Galaxy
A galaxy is a vast, gravitationally bound system of stars, stellar remnants, gas, dust, and dark matter. They range in size from dwarf galaxies with a few thousand stars to giants containing over a trillion stars and spanning more than a million light‑years in diameter.
There are thought to be over 100 billion galaxies in the universe, but recent studies—including data from NASA and ESA—suggest this number may exceed two trillion.
Galaxies are classified by shape into three main types: spiral (like the Milky Way), elliptical, and irregular. Active galaxies—those emitting exceptionally bright radiation from their cores—make up around 10 % of observed galaxies.
The Milky Way is a barred spiral galaxy, part of the Local Group of over 20 galaxies. It contains over 100 billion stars, a stellar disk ~100,000 light‑years across, and a central supermassive black hole.
Galaxies often cluster together in groups and clusters, which themselves are components of larger structures like superclusters and filaments. These structures form the cosmic web that defines the large‑scale structure of the universe.
Galaxies evolve through processes like mergers—where two galaxies interact and combine—and internal star formation driven by gas dynamics. Interactions can trigger starbursts, dramatically increasing a galaxy’s stellar birth rate.
Supermassive black holes, found at the centers of most large galaxies, power active galactic nuclei (AGN), emitting extreme energies and sometimes launching relativistic jets. These AGN influence both galactic and intergalactic environments.
Dark matter makes up most of a galaxy’s mass. Its presence is inferred from phenomena like flat rotation curves and gravitational lensing—despite being invisible, it profoundly shapes galaxy formation and dynamics.
Our understanding continues to expand thanks to advanced observations from missions like ESA’s Herschel, Gaia, and NASA/ESA’s Hubble and James Webb Space Telescopes, along with theoretical frameworks and cosmological simulations.
Sources
- ESA – Galaxies (CESAR booklet)
- NASA Science – Galaxy Basics
- ESA – Every point is a galaxy (Herschel)
- NASA Science – Galaxy Evolution & Mergers
- NASA Science – Galaxy Types
- ESA – Galaxy groups and clusters (The scale of the Universe)
- NASA Science – Large Scale Structures
- NASA – Hubble’s Galaxies
- ESA – The Galaxy in Context (arXiv)
- ESA – The Fundamental Properties of Galaxies (arXiv)
APODs including "Galaxy"
GLAST Gamma-Ray Sky Simulation
12 November 1998
This simulated image models the intensities of gamma rays with over 40 million times the energy of visible light, and represents how the sky might appear to the proposed Gamma-ray Large Area Space Telescope (GLAST) after its first year in orbit. Familiar steady stars are absent from the dramatic 80x80 degree field which looks directly away from the center of the Galaxy. Instead, the Geminga and Crab pulsars - bizarre, spinning stellar corpses known to be neutron stars - are the two brightest gamma-ray sources. These and other bright objects in the field, dense pulsars, monstrous active galaxies, and still unknown sources, have been detected by the Energetic Gamma-Ray Experiment Telescope (EGRET) on the orbiting Compton Gamma-Ray Observatory. However, most of the simulated point sources are new - extrapolating current ideas and anticipating discoveries resulting from GLAST's improved gamma-ray vision. The central broad band of faint gamma-ray emission is due to high-energy cosmic rays colliding with interstellar gas in the outer spiral arms of the Milky Way, while below is a diffuse energetic glow from prominent molecular clouds in Monoceros, Orion, Auriga, and Taurus. Intended to explore the most extreme energy sources in the distant cosmos and planned for launch in 2005, the GLAST mission is under development by NASA and a collaboration of U. S. and international partners.