(Submitted January 16, 1998)
After reading a book about galaxies and learning that galaxies
can sometimes change shape when one galaxy brushes past another
galaxy, one of the students in our Third Grade asked if part
of a galaxy can break off and join another galaxy when they
brush past each other?
This is a very good question. Unfortunately I can't give a definitive
answer. Here is what I can tell you: galaxies consist of 3 kinds of material:
gas, stars, and 'dark matter' (material that we know must exist because
its gravity is needed to hold the galaxy together, but we can't observe it
directly). When 2 galaxies interact at a distance, they affect each other
through their gravitational forces. These are of 2 types: first is the
ordinary gravitational attraction which holds us onto the earth, and
which holds solar system together; second is the tidal force, which is
due to the fact that gravity decreases with distance. The tidal forces
due to the moon and the Sun are responsible for the earth's tides. If they
were very much stronger, they could actually rip the oceans off of the earth,
or rip the earth apart. There is no danger of this, but
it can happen in galaxy interactions -- the tidal forces
can disrupt one of the galaxies, or remove the gas from one of them.
This has been suggested as a way of explaining why some galaxies
(ellipticals) have little gas while others (spirals) have a lot more.
In this case some of the gas is probably transferred to the bigger
galaxy. Other possible interactions include total disruption of
one of the galaxies, or merging of the two galaxies. Which of these
occurs depends on how closely the galaxies approach each other and the
masses of the two galaxies.
I hope this helps,
for the Ask an Astrophysicist Team