Black hole: A dense, compact object whose gravitational pull is so strong that — within a certain distance of it — nothing can escape, not even light.
Galaxy: A system of stars, gas, dust and dark matter all held together by gravity. Our home galaxy is the Milky Way.
Light year: The distance that light, moving at a constant speed of 300,000 km/s, travels in one year. One light year is about 10 trillion kilometers.
Star: A glowing ball of gas held together by its own gravity and powered by nuclear fusion in its core.
Supernova: Explosive death of a star -- one of the most energetic events of the universe and may temporarily outshine the rest of the galaxy in which it resides.