A ground-based telescopic view(left) and Hubble Space Telescope view (right) of the Antennae galaxies (known formally as NGC 4038/4039) - so named because a pair of long tails of luminous matter, formed by the gravitational tidal forces of their encounter, resembles an insect's antennae. The galaxies are located 63 million light-years away in the southern constellation Corvus. The respective cores of the twin galaxies are the orange blobs, left and right of image center, crisscrossed by filaments of dark dust. A wide band of chaotic dust, called the overlap region, stretches between the cores of the two galaxies. The sweeping spiral-like patterns, traced by bright blue star clusters, shows the result of a firestorm of star birth activity which was triggered by the collision.
Contrary to what you might think, when galaxies collide it is very unlikely that even a single pair of stars would collide within the two galaxies as they interact. As the galaxies approach one another, the changing gravitational forces can distort the galaxies, pulling spiral arms containing billions of stars out of the galaxies and sending the stars into completely different orbits. The collisions take millions of years, and the outcomes change depending on the sizes of the two galaxies and their trajectories. If the collision is between two galaxies of very different sizes, such as the Milky Way and its two Magellanic Clouds, the collision can shred the magellanic clouds, causing the stars in them to get incorporated into the Milky Way galaxy. The gravitational forces can deform the spiral arms of the Milky Way as the Magellanic Clouds pass by, and can 'warp' the plane of the Milky Way so that the Milky Way's pinwheel spiral pattern no longer lays flat in a common plane. Encounters between galaxies that are roughly the same size, but where one is an elliptical and the other is a spiral-type, can transform the spiral galaxy into a rare 'ring galaxy' if the collision is head-on and the two galactic nuclei pass directly through each other. If the collision is off-center, the two galaxies get completely shredded by their mutual tidal gravitational fields. The end product may be a single new galaxy created from the remains of the two that collided, or the two galaxies may continue on their ways, but with their new shapes very different from what they looked like before.
The entire process is very complicated to study but there are hundreds of examples of galaxies in just about every stage of collision, and computer modeling of these encounters has been extremely successful in accounting for many of the common morphologies that we actually see.
Copyright 1997 Dr. Sten Odenwald
Return to Ask the Astronomer.