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 Tail of a Wonderful Star
17 August 2007
To seventeenth century astronomers, Omicron Ceti or Mira was known as a wonderful star, a star whose brightness could change dramatically in the course of about 11 months. Mira is now seen as the archetype of an entire class of long-period variable stars. Surprisingly, modern astronomers have only recently discovered another striking characteristic of Mira -- an enormous comet-like tail nearly 13 light-years long. The discovery was made using ultraviolet image data from the Galaxy Evolution Explorer (GALEX) satellite. Billions of years ago Mira was likely similar to our Sun, but has now become a swollen red giant star, its outer layers of material blowing off into interstellar space. Fluorescing in ultraviolet light, the cast off material trails behind the giant star as it plows through the surrounding interstellar medium at 130 kilometers per second. The amount of material in Mira's tail is estimated to be equivalent to 3,000 times the mass of planet Earth. About 400 light-years away toward the constellation Cetus, Mira is presently too faint to be seen by the unaided eye, but will become visible again in mid-November.