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A galaxy is a gravity-bound system of stars, gas, dust, stellar remnants, and dark matter.
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).
A galaxy is a gravity-bound system of stars, gas, dust, stellar remnants, and dark matter.
Galaxies are commonly classified by visual morphology (elliptical, spiral, irregular) and by special behaviors such as starbursts or AGN activity.
Most galaxy mass is dark matter, while only a small fraction is visible as stars and nebulae.
Because galaxies fade gradually with distance from their centers, their sizes are measured using observational conventions like angular diameter, isophotal diameter, and effective (half-light) radius.
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas and dust, and dark matter.
The interstellar medium is the gas and dust that exists between stars within a galaxy.
Dark matter is an unseen form of matter that makes up most of a galaxy’s mass and provides much of its gravitational influence.
A supermassive black hole is a very massive black hole commonly found at the centers of galaxies.
Galactic morphology refers to the visual structural classification of galaxies, such as elliptical, spiral, or irregular.
An AGN is a compact, extremely luminous region at a galaxy’s center powered by accretion onto a central supermassive black hole.
A starburst galaxy is a galaxy experiencing an unusually high rate of star formation, often triggered by interactions or mergers.
Isophotal diameter is a conventional galaxy size measured by the extent of light down to a specified surface brightness level (an isophote).
Effective radius is the radius within which half of a galaxy’s total observed brightness (flux) is emitted.
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