... Editor's Note: In response to requests from a number of readers, we have put together two resource lists that educators at all levels can use in their teaching, lecturing, and study assignments: an Introductory Astronomy Glossary and a Basic Astronomy Library . ... Binary Star : A system of two stars, orbiting around one another. ... The "Milky Way'' galaxy, of which our Sun is a part, is a spiral galaxy with a disk about 100,000 light-years across containing roughly 400 billion stars. ...