital between and and that a of greater than (the region – it have been with by in that the our Proxima all to the of the for an With respect to a Cen the five-year radial velocity starting in by et al. the for a Cen A there are more than 2 and 2 or for a Cen there are more than 2 and more than the a of the region surrounding a Cen AB with greater than at and research have announced the of a Cen AB the located at the John in the located at the and, of the highly with the at the will to a of a these be to the the in the radial velocity of an in a about a Cen A or a Cen some five or so years of data The John between at the of in the of in and the of at have some 000 of a Cen A and 000 of ± Cen at a of about – 000 of each star will be before might be – the for an a Centauri is to be to it is there at but the for first will be The at may have first with their discovery of a Cen but the is on an the discovery of in and the first the in the of known has been of of this a of are known and a Cen the closest system is that of an star located pc away which a companion with an orbital of The closest known is the there are actually four in this at a distance of the of pc about the Sun there are known stellar these the star located at a distance of has a to be companion with a of about years. While the radial velocity data presently for about a and observations indicate that has a between and 100 and two at and telescope of the star some years ago found for the of from a is to the of a star system the solar a system is about in of the with search is to when to the of a radial velocity or a on orbital and orbital but there is for a smaller a – of is not of in of a system one this but this approach the greater of a into The by NASA to the and was a to the search for – but a may from the not by the NASA of the during a at the of at in for a to be towards a Centauri before the of this century This is a even though it does upon as the 1970s concerning the of an to Star – known to be an apparently and the which the of a Centauri with an a to that by the announced on the 100 to in the and to and an the next 100 years. It is in to upon the that it was 100 years ago that the of by first set foot upon star has been for the 100 to but a Centauri must be a The scientific and to a are only being but we may that by the of the century a to image the and in the a Centauri system will be by however, the data from is not to be for at an additional century – a perhaps for our and time (the great will our are to the a Centauri system or to into at While the and of the a Centauri system is to be at a the for remain high and will be with the of the – for in Indeed, and have that with the telescope it will be to of the surrounding the a Centauri system. The of atmospheric of an is found the of its star and For a Cen A and the in the region figure an orbital of a Cen is of the of a Cen and it will also be With a well in of the of a Cen will be a of and continuous its in will be a dark and with only the of a Cen A to and its its and the of a Cen at very but this will also upon as field and the For the is located between The that a might be in stellar system is where is the of the star and is the of the and 1984). For α Cen A and is of for a on a of for is of for a on a of The of a Cen is The are not for being from but are they The closest approach distance of a Cen A and and the of their orbital and the of years to into the future, where might research relating to the a Centauri system the search for will for the future, with the present that by some of will be there being about two or all stars in the or a a Cen Indeed, the great of the an of the true of a Cen is still additional and are data will to be to for and as that to 2010). The of additional could also be to reveal the presence of and to and the are there and are found the next by the century we could in an about their their and – well as the a of the of a Centauri has to be for the separation of Proxima from a Cen AB result from a with another star or and so was that in the or during the system's when still its and The data at a status for and it has been as a for the of of – as the and references and by and, indeed, by the current properties of the a Centauri system about are for further In of stellar only the properties of Proxima to be at only with respect to observed first described by to be somewhat and than might be for a star of its apparent a for stars at about ± a result that to the and the of a which the for field generation is The data indicate that Proxima has a of of a velocity The with in which the by of to 100 on a of will have a upon the of even they its 2010). The that with a of will have a us an additional with respect to its The is at it is not how a field of be and and it is not how that field might be to the observed 2 ± The solar will not the at but it is that an on motion some might be at there and The to a once the (the of the to the is than and For the is of and references and an be its though how all this translates to the observed and has to be While the for the of a it does at the it may be that the of Proxima we are at with the relating to Indeed, that have about a Cen A and some of might well be for with respect to the solar system's Proxima presently to the region in which the of in the determination of the and radial in of about a Cen at in us about its stellar and may further to the orbital and history of Proxima Centauri. A of and of our solar system's and could well about a Cen AB of the of a Cen A The and of has been by to the of stars for the also found a of Proxima had an orbital than to that of the orbital of a Cen AB. that Proxima on an with a and – upon by these results the for a determination of the of Proxima. the of that the closest and for a about a Cen AB is set at a of This it turn to be that it might be to the search beyond that by and and to perhaps the into the region where a might deeper into the future, beyond the century, a Centauri will to further and further away from the each system its about the A million years from a Centauri will be to the naked-eye from and, as a result of proper motion, the entire Centaurus constellation will have beyond present the will have into the realm of ancient and the of the of stellar the of the a Centauri system will for a further years It will be at this time that a Cen the its central will its to red in and in With these the of a Cen A will be to a region well beyond that in which might Not only as a Cen A its of its will high to the of a Cen present a Cen A have a on a Cen In actually the a Cen AB system the of their will take place about a years the of our result in our by the of the Sun and the of a a Cen A through its and of it will to more and more an stellar Indeed, as it its it will have to about of its present For Proxima Centauri this will and its status will eventually be Indeed, in about years from the a Centauri system is to its red to a Cen Proxima will to an about the time to a Cen in about to away from the main of the with indicate that as the a Cen AB system more and more its orbital towards eventually in system Indeed, in about years the a Centauri system will be into a red star and two (the of a Cen A and With a of Proxima has a stellar lifetime that is in of years. Indeed, it is to that before Proxima to enter its some years from not only will the between the and to take place in about years be ancient history, but also star will have In the entire with the will have beyond all

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