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"
The Porpoise Galaxy from Hubble
10 May 2020
What's happening to this spiral galaxy? Just a few hundred million years ago, NGC 2936, the upper of the two large galaxies shown, was likely a normal spiral galaxy -- spinning, creating stars -- and minding its own business. But then it got too close to the massive elliptical galaxy NGC 2937 below and took a dive. Dubbed the Porpoise Galaxy for its iconic shape, NGC 2936 is not only being deflected but also being distorted by the close gravitational interaction. A burst of young blue stars forms the nose of the porpoise toward the right of the upper galaxy, while the center of the spiral appears as an eye. Alternatively, the galaxy pair, together known as Arp 142, look to some like a penguin protecting an egg. Either way, intricate dark dust lanes and bright blue star streams trail the troubled galaxy to the lower right. The featured re-processed image showing Arp 142 in unprecedented detail was taken by the Hubble Space Telescope last year. Arp 142 lies about 300 million light years away toward the constellation, coincidently, of the Water Snake (Hydra). In a billion years or so the two galaxies will likely merge into one larger galaxy.