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, 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. Many galaxies are believed to have Supermassive black holes (SMBH) at their center. These are thought to be the primary driver of active galactic nuclei found at the core of such galaxies.
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.
