Galaxy

A large system of stars, interstellar gas, dust, and dark matter bound together by gravity

A distant galaxy captured by the Sloan Digital Sky Survey

A galaxy is a massive, gravitationally bound system consisting of stars, stellar remnants, interstellar gas, dust, and dark matter. The word is derived from the Greek "galaxias" (γαλαξίας), literally meaning "milky", a reference to our own Milky Way galaxy. Galaxies range in size from dwarfs with just a few billion stars to giants with one hundred trillion stars, each orbiting their galaxy's center of mass.

Types of Galaxies

The Hubble sequence, a morphological classification scheme for galaxies

Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926.

Elliptical Galaxies

Elliptical galaxies have an ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly elongated structures. These galaxies are generally composed of older, low-mass stars, with little interstellar matter, and thus have minimal star formation activity.

Spiral Galaxies

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The Andromeda Galaxy, a spiral galaxy

Spiral galaxies consist of a flat, rotating disk containing stars, gas, and dust, and a central concentration of stars known as the bulge. These are often surrounded by a halo of stars, many of which reside in globular clusters. Spiral galaxies are further divided into normal spirals and barred spirals, depending on the presence of a central bar structure.

Irregular Galaxies

Irregular galaxies do not fit into the standard Hubble classification scheme. They lack a distinct shape and structure, often appearing chaotic. These galaxies may have been distorted by gravitational interactions with other galaxies.

Galaxy Formation and Evolution

Galaxy Cluster Abell 1413, a massive cluster of galaxies

Galaxies are thought to have formed from small fluctuations in the density of matter in the early universe. Over time, these fluctuations grew under the influence of gravity, leading to the formation of stars and galaxies. The process of galaxy formation and evolution is complex and involves the interplay of various physical processes, including star formation, supernova explosions, and interactions with other galaxies.

Components of Galaxies

Galaxies are composed of several key components:

• Stars: The primary constituents of galaxies, stars are formed from the gravitational collapse of gas clouds.
• Interstellar Medium: The gas and dust that exist in the space between stars, playing a crucial role in star formation.
• Dark Matter: A form of matter that does not emit or interact with electromagnetic radiation, making it invisible and detectable only via its gravitational effects.

Galaxy Interactions

NGC 3923, an elliptical shell galaxy

Galaxies frequently interact with one another, leading to a variety of phenomena such as galaxy mergers, tidal interactions, and the formation of new structures. These interactions can trigger bursts of star formation and significantly alter the morphology of the galaxies involved.

Rotation Curves and Dark Matter

Rotation curve of spiral galaxy Messier 33 (Triangulum)

The rotation curves of spiral galaxies provide evidence for the existence of dark matter. Observations show that the outer regions of galaxies rotate at higher velocities than would be expected based on the visible mass alone, suggesting the presence of an unseen mass component.

Related Pages

• Milky Way
• Andromeda Galaxy
• Dark matter
• Star formation
• Hubble sequence