Perspective

Sometimes it is nice to have perspective.  Our Milky Way Galaxy is 105,000 light-years (617 quadrillion miles) across.  

This is an artist's rendering based on astronomical data.  It is difficult to see everything because interstellar dust gets in the way.  We used to think that our solar system was closer to the edge than it actually is.  This was revised a few years ago.

I wondered why there are spiral arms.  All the stars in the spiral arms should be moving at different speeds depending upon their distance from the center, so you would not expect them to clump together into a structure.  The technical explanation as to why this happens is quite complicated, and there are multiple reasons for it.  The Milky Way is so big that it has smaller galaxies orbiting around it, and their gravitational influence is part of the reason that the arms form.

In about 2.5 billion years, the Milky Way Galaxy will begin to collide with the bigger Andromeda Galaxy.  Eventually, the two galaxies will merge into a larger galaxy.