Starship Asterisk*

Many of the beautiful effects shown in some of the APOD photos are the result of a collision or near miss of two galaxies.

The fact that the universe is expanding implies (at least to me) that its galaxies and other such large objects are steadily moving away from each other.

How is it possible, then, for two galaxies to collide or pass near each other?

For example, in today's photo (09/24/04), the explanation says that "the Fornax cluster core is moving toward the lower right." Moving in relation to what? And if everything is expanding away from the Big Bang singularity, like shrapnel from a grenade, what caused Fornax to change direction and travel sideways?

Good question.

The answer is actually surprisingly simple though, but not obvious.

Galaxies are divided up into groups--for example, our Milky Way Galaxy along with Andromeda Galaxy and several other smaller ones are in the "Local Group."

While the entire group as a whole may be expanding outwards away from other groups, each individual galaxy may have different motion toward each other--thus, galactic collisions can still happen, even as the universe as a whole is expanding.

as well as the unseen force called repulsion,which is spreading the universe out their is also the unseen force called attraction, which is pulling parts of it together.
an example would be the clusters of galaxies spread throughout the universe. these are groupings of galaxies which have a common 'attraction' yet are also propelled by repulsion. evidence indicates that most galaxies are in some sort of a 'stream' of galaxies moving along a common and often paralell path. the minute differences in their course causes galaxies to 'Collide' and their attraction defeats the repulsion many times over This causes galactic mergers as evidence by many APOD photos http://antwrp.gsfc.nasa.gov/apod/ap040624.html