New! Sign up for our free email newsletter.
Reference Terms
from Wikipedia, the free encyclopedia

Galaxy

A galaxy is a massive, gravitationally bound system consisting of stars, an interstellar medium of gas and dust, and dark matter. Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion stars, all orbiting a common center of mass. Galaxies can also contain many multiple star systems, star clusters, and various interstellar clouds.

Historically, galaxies have been categorized according to their apparent shape (usually referred to as their visual morphology). A common form is the elliptical galaxy, which has an ellipse-shaped light profile. Spiral galaxies are disk-shaped assemblages with curving, dusty arms. Galaxies with irregular or unusual shapes are known as peculiar galaxies, and typically result from disruption by the gravitational pull of neighbouring galaxies. Such interactions between nearby galaxies, which may ultimately result in galaxies merging, may induce episodes of significantly increased star formation, producing what is called a starburst galaxy. Small galaxies that lack a coherent structure could also be referred to as irregular galaxies.

There are probably more than one hundred billion galaxies in the observable universe. Most galaxies are 1,000 to 100,000 parsecs in diameter and are usually separated by distances on the order of millions of parsecs (or megaparsecs). Intergalactic space (the space between galaxies) is filled with a tenuous gas of an average density less than one atom per cubic metre. The majority of galaxies are organized into a hierarchy of associations called clusters, which, in turn, can form larger groups called superclusters. These larger structures are generally arranged into sheets and filaments, which surround immense voids in the universe.

Although it is not yet well understood, dark matter appears to account for around 90% of the mass of most galaxies. Observational data suggests that supermassive black holes may exist at the center of many, if not all, galaxies. They are proposed to be the primary cause of active galactic nuclei found at the core of some galaxies. The Milky Way galaxy, home of Earth and the solar system, appears to harbor at least one such object within its nucleus.

Related Stories
 


Space & Time News

June 9, 2025

Supermassive black holes might naturally replicate the colossal energies of man-made particle colliders possibly even revealing dark matter offering a cosmic shortcut to discoveries that would ...
Astronomers have revealed new research showing that millions of new solar system objects are likely to be detected by a brand-new facility, which is expected to come online later this ...
Scientists have identified a new source of energetic particles near the Sun. These definitive observations were made by instruments aboard NASA's Parker Solar Probe, which detected the powerful phenomena as the spacecraft dipped in and out of the ...
Tracing the origin of an ultra-hot exoplanet: The chemical composition of WASP-121b suggests that it formed in a cool zone of its natal disc, comparable to the region of gas and ice giants in our Solar System. Methane indicates unexpected ...
Researchers have engineered a laser device smaller than a penny that they say could power everything from the LiDAR systems used in self-driving vehicles to gravitational wave detection, one of the most delicate experiments in existence to observe ...
A series of studies sheds light on the origins and characteristics of intermediate-mass black ...
The process of necrosis, a form of cell death, may represent one of the most promising ways to change the course of human aging, disease and even space travel, according to a new ...
A multi-sensor detection of an intense gamma-ray flash, which occurs when two lightning leaders collided, has been observed. Observations across a wide radiation spectrum enabled precise measurement of the electric current produced during this ...
Astronomers have explained how a galaxy cluster maintains its heat, despite emitting X-rays that cool the hot gas at its center. The group discovered the existence of a fast-moving, high-temperature ...
A team of international astronomers have discovered a new cosmic object emitting both radio waves and ...
A series of experiments support spectral data recently collected by the James Webb Space Telescope that found evidence that the icy surface of Jupiter's moon Europa is constantly changing. Europa's surface ice is crystallizing at different rates in ...
Scientists have observed anyons -- quasiparticles that differ from the familiar fermions and bosons -- in a one-dimensional quantum system for the first time. The results may contribute to a better ...

Latest Headlines

updated 12:56 pm ET