Definition of a galaxy

Summary

A galaxy is a gravitationally bound physical system made up of stars, stellar remnants, interstellar gas and dust, and dark matter. It is held together by gravity and can contain anywhere from fewer than a thousand stars in dwarf galaxies to around a hundred trillion stars in the largest known galaxies. Although stars and nebulae are visible, most of a galaxy’s mass is typically dark matter, and many galaxies also host supermassive black holes at their centers. Galaxies are classified mainly by their visual morphology—such as elliptical, spiral, or irregular—but they can also be distinguished by special properties like interactions with other galaxies, unusual star-formation rates, or the presence of an active galactic nucleus (AGN). The Milky Way is an example of a spiral galaxy, and the observable universe is estimated to contain from hundreds of billions to trillions of galaxies, with typical diameters ranging from about 1,000 to 100,000 parsecs (roughly 3,000 to 300,000 light-years).

Key Points

• 1

  A galaxy is a gravity-bound system of stars, gas, dust, dark matter, and stellar remnants.

• 2

  Galaxies vary widely in size and are commonly classified by visual morphology (elliptical, spiral, irregular).

• 3

  Many galaxies contain supermassive black holes and may show special behaviors such as interactions, starbursts, or AGN activity.