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"
Historic Optical Flash Fades Credit and Copyright:
22 April 1997
The largest telescopes in the world have scrambled to point toward this faint, fading object. Why? Because it may well be the first active optical counterpart ever found for a gamma-ray burst, and could hold the clue to the distance scale to this most enigmatic class of astronomical objects. During the past two months, multiwavelength observations, claims, and counterclaims have been exchanged at a torrential pace, as astronomers wait impatiently to see what setting houses the fading transient. Above are two pictures of the optical transient (OT), one taken on February 28th, the other about a week later on March 8th. The OT arrows show the source that faded from view during this time. The OT has been examined by large telescopes including the Hubble Space Telescope and Keck, yet researchers are still scrambling to get answers to fundamental questions about its true nature. Does the OT actually move (exhibit proper motion)? Is there a faint galaxy superposed behind the OT or is this extended emission fading too? In the face of intense scrutiny with the world's most capable telescopes, the origin of gamma-ray bursts so far remains mysterious!