Cartwheel Galaxy 
Galaxies are “island universes” — immense collections of stars floating through the vast cosmos. Large galaxies come in a variety of shapes. Some, like the Whirlpool galaxy, are beautiful spirals. Others, like our own Milky Way, are spirals with “bars” of stars across their centers. And still others look like fat, fuzzy footballs.
But some large galaxies are a bit different. Ring galaxies are so rare that we’d have to travel a long way to find an example.
As the name implies, a ring galaxy looks like a circle of starlight.
It owes this unusual appearance to a collision. A small galaxy plunges through the heart of a large spiral galaxy and comes out the other side. Just as a pebble dropped into a pond generates an expanding wave of water, the gravity of the intruder galaxy creates a wave in the spiral galaxy. The wave causes gas and dust to pile up in a ring that expands away from the galaxy’s center. The gas and dust then give birth to bright new stars, creating a galaxy whose brightest stars reside in a ring.
Such head-on collisions don’t happen often, so ring galaxies are rare. In fact, one of the closest, the Cartwheel galaxy, is 420 million light-years away. It’s in Sculptor, a faint constellation that scoots low above the southern horizon tonight. Pictures from Hubble Space Telescope show the galaxy’s ring of newborn stars — the product of a collision that took place before the reign of the dinosaurs.
Script by Ken Croswell, Copyright 2011