On Feb 5, 9:12 pm, (Paul Schlyter) wrote:
In article ,
Henri Wilson HW@....... wrote:
I was really wondering about well separated binary pairs...why they weren't
seen to be changing places more frequently...but again 'distance' probably
provides the answer.
I know some such binaries are recorded,
...not only "some" - it's actually a quite large number of binaries
which have had their orbital motion measured and their orbits
determined. Thousands of binaries have had their orbits
determined..... btw the first person who measured orbital motions of
binary stars was William Herschel, several centuries ago.
but generally, those in resolvable orbits will be moving very slowly around
their orbits.
It seems you have a quite small telescope. Of course whether a binary is
resolvable depends a lot on your telescope: larger scopes will be able
to resolve many more binary stars.
However, for very heavy stars, the orbit period at say 0.01 LY radius could
conceivably be less than one hundred years....and movement should be
observable.
If you want to observe orbital motions in binary stars most easily, you should
pay attention to nearby binary stars.
Alfa Centauri, our most nearby star system at some 4.2 LY distance,
will show considerable orbital motion during a human lifetime (its
orbital period is some 80 years). But it's too far south for most
northern hemisphere observers to see.
I have myself seen orbital motion in two binaries, with causal visual
observation:
70 Ophiuchi: near its perihelion in the 1980's I observed and drew
this binary once a year. After only some 4-5 years it had changed its
PA by some 90 degrees. Now it's away from perihelion and therefore
moving more slowly, but keep an eye on this pair anyway and you'll see
orbital motion. Although now it may take a decade or two. IF you
attach a micrometer to your eyepiece, so you can detect smaller
changes in PA or separation, you'll detect the motion sooner of
course.
Gamma Virginis: In my youth in the 1960's, this binary was easily
resolvable with a separation of some 6 arc seconds. Today it's near
perihelion, with a separation of a fraction of an arc seconds and
most telescopes will be unable to resolve it. Within several years
the pair will widen again, making Gamma Virginis resolvable also
with smaller telescopes.
I guess another good one to try is Sirius and its' "Pup". The orbit is
around fifty years. The main problem with this one is the difference
in brightness of more than nine magnitudes.
Bill
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