Galaxy structure, classification, and observation

Definition of a galaxy and its components

A galaxy is a gravitationally bound system made of stars, stellar remnants, interstellar gas and dust, and dark matter. Its components are held together by gravity, and most of a galaxy’s mass is typically in the form of dark matter, while only a small fraction is visible as stars and nebulae. Galaxies vary widely in size, from dwarf galaxies with fewer than a thousand stars to the largest known galaxies containing on the order of hundreds of trillions of stars. Galaxies can be described by both their physical structure and their observable properties. They are commonly categorized by visual morphology—such as elliptical, spiral, or irregular—and may also be distinguished by special behaviors or features, including interactions with other galaxies, unusually high star-formation rates (starbursts), and the presence of an active galactic nucleus (AGN). Examples include the Milky Way (a spiral galaxy) and galaxies with energetic radio emission (radio galaxies), which are powered by jets from their central regions.

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Etymology of the word galaxy

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