An astronomer's sighting may include a variety of celestial objects, such as meteors, planets, comets, stars, novas, and nebulae. Meteors are small pieces of rock or metal that enter the Earth's atmosphere and burn up, leaving a streak of light in the sky. Planets are large bodies that orbit around stars, such as our own PLANET Earth. Comets are icy bodies that orbit around the sun, often leaving a tail of gas and dust behind them. Stars are large, bright balls of gas that produce their own light and heat. Novas are stars that suddenly become much brighter for a short period of time. Nebulae are clouds of gas and dust in space that can form stars, planets, and other objects. All of these objects can be seen by an astronomer with the right equipment.