University of Wisconsin
The Why Files: Things That Go Bump in Space
Contains an introduction to x-ray astronomy, describing how x-rays are being used by scientists to learn about the universe. Also contains links to sites describing black holes, dark matter, neutron stars, and how they are investigated...
Curated OER
Sky Server: Scavenger Hunt
In this site, from the SkyServer of the Sloan Digital Sky Survey users hunt for stars, galaxies, quasars, asteroids and meteors. Learn what they look like and how to find them.