An astronomer's tool is a TELESCOPE, which is an instrument used to observe distant objects in the sky. Telescopes allow astronomers to see stars, planets, galaxies, and other celestial bodies that are too far away to be seen with the naked eye. Telescopes come in many different sizes and shapes, and can be used to observe both visible light and other forms of radiation. Telescopes have been used for centuries to study the night sky, and they continue to be an essential tool for astronomers today.