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.
Prof Brian Cox takes a closer look at the spiral galaxy Andromeda.
Prof Brian Cox takes a photograph of the spiral galaxy Andromeda, seeing the light it emitted 2.5 million years ago.
BBC News reports on dramatic Hubble pictures.
In 2005, to mark Hubble's 15th birthday, new pictures were released showing stunning views of the Whirlpool galaxy and the Eagle Nebula. The servicing mission mentioned at the end of this clip eventually took place.
Prof Brian Cox looks back 13 billion years to distant galaxies.
An image from the Hubble Space Telescope reveals the distant light from galaxies 13 billion years old; the oldest ever found.
Prof Brian Cox studies the colour of stars to understand how the Universe began.
Prof Brian Cox explains how we can understand the origins of the Universe through differing wavelengths of light emitted by stars.
Use the constellation Pegasus to find interesting objects in the autumn night sky.
Patrick Moore and his guest Dr John Mason discuss the main constellation northern hemisphere stargazers can see when looking south in autumn - Pegasus. In mythology Pegasus was a flying horse, in the night sky it is a square. Sights in this region of the sky include star clusters and a group of five galaxies called Stephan's Quintet.
A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, an interstellar medium of gas and dust, and dark matter, an important but poorly understood component. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Examples of galaxies range from dwarfs with as few as ten million (107) stars to giants with one hundred trillion (1014) stars, each orbiting their galaxy's own center of mass.
Galaxies contain varying numbers of planets, star systems, star clusters and types of interstellar clouds. In between these objects is a sparse interstellar medium of gas, dust, and cosmic rays. Supermassive black holes reside at the center of most galaxies. They are thought to be the primary driver of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy is known to harbor at least one such object.
Galaxies have been historically categorized according to their apparent shape, usually referred to as their visual morphology. A common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped with dusty, curving arms. Those with irregular or unusual shapes are known as irregular galaxies and typically originate from disruption by the gravitational pull of neighboring galaxies. Such interactions between nearby galaxies, which may ultimately result in a merger, sometimes induce significantly increased incidents of star formation leading to starburst galaxies.
There are probably more than 170 billion galaxies in the observable universe. Most are 1,000 to 100,000 parsecs in diameter and usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic meter. The majority of galaxies are organized into a neither fully random nor fully deterministic set of associations known as galaxy groups and clusters, which, in turn usually form larger superclusters. At the largest scale, these associations are generally arranged into sheets and filaments, which are surrounded by immense voids.