Celestia is an explorable, 3D rendering of our universe that allows users to travel vast distances at vast speeds from the comfort of their desktops. The program contains over 100,000 stars and 10,000 galaxies and allows for user modifications and add-ons.
If you haven’t played around with Celestia, get the free download from their website, and start with the Demo. The Demo takes you on a quick tour of our solar system, zips across the galaxy to a huge red giant, then zooms out for an outside view of the Milky way.
Ibiblio hosts the Celestia Motherlode, a collection of addons surface textures, models and astronomical data, which enables Celestia users to easily create their own detailed, realistic copies of the known (and imagined) universe. Add atmospheres to planets, chase some well-known spaceships around (USS enterprise, anyone?) and follow realtime orbits of planets, moons, and stars.
Go to ibiblio’s Celestia page and start exploring!