Pebble, a smooth, rounded stone usually less than 2 inches (64 mm) in diameter. (A larger stone of this type is called a cobble, or cobblestone.) Pebbles are fragments of any kind of rock that have become worn by rubbing together in rivers or on beaches. A loose accumulation of pebbles is called gravel. Conglomerate is rock composed of pebbles cemented together.
When a star dies, it becomes a black hole. Once something passes through the it's gone for good, never to be seen or heard from again. If we lived next to a black hole, would it suck us in too?
Every day, astronomers unravel a little more of the universe's inner workings, but the jury is still out on 95 percent of its contents.