In addition to planets and stars, the universe contains gas, dust, magnetic fields, and high-energy particles called cosmic rays. Stars are grouped into galaxies. The sun is one of more than 100 billion stars in a giant spiral galaxy called the Milky Way. This galaxy is about 100,000 light-years across. Galaxies tend to be grouped into clusters.
Some clusters appear to be grouped into superclusters.
The Milky Way is part of a cluster known as the Local Group. This cluster is about 3 million light-years in diameter. Also in the cluster are two giant spirals known as the Andromeda Galaxy and M33 and about 30 small galaxies, also known as dwarf galaxies. The Local Group is part of the Local Supercluster, which has a diameter of about 100 million light-years. No one knows whether the universe is finite or infinite in size.