Ronn!Blankenship wrote: > >>That's the problem, isn't it? But maybe if we don't expect any two >>stars to get close enough, > >An illustration I have heard (not necessarily precise) is that two stars in >our Galaxy have about the same chance of colliding as two flies which start >from opposite ends of the Grand Canyon. > But I am not talking about _collision_. I am talking about getting close enough for enough time to change their speeds more than the initial errors in the measurement of those speeds.
And there are 100 thousand million flies, or 10 thousand trillion pairs of stars :-)
With the average distance between closest pairs comparable to the size of the Grand Canyon. The whole Galaxy at that scale would likely fill a fair amount of the solar system . . .
Ok, so we can assume that the Sun will keep its current orbit around the Galaxy. But what about other stars?
Then you could always do what you do to extrapolate solar system positions: start with the current positions and velocities of all the stars for which you have data, compute the gravitational forces between each pair of stars, then do a stepwise integration. (Of course, you do have access to large blocks of time on a Cray, don't you?) And you'll need to approximate the general gravitational field of the galaxy as a whole. For a description of that, as well as the kind of orbits stars follow in the galactic gravitational field, probably the standard text is:
<<http://pup.princeton.edu/titles/2537.html>>
and this book goes along with it:
<<http://pup.princeton.edu/titles/6358.html>>
However, I suspect that over the time periods you're talking about (a few x10^6 years), there will be at most only a few close enough encounters between stars to significantly affect the paths of the stars. Stars are really w�i�d�e�ly separated in our neighborhood of the galaxy. (You are not concerned with the stars in the central regions of a globular cluster or near Sgr A*, which are about the only places they are really crowded . . . )
>> m = 1/1000. Do you mean My or ky? > > "mybp" is frequently used by geologists as an abbreviation for "million > years before present." > Bloody non-standard bastards. What is the abbreviation of year? yr? _Certainly_ m stands for 1/1000, not 10^6
OK, it probably ought to be Mybp. But what can you expect out of geologists? After all, they have rocks in their heads . . .
> If so, are you familiar with the Hip/Tyc catalog > and its data format, or do you need more information? > No; but I guess I can find it quickly with a google search. It's free, isn't it? Probably some Megabytes of data
Home page: <<http://astro.estec.esa.nl/SA-general/Projects/Hipparcos/hipparcos.html>>
Actually, the "Celestia 2000" program mentioned there fits on a CD-ROM, and I think that must include the data from the catalog, since it runs without needing anything else. That program is mainly for statistical searches, e.g., "list all the stars in the HIP/TYC catalog within <n> parsecs of the Sun (i.e. with parallax greater than 1/n arcseconds) whose annual motion is between <lower limit> and <upper limit>," but that's not really what you need. So you'll need to download the catalog and write a program to read it and do the calculations you need.
-- Ronn! :)
Ronn Blankenship Instructor of Astronomy/Planetary Science University of Montevallo Montevallo, AL
Disclaimer: Unless specifically stated otherwise, any opinions contained herein are the personal opinions of the author and do not represent the official position of the University of Montevallo.
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