Abdul Ahad wrote:
Firstly, how does one go about making such future positional guesses
and secondly, how long has Alpha Centauri been in close proximity to
the Sun? Is there any projections as to how long Alpha Centauri will
stay this close?

The gravitational interactions between stars are pretty small. Unless
one star is predicted to pass very close indeed--say, less than a tenth
of a light-year or so--the impact on their orbit around the galactic
center is minimal. Testament to this is the relative stability of the
galactic disc. You can map the radial and proper velocities of the
star in three dimensions and get reasonably accurate results into the
fairly distant future (say, about a million years) for the nearest stars.

Could it be that Alpha Centauri (A+B+C) and the Sun are
gravitationally *locked* together and share a common proper motion
around the galaxy?

No. I forget the exact figures, but the relative velocity of the
alpha Centauri system is considerably more than permitted for a locked
system.

I know we observe distinct radial velocity and
proper motions that Alpha Centauri has relative to the Sun, but they
are based on short term measurements in the current era... and I don't
expect you can simply *extrapolate* forwards/backwards in time simply
on the basis of their present values... or can you?

You sure can, because the gravitational interactions between individual
stars separated by light-years are quite weak.

the next nearest large star system to have any *significant*
interaction with both the Sun and Alpha Centauri(A+B+C) is Sirius
(A+B) - which is quite far removed at 8.6 LY away from the Sun and 9.5
LY away from from Alpa Cen.

This means the Sun and Alpha Centauri system are relatively isolated
in space...

How do you figure? The Sun and alpha Centauri are 4.3 light-years apart,
or half the distance from here to Sirius. Since Sirius has a bit more
than twice the mass of the Sun, and is more massive than the entire alpha
Centauri system, I don't think you can consider the Sun and alpha Centauri
much more isolated from Sirius than they are from each other.

The essential paucity of gravitational interaction is why the "stellar
rape" hypothesis of the solar system's formation had such a short life
in the middle of the 20th century. It's just too darned unlikely for
stars to pass that close to one another. To give you an idea of how far
apart the stars are from one another, consider that the Sun and alpha
Centauri A are about the same size (1.4 million km across) and are
separated by about 42 *trillion* km. That is, the distance between them
is about 30 million times their diameters.

To put it another way, if the Sun is a golf ball in San Francisco, then
alpha Centauri is another one (or two golf balls and a marble) in Los
Angeles, and Sirius is a slightly larger racquetball, accompanied by an
incredibly dense BB pellet, in Boise. Picture that for a moment, and
ask yourself how likely it is that they will interact with one another.
And remember, they move in three dimensions, not two.

Its important I think...

Steady, Brian, steady...

Its important I think to study our nearest triple star system in
greater depth (if only it rose above my horizon... but then I can't do
a lot with my tiny 8-inch Newtonian!). How much Hubble or other
space/ground-based telescope time is devoted to Alpha Centauri,
compared to all other stellar astronomy, I wonder...

What you require is accurate astrometry, and this has all been done
previously--most recently by the Hipparcos mission.

Brian Tung
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