Hi Supernova 1965 Rob & All,
Quote:
Originally Posted by Robh
Assuming the components of alpha Centauri are separated by 20 arcsecs, ...
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The problem is the current separation is actually 7.5 arc-seconds and closing down to about 4 arc-seconds in 2016. Appliying the Dawes limit, even generously so, places it very clearly beyond human capacity in 2009.
Even at a separation of 20" (about three times wider than current) it is impossible in practice to resolve this pair naked-eye. The components are very bright 1st and 0th magnitude stars and their bright, spiky, overlapping halos as seen in our imperfect eyes mean it can't really be done -- even in perfect conditions.
Epsilon Lyrae (the famous "double-double") is a good example. A truly keen-eyed human can
barely split it into two components naked eye. The separation there between A and B is 3.4
arc minutes -- about 27 times larger than the apparent gap between the components of Alpha Centauri. The separation between the pretty well-known optical double Mu Scorpii (along the back of the Scorpion is about 5.9 arc-minutes. Virtually everyone can see that gap but it is about
48 times larger than the separation between Alpha Centauri A&B.
That's not to say for one moment that anyone is telling fibs here -- and I mean that! People can be sincere, but sincerely wrong at the same time.
I've lost track of the number of members of the public at Sydney Observatory and elsewhere who,
after seeing Alpha Centauri split through the telescope then claim almost immediately afterwards (sincerely, soberly and apparently credibly) that they can just see it split naked eye -- even in poor seeing. On the other hand, when I do a naked eye tour
before we look through the 'scope, no-one has ever said words to the effect "gee, I can see two ..."
After looking through the 'scope, many really believe it must be possible. They look and sincerely believe they can see it, but I think it is an unconscious trick of the mind. For better or for worse, the human mind is a very powerful thing.
Conventional wisdom holds that Epsilon Lyrae pretty-much represents the edge of the envelope. In practice, I believe the keenest of eyes in absolutely perfect conditions might possibly resolve pairs of exactly equal magnitude down to perhaps a little over an arc-minute -- provided the stars are not bright (say 4th magnitude) and therefore do not have spiky, bright halos. A gap of 7.5 arc-seconds is 8-times smaller than this. With Alpha Centauri, on top of that, there is the unequal nature of the pair (more than one magnitude difference between them) and their bright halos ...
Best,
Les D
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