Astronomy 162: Professor Barbara Ryden

Thursday, February 27


``To study the abnormal is the best way of understanding the normal.'' - William James

Key Concepts

(1) Galaxies are close enough together to collide occasionally.

Stars collide with each other very rarely. The distance between neighboring stars (at our position in the Milky Way Galaxy) is approximately equal to 10 million times the diameter of a star. By contrast, galaxies collide with each other quite frequently. The distance between neighboring galaxies is approximately equal to 20 times the diameter of a galaxy.

To illustrate this difference, consider building a scale model of our galaxy in which the stars are represented by ping-pong balls. In this model, the distance between the Sun and Alpha Centauri will be 1100 kilometers (the distance between Columbus and Jacksonville, Florida). Now consider a scale model of the universe in which individual galaxies are represented by ping-pong balls. In this model, the Milky Way Galaxy and the Andromeda Galaxy will be a pair of ping-pong balls only 1 meter apart.

If two galaxies pass each other at close range, then the tidal forces between the two galaxies will produce interesting distortions. For instance:

[Image credit: Kirk Borne (STScI) & NASA]
Low-speed, head-on, encounters between galaxies result in mergers. The individual stars in the two galaxies pass harmlessly by each other, without slamming into each other. A few stars attain high speed, and are lost to intergalactic space. Most of the stars, however, are traveling slowly enough to settle back down to form a single large elliptical galaxy. The merging process takes hundreds of millions of years, so merging galaxies are often caught `in flagrante', as it were.

Computer simulations of merging galaxies are amusing to watch -- they compress hundreds of millions of years into a few seconds of screen time. The Space Telescope Science Institute has both Mpeg and QuickTime videos of a computer simulation of the merger of the ``Mice''.

(2) Galaxy collisions often trigger bursts of star formation.

Although stars don't collide when two galaxies merge, the much larger gas clouds do. The clouds in the two galaxies slam into each other violently. Shock waves from the collision run through the clouds and trigger the collapse of dark nebulae to form stars. Thus, if the two colliding galaxies are rich in gas, their merger will be accompanied by a burst of star formation.

Gas-rich galaxies undergoing mergers generally show the signs of recent star formation. For example:

[Image credit: Ohio State Galaxy Survey Project]

(3) The merger of spiral galaxies produces elliptical galaxies.

One of the basic principles of physics is, ``Entropy tends to increase.'' Entropy is a measure of how disordered a system is. Thus, the statement ``Entropy tends to increase with time'' is merely a statement that things tend to become more disordered as time goes on. (If you shuffle a sorted pack of cards, it becomes unsorted. If you shuffle an unsorted pack of cards, however, the chances of its spontaneously sorting itself into suits is minuscule.)

Spiral galaxies are ordered systems. All the stars in the spiral's disk go around the center on circular orbits, in the same direction.
Elliptical galaxies, by contrast, are very disordered systems. The stars in the elliptical galaxy are on orbits of all eccentricities, oriented randomly. Elliptical galaxies, in sort, are not tidy.

Galaxy mergers are like car crashes. When you collide two neat, orderly sports cars, you don't get a neat, orderly SUV. Instead you get a disordered, chaotic tangle of metal. Similarly, when you collide two neat, orderly spiral galaxies, you don't get a neat, orderly bigger spiral. Instead you get a disordered, chaotic elliptical galaxy.

Elliptical galaxies are found most frequently in rich clusters because rich clusters are crowded with many galaxies, and collisions are frequent.
Spiral galaxies are found most frequently in poor clusters because poor clusters contain few galaxies, spaced relatively far apart, and collisions are less frequent.

The giant elliptical galaxies found near the center of rich clusters are huge, containing about a trillion stars apiece. They have gradually grown to this immense size by ``cannibalizing'' smaller galaxies. In many cases, you can see the partially ``digested'' smaller galaxies as luminous spots within the giant galaxy that has engulfed it.

Collisions do occur from time to time within poor clusters. For instance, within the Local Group itself, our galaxy and the Andromeda Galaxy are falling toward each other at a speed of 300 kilometers/sec. About 3 billion years from now, they will be close enough together to distort each other tidally, producing long tails. About 4 or 5 billion years from now, they will have merged into a single (badly mis-shapen) galaxy. About 6 billion years from now, all the stars from the two galaxies, as well as the stars that formed when they merged, will have settled down into a single giant elliptical galaxy.

In the past, the average size of galaxies was smaller than it is now. The average size has grown thanks to mergers.
In the past, the universe had more spiral galaxies than it does now. The spiral galaxies have been converted to elliptical galaxies thanks to mergers.

Prof. Barbara Ryden (

Updated: 2003 Feb 26

Copyright 2003, Barbara Ryden