Sun
Moon
Any of the nine planets
Asteroids
Comets
DNA
Genetic Engineering
Biography of Sagan or other astronomer or scientist
SETI, CETI
Life of a star
Supernovae
Pulsars
Quasars
Galaxies
Black Holes
Satellites
Terraforming
Meteorites
Ozone Layer
Greenhouse Effect
Nuclear winter
Stonehenge and other ancient observatories
Constellations
Manned space travel
Unmanned space travel/Robotic space travel
Big Bang
How the brain stores information
Libary of Alexandria
Ptolemaic system
Copernican system
Kepler's laws
Relativity
Evolution
Tools of astronomy (Radio telescopes, IR, gamma ray, ultraviolet, etc.)
Astrology
History of astronomy