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Alpha Centauri is a triple-star system.

Alpha Centauri is the third-brightest star in our night sky – a famous southern star – and the nearest star system to our sun. Through a small telescope, the single star we see as Alpha Centauri resolves into a double star. This pair is just 4.37 light-years away from us. In orbit around them is Proxima Centauri, too faint to be visible to the unaided eye. At a distance of 4.25 light years, Proxima is the closest-known star to our solar system.

Astronomers have been searching for planets around the Alpha Centauri stars. So far, two planets have been found orbiting Proxima Centauri, one in 2016 and another in 2019. A paper published in February 2021 reported tantalizing evidence of a Neptune-sized planet around Alpha Centauri A, but so far, it has not been definitively confirmed.

The two main stars are Alpha Centauri A and Alpha Centauri B, which form a binary pair. They are about 4.35 light-years from Earth, according to NASA. The third star is called Proxima Centauri or Alpha Centauri C, and it is about 4.25 light-years from Earth, making it the closest star other than the sun.

Alpha Centauri (Hubble Telescope)

Observation[]

To the naked eye, Alpha Centauri AB appears to be a single star, the brightest in the southern constellation of Centaurus. Their apparent angular separation varies over about 80 years between 2 and 22 arcsec (the naked eye has a resolution of 60 arcsec), but through much of the orbit, both are easily resolved in binoculars or small telescopes. As seen from Earth, Proxima Centauri is 2.2° southwest from Alpha Centauri AB, about four times the angular diameter of the Moon. Proxima Centauri appears as a deep-red star of a typical apparent magnitude of 11.1 in a sparsely populated star field, requiring moderately sized telescopes to be seen. Listed as V645 Cen in the General Catalogue of Variable Stars Version 4.2, this UV Ceti-type flare star can unexpectedly brighten rapidly by as much as 0.6 magnitudes at visual wavelengths, then fade after only a few minutes. Some amateur and professional astronomers regularly monitor for outbursts using either optical or radio telescopes. In August 2015, the largest recorded flares of the star occurred, with the star becoming 8.3 times brighter than normal on 13 August, in the B band (blue light region).

Alpha Centauri is inside the G-cloud, and its nearest known system is the binary brown dwarf system Luhman 16 at 3.6 ly (1.1 pc).

Kinematics[]

All components of Alpha Centauri display significant proper motion against the background sky. Over centuries, this causes their apparent positions to slowly change. Proper motion was unknown to ancient astronomers. Most assumed that the stars are permanently fixed on the celestial sphere, as stated in the works of the philosopher Aristotle. In 1718, Edmond Halley found that some stars had significantly moved from their ancient astrometric positions.

In the 1830s, Thomas Henderson discovered the true distance to Alpha Centauri by analysing his many astrometric mural circle observations. He then realised this system also likely had a high proper motion. In this case, the apparent stellar motion was found using Nicolas Louis de Lacaille's astrometric observations of 1751–1752, by the observed differences between the two measured positions in different epochs.

The Future[]

Moving ever deeper into the future, far beyond the 21st century, Alpha Centauri will continue to track further and further away from the Sun; each system ploughing its independent course about the galactic centre. A million years from now, Alpha Centauri will no longer be visible to the naked-eye from Earth and, as a result of proper motion, the entire Centaurus constellation will have shifted beyond present recognition. Once again, the Centaur will have passed into the realm of ancient memory and the shape-shifting world of mythology. On the timescale of stellar evolution, the characteristics of the aCentauri system will hardly change for a further 3 billion years (Beech 2012). It will be at this time that aCenA, having exhausted the hydrogen within its central core, will begin its advancement to red giant status, swelling dramatically in size and greatly increasing in luminosity. With these changes the habitability zone of aCenA will be swept outwards to a region well beyond that in which stable planetary orbits might exist. Not only this, as aCenA enters its advanced helium burning, asymptotic giant branch, phase of evolution, its energy output will become sufficiently high to sterilize the habitability zone of aCenB.

At present aCenA would have a minimal heating effect on any habitable Earth�analogue planet orbiting aCenB (Forgan 2012). In apocalyptic resonance, should habitable planets actually exist within the a Cen AB system then the demise of their biospheres will take place about a billion years after the destruction of our own (a result brought about, in our case, by the increased luminosity of the Sun and the onset of a runaway moist greenhouse heating effect). As a Cen A moves through its giant and asymptotic giant branch phases of evolution, it will begin to lose more and more mass via an enhanced stellar wind. Indeed, as it enters its final white dwarf configuration it will have slimmed down to about 60% of its present bulk. For Proxima Centauri this mass loss will prove critical, and its gravitationally bound status will eventually be compromised. Indeed, in about 3.5billion years from now the aCentauri system is likely to lose its outermost red dwarf component. No longer gravitationally bound to aCenAB, Proxima will begin to move along an independent path about the galactic centre. As time continues to tick by, aCenB will, in about 10billion years, begin to evolve away from the main sequence.

Refrences[]

Astronomy & Geophysics, Volume 53, Issue 6, December 2012, Pages 6.10–6.16, https://doi.org/10.1111/j.1468-4004.2012.53610.x[1]

https://earthsky.org/brightest-stars/alpha-centauri-is-the-nearest-bright-star

https://www.space.com/18090-alpha-centauri-nearest-star-system.html

https://en.wikipedia.org/wiki/Alpha_Centauri

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