Spiral galaxies like the Milky Way form new stars and are shaped like discs with central bulges.
Elliptical galaxies look like the bulges in spiral galaxies without the surrounding disc and mainly contain old stars.
Irregular galaxies do not have a standard shape and tend to have many new stars. Another type of galaxy called lenticular (also called S0) has a smaller disc, central bulge and is primarily full of older stars.
Image: A Hubble Space Telescope image of the Sombrero galaxy, M104 (credit: NASA)
Gravity gathers stars, gas, dust and dark matter.
Vera Rubin's star velocity measurements support theory.
Measurements of the velocities of stars orbiting in galaxies made by Dr Vera Rubin in the 1970s are evidence of dark matter's existence. Dark matter is thought to provide the "extra gravity" needed to reconcile the orbits of stars with Newton's law of gravity.
Patrick Moore and his guests discuss galaxies.
Sir Patrick Moore and his guests explain what galaxies are and discuss some of their interesting features.
The great astronomer worked at Mount Wilson Observatory.
This clip was filmed at a time when the Hubble Space Telescope had been discovered to have a flawed mirror, hence the reference to "failure". The Hubble Space Telescope went on to become one of the most successful NASA missions. This clip looks at some of the American astronomer Edwin Hubble's most important contributions to astronomy and the equipment he used at the Mount Wilson Observatory. The age of the Universe is now thought to be 13.7 billion years.
Hubble sees the Universe 700 million years after the Big Bang.
The Hubble Space Telescope has been used to create images of the furthest reaches of the Universe - only 700 million years after the Big Bang.
Researchers find a black hole at the galaxy's centre.
Professor Reinhard Genzel explains how he found strong evidence of a supermassive black hole at the centre of our galaxy, the Milky Way.
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few billion (109) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy's center of mass. Galaxies are categorized according to their visual morphology as elliptical,spiral and irregular. Many galaxies are thought to have black holes at their active centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the Big Bang. Previously, as of July 2015, EGSY8p7 was the most distant known galaxy, estimated to have a light travel distance of 13.2 billion light-years away.
Approximately 200 billion (2.0 × 1011) or, more recently, at least 2 trillion galaxies exist in the observable universe. Most of the galaxies are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs). The space between galaxies is filled with a tenuous gas having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into associations known as galaxy groups, clusters, and superclusters. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. The largest structure of galaxies yet recognised is a cluster of superclusters, that has been named Laniakea