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"
Galaxy And Gamma-ray Burst
25 January 1999
Gamma-ray bursts rule the high-energy sky and Saturday another brief, intense flash of gamma-rays from the cosmos triggered space-based detectors. The orbiting Compton Observatory's BATSE instrument quickly relayed the burst's approximate location to fast-slewing, ground-based cameras primed to search for an elusive optical flash. The result -- the GCN coordinated ROTSE-I telephoto array recorded a breakthrough detection only 22 seconds after the burst trigger. Larger telescopes too captured the burster's optical counterpart and at right is an image identifying the candidate from the 60 inch Palomar reflector. At left is a prior sky survey image of the region which astonishingly shows a discernible smudge near the same position, likely the burster's host galaxy. The galaxy and bright burst suggest that this is the closest yet localized gamma-ray burst! Still, believed to be the most powerful explosions in the Universe, the source of the incredible energy of gamma-ray bursts remains a mystery. Update: Reported redshift measurements now suggest that the burst is instead a very distant one. The galaxy in the survey image may not be the gamma-ray burst host but a foreground galaxy which by chance lies along the line of sight from Earth to the burster.