Galaxies are made of stars, dust and dark matter, all held together by gravity. They come in a variety of shapes, sizes and ages, and many have black holes at their centers.
Galaxies contain a different number of planets, star systems, star clusters and types of interstellar clouds. In between them is a sparse interstellar medium of gas, dust and cosmic rays. The black holes at the center of most galaxies are considered to be the primary driver of active galactic nuclei found at the core, and their surroundings produce enormous amounts of energy that astronomers can see over great distances. Material surrounding the black hole is accelerated outwards by its jets. Other galaxies contain objects like quasars, the most energetic bodies in the universe, at their cores.
Galaxies are categorized according to their apparent shape, 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, and those with irregular shapes are known as irregular galaxies and typically originate from disruption by the gravitational pull of neighboring galaxies. Interactions between neighboring galaxies, which can result in a merger, sometimes induce significantly increased incidents of star formation leading to starburst galaxies.