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"
A Galactic Cloud of Antimatter Credit:
1 May 1997
The center of our Milky Way Galaxy is full of surprises. Its latest spectacular is a mysterious cloud glowing in gamma rays produced by annihilating antimatter particles! Star Trek fans are all too familiar with the consequences of mixing matter (electrons) and antimatter (positrons) - the particles catastrophically annihilate converting their masses to energy according to Einstein's famous E=mc2. Positron/electron annihilation energy is emitted as gamma rays with photon energies of 511,000 electron volts. Searching for these high energy photons, the OSSE instrument onboard NASA's orbiting Compton Gamma Ray Observatory has recently produced this map of the Galactic Center (GC) region. As anticipated, it shows annihilation gamma rays as a bright spot at the GC with fainter horizontal emission from the galactic plane. Astoundingly, it also reveals a large and unexpected cloud of annihilation radiation, probably about 4,000 light years across, extending nearly 3,500 light years above the GC. What could have created this cloud? Associated with no previously known object, it seems to imply that a fountain of antimatter positrons streams from the GC. Present guesses about the source of the positrons include the violent and exotic environments surrounding starbirth, neutron star collisions, and black holes at the GC. Are there other such clouds in our Galaxy?