Ripples in the Milky Way's Disk

A Galaxy Hit-and-Run

Ripples found in the Milky Way’s disk reveal our galaxy survived an ancient hit-and-run. Now, astronomers might have caught the culprit.

Galaxies in the Universe: How Many are There?

Astronomers determine the number of galaxies in the universe by counting up the number visible in a tiny portion of the sky, and then accounting for all the regions of the observable universe. A 2013 study estimates that there are 225 billion galaxies in the observable universe.