A real-time visual space simulation

Celestia is a free real-time space simulation that lets you experience
our universe in three dimensions. Unlike most planetarium software,
Celestia doesn't confine you to the surface of the Earth.
You can travel throughout the solar system, to any of over 100,000 stars,
or even beyond the galaxy. Travel in time to witness past and future
astronomical events. Celestia is expandable via add-on packages.