From all this detailed information, we can now paint a general picture of the nature of the system. Hypothetically any Earth-like planet orbiting at one astronomical unit from α Centauri A would have skies similar to here on Earth. In size, the primary G2 star would be about 1.61 million kilometres across and 42′arcmin. in apparent diameter - only 10′ arcmin. larger than our Sun. Naked-eye observations would likely just see the small 2.3% difference in luminosity - similar to the dullness observed during 60% partial solar eclipses. An average apparent magnitude would be about -27.5 one magnitude brighter than our Sun.
From the same planet, α Cen B would be seen to make its eighty-odd year journey slowly against the background stars. If our hypothetical planet also was on the ecliptical plane and similarly tilted by 23.5o, α Cen B would be highly inclined to the ecliptic. The system would have both ‘stellar oppositions’ and ‘stellar inferior conjunctions’. (I don’t really know the correct terms for these, and the “Star Trek Manual on Extragalactic Terms” didn’t help either!)
Orbital distances would vary somewhere between 11.9 AU, just larger than the orbit of Saturn, and 34.9 AU - 1.78 billion and 5.24 billion kilometres, respectively. At the present distance, 1″ arcsec. is very approximately 1 AU in size. At stellar opposition, night as we know it would disappear, leaving one very bright coppery-orange star and similarly coloured on the ground and local vistas. The naked-eye would perhaps see stars down to about 2.0 to 2.5 apparent magnitude. For c. 25o, the surrounding corona of α Cen B would cause some difficulty for serious observation, mainly due to the atmospheric scattered light. The observed colouration would come from the K2 spectral type 4 800K surface temperature some 900K cooler than the Sun. At stellar inferior conjunction, α Cen B would be some 56o above the primary.
Looking at α Cen B no doubt would hurt the eyes, while the apparent size would be 0.9′ arcmin. or 52″arcsec. when at 35 AU. increasing in size to 2.7 arcmin. or 195″arcsec. at 11.9 AU. Both these sizes lie between the telescopic observed diameters of Jupiter and Saturn. Physically the α Cen B component is 1.02 million kilometres across, with the apparent visual magnitude from our hypothetical planet varying between c.-22 and -19 - roughly 4% in variation.
Effects on day and night from both stars throughout the orbit would be profound. causing serious problems in the organisation of most of life’s activities - especially with optical observations. The amount of light would vary on our hypothetical planet with both latitude and longitude. For example, near either pole, over an eighty year period, it would be possible to have seasonal variations of:
• Duolight with the both stars never setting.
• Daylight the primary being circumpolar while the secondary lies below the horizon.
• Daynight with the primary above the horizon and the secondary below the horizon.
• Twinight the secondary being circumpolar while primary lies below the horizon.
• Alnight both stars never rising above the horizon.
In mid-to-equatorial latitudes. this would be equally complex. Although both stars would still rise and set, the description of the various ‘twilights’ would be truly difficult, as there are quite a few different combinations. Adding a moon, similar to our own, would compound these complications, including combinations of both solar and lunar eclipses. (Note: The “Star Wars” home of Luke Skywalker on Tatooine would also have similar problems with the dual suns.)
In the night sky, Proxima Centauri would be a faint red star just a magnitude or so above naked-eye visibility (c.5.1), and would only be visible to the naked-eye during the rare times when both bright stars were below the horizon. Detection of Proxima’s association with the main double system would require telescopic observations, specifically from the observed proper motions. This would be in the order of 4.6′ arcmin. per year - too obvious for a keen alien observer.
Conversely from Proxima, Alpha Centauri would appear as a absolutely stunningly brilliant pair in the night’s sky. The duo would be visible to the naked-eye - though a glary to the eyes. At the distance of 13 000 AU or 1.9x1012 km. or 0.205 ly., the visual brightness of both α Cen A and α Cen B would be -6.3 and -4.9 assumed on the absolute magnitudes of +4.7 and +6.1, respectively. In some reference, like the I have used in Table 4-1, give the absolute magnitudes as +4.38 and +5.74, which would make the visual magnitudes maginally brighter as -6.6 and -5.2. Regardless this of course is much brighter than Venus as seen from Earth at its best. Much of this is clear cut, but as for the problems of the eccentricity of the orbit of Proxima Centauri, we presently know very little. If the orbit was near circular then little would change except for the apparent separation of α A and B. If the orbit was eccentric, then the magnitude of these main stars would vary.
From Proxima the observer would see at maximum distance between the stars would be around 7.4′ or 450″ apart, but this could, at times, range down to several tens of seconds of arc.) Describing this in useful terms is not straight forward. The apparent positions would change over the eighty year orbit, and this must be combined with Proxima’ own place in the 100 000 year period around the main pair. At the moment, Proxima lies in the sky 1.9o away along position angle 229o. This almost matches today’s (2005) position angle of about 220o from α A and B of today, so that both stars would appear nearly along the same line of sight. I have placed in Figure 5-1 to show the brightness of the main pair as seen from Proxima itself, with the separation of 7.4 ′, as given above. I estimate will occur somewhere between 2015 AD and 2020 AD.
Our Sun would appear as a +0.12 magnitude star in the southern part of the northern constellation of Cassiopeia, being the fifth or sixth brightest star. All visible stars that surround us in space would look the same as from Earth, except perhaps the nearby Sirius, Procyon and Arcturus, which would have slightly different magnitudes and positions.
If our Earth-like planet orbited in the same plane as the two bright components, highly unstable planetary orbits would be produced, caused mainly by large perturbations from the unequal gravitational pull by the two stars. The climate would likely be slightly affected, perhaps with higher day temperatures and lower night temperatures. Over the aeons these differences could be quite varied, as the combined gravitational effects of perturbations, coupled with the eccentricity of the planetary orbits, would be both chaotic and highly variable on the planetary climate. According to Ken Croswell of Harvard University (“Does Alpha Centauri Have Intelligent Life.” Astronomy, 19, 4; p.28-37 April (1991), stable planets could possibly orbit α Cen A as far as ~2 AU, and α Cen B up to ~1.6 AU. (As yet no evidence of these planets exists, but there are some ideas in the pipeline to attempt this long interstellar journey in the mid-21st Century.)
As regards the stability of the orbits, it seems to me that he brushes off the significant problems of perturbations. Another minor hole in his speculations is the problem of how planets could form in such extraordinarily harsh circumstances, as the formation of stellar disks would be significantly destroyed by the gravitation of each star. The main objections to planetary formation would also be based on angular momentum. Formation of binaries, for example, is far more efficient in reducing the problem of wasting angular momentum during formation than making solitary stars like the Sun. A binary stores some 107 times more gravitational energy than the Sun, suggesting that the larger this value, the larger the mean particle velocities during formation. This translates into either mass loss into interstellar space or disruption of the planetary formation process. Evidence of this is the number of stars that have circumstellar disks - none are visual binaries.
Another result of these effects would be regarding the influence of comets on the evolution of life. A binary would be very disruptive on the stability of the ana-stellar Oort cloud, causing the cometary material to be rapidly eradicated. In the Alpha Centauri system, for me Proxima influence would literally vacuum clean and quickly destroy this cloud!
Although the current ‘flavour of the month’ seems to show that comets and asteroids are quite ‘dangerous’ objects for us fragile human beings, perversely, they may have had some significant influence on why we are here in the first place! Although comets crashing into the Earth may cause absolute havoc for life and mass extinctions, they act as significant stimuli for driving evolution to ever more complex life-forms. Some, like Fred Hoyle. have speculated that comets may have brought life to the Earth, or at the very least, introduced the materials of life - amino acids, water, and possibly forming the Earth’s oceans, etc.
For our hypothetical Alpha Centauri planet, such problems may mean that life is far less likely. Development of life (Rigel Kentaurians?) may be limited in binary star planetary scenarios. If life forms do exist, their exobiology would likely be quite different from Homo Sapiens. I.e. They would not have our natural life cycles that follow day and fight. The species would be far more robust than the frail creatures of the Earth (views strongly developed in films like the sci-fi classic “Alien”.
Ken Croswell seems to have some remarkable faith in his speculations. In his concluding remarks, in the same issue of Astronomy, he says:
“Alpha Centauri is therefore special special nor only for its proximity but also for its promise. If Alpha Centauri has planets, it is even more special since one or more of those planets could resemble Earth. And if we could ever launch missions to other stars in the Galaxy, Alpha Centauri will certainly be our first target.”
(NO ADS!)