Sun

The Sun is the star at the center of the solar system. It is a nearly perfect sphere of hot plasma, and is by far the most important source of energy for life on Earth. All planets in the solar system orbit the Sun, and it's effects on Earth have been recognized since prehistoric times. Some cultures (both in the old-world and in the post-apocalyptic wasteland) regard it as a deity or other religious symbol, and the synodic rotation of Earth and it's orbit around the sun are the basis of solar calendars.

Effects on Earth
The sun produces electromagnetic radiation that travels outwardly from it; the most well-known being sunlight. Sunlight not only acts as illumination, but also as fuel for life on Earth. The energy from the sun is vital to all life, but too much exposure can cause sunburns (a type of radiation burn).