Understanding Galaxies: Structure, Types, and Evidence

Definition of a galaxy and its components

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 supermassive black holes at its center. Galaxies vary widely in size and star count, from dwarf galaxies with fewer than a thousand stars to the largest galaxies with up to around a hundred trillion stars, and most of their mass is typically dark matter rather than visible matter. Galaxies are described not only by what they contain but also by how they look and behave. Common morphological categories include elliptical, spiral, and irregular galaxies, with the Milky Way serving as an example of a spiral galaxy. Some galaxies show special properties such as interactions with other galaxies, unusually high star-formation rates (starbursts), or the presence of an active galactic nucleus (AGN), which can produce powerful energy output and jets. Because galaxies lack a sharp physical boundary, their “components” and “sizes” are often characterized using observational conventions (e.g., angular or isophotal diameters, and half-light/effective radius).

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Etymology of the word galaxy

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