LECTURE 2: ALPHA CENTAURI
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 .
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.
α Centauri AB is well observed and has an established orbit of good accuracy, but observationally remains too close to be resolved by the naked-eye. Its duplicity, according to renown double star observer Robert Aitken (1961) and the 6th Catalog of Binary Stars (2008), was discovered by the Father Richaud in December 1689 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 photographic plates taken a different times during a proper motion survey. Proxima was revealed by the slight movement against the background stars due to its very large proper motions.
Alpha Centauri is famously known the world over as the closest star system to our Solar System at 4.37 light-years distant (about 41.5 trillion km, 25.8 trillion miles or 277,600 AU). This fact was discovered by Thomas Henderson who had made many stellar observations of both stars of the Centauri AB, measuring their trigonometric parallaxes of between April 1832 and May 1833, but did not formally published them because he seriously doubted his own results. Henderson eventually published them in 1839, after Friedrich Wilhelm Bessel published his own results for 61 Cygni in 1838. Hence, Alpha Centauri is now considered the second star to have a distance calculated, even though it is presently importance 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 of 1991.
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.
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.[1]
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 stars is 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.[1] A solar-like rotation period of some 36.8 days has been determined.[4]
Both stars are gravitationally attached, whose orbit is moderately elliptical (e=0.5179), [5] 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 about 5 to 6 billion years old, or some 30% 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. )
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 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.
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.
Discovery of additional planets in both single stars and binary star systems (such as the binary Gamma Cephei), leaves the real possibility of finding new planets orbiting the Alpha Centauri system, or revolving close to either the stars α Cen A or α Cen B. With additional evidence, like high metallicity and both the principle stars being similar in nature to the Sun, reinforces astronomers view that it is important and worthwhile to make detailed searches for planetary bodies around Alpha Centauri. So far, various Radial velocity methods used by several planet-hunting teams have all failed to find any giant planets or brown dwarfs in the system.
Others consider, based on computer simulations, that any potential terrestrial planets that did once orbit near the stars' habitable zones are now no longer there. The cause of the loss of these bodies may have happened during the system's original formation, or have 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 may very likely be at the top of the list for exo-planetary exploration. Needless to say, such lengthy trips to cross the huge voids between the stars - assuming spacecraft could obtain high enough velocities, would likely still take several centuries or more before direct evidence of such planets was obtained.
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.
Some computer models of planetary formation predict terrestrial planets around both Alpha Centauri A and B[6][7][8], 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.[9] 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.[10] 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. [11][12][13]
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.[14]
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] [15]
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. [16]
The final fixed merge point will occur more than 100,000 years hence, where this once dominant bright star will finally disappears 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.
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