The light of a fireball comes from the glowing gas surrounding the solid meteoroid, rather than from the meteoroid itself. Each element glows with its characteristic set of colours. The eerie green light so common in many fireballs comes from atmospheric oxygen. Nitrogen from our atmosphere contributes blue, and sodium from the meteoroid can add yellow to the light. Magnesium, iron, calcium, and dozens of other elements add their own colours.
Many fireballs change colour as they descend and slow down.
A fireball that glows as bright as Venus, the bright evening star, may come from a pebble of iron or stone weighing barely an ounce. Astronomers estimate that a fireball must be somewhere between those two extremes if a piece will survive to reach the surface. An exploding fireball is technically called a bolide, from the Greek bolides, meaning a thrown spear. When fireballs explode near the end of their flight, the meteoric debris from the fall can scatter in hundreds of fragments spread over a wide area. The fragments of a fireball that reach Earth, called meteorites, are the best sources of pristine, or pure, interplanetary material.