Milky Way & Andromeda Merger

" writes:

It's all a matter of proper semantics in characterising the
phenomena. I feel merger is more appropriate for galaxies and
collision for billiard balls. Gravitational interactions and material
contact are vastly different. Call them what you will so long as you
understand what is happening. I think collision is misleading for
galaxies since nothing is actually colliding in the common sense of
the term.

Of course, this assumes every galactic gravitational interaction leads
to merger.

In a thought experiment, two galaxies are on a collision course with two
different velocities. Their centers of mass or gravitation may be
sufficiently distant such that they won't merge. A barred spiral arm
collides with a spiral arm of the other galaxy. It'd still be a
collision since the result would never yield two completely intact and
unchanged galaxies. They would have survived a galactic collision and
travel along their merry way.

--
Daniel
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