Our Solar System consists of the Sun, its family of planets, and some smaller bodies such as moons, asteroids, comets, and dwarf planets. Planets are spherical, nonluminous bodies that orbit a star and shine by reflected light. The Sun is a star, a self-luminous ball of hot gas. A galaxy is a great cloud of stars, gas, and dust held together by the combined gravity of all its matter. Our home galaxy is called the Milky Way and contains more than 100 billion stars. The Milky Way is just one of billions of galaxies in the universe. Our galaxy is part of a group of a few dozen galaxies. Galaxies are commonly grouped together in such clusters. Clusters of galaxies are grouped into superclusters. Superclusters are linked to form long filaments and walls outlining nearly empty voids. These filaments and walls appear to be the largest structures in the Universe.
The universe began approximately 14 billion years old in an event called the Big Bang. The first stars began to shine about 400 million years after the Big Bang. The Solar System formed about 4.6 billion years ago.
The astronomical unit (AU) is the average distance from Earth to the Sun. The light-year (ly) is the distance light travels in one year.
—January 2021
—January 2023
—November 2023
—March 2024
—July 2025
Lance Jones 