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In this model, two large disk galaxies collide to form a galaxy pair much like The Antennae.Yellow/white shows the stars in the model. The whole sequence covers approximately 500 million years.
Upper Left: The galaxies approach one another before
collision.
Upper Middle: The galaxies first collide on their
orbit.
Upper Right: After the first collision, tidal
forces strip long streamers of stars (called "tidal tails") from the
galaxies. While the galaxies appear close to one another in this projection,
they are actually moving apart along the line of sight.
Lower Left: The galaxies begin to fall back and
recollide as the tails continue to expand away from the center. At this
point the model best matches the Antennae.
Lower Middle: The galaxies begin to coalesce,
forming a single "merger remnant."
Lower Right: As the remnant settles down and the
merger is complete, long tidal tails and loops remain visible around the
remnant. Many elliptical galaxies sport such tidal features, hinting at
some sort of merger origin.
Image Credit: Chris Mihos and Sean Maxwell, Case Western Reserve University
For comparison, here is the Hubble Space Telescope image of the Antennae Galaxy.