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"
NGC 4449: Star Stream for a Dwarf Galaxy
26 January 2012
A mere 12.5 million light-years from Earth, irregular dwarf galaxy NGC 4449 lies within the confines of Canes Venatici, the constellation of the Hunting Dogs. About the size of our Milky Way's satellite galaxy the Large Magellanic Cloud, NGC 4449 is undergoing an intense episode of star formation, evidenced by its wealth of young blue star clusters, pinkish star forming regions, and obscuring dust clouds in this deep color portrait. It also holds the distinction of being the first dwarf galaxy with an identified tidal star stream, faintly seen at the lower right. Placing your cursor over the image reveals an inset of the stream resolved into red giant stars. The star stream represents the remains of a still smaller infalling satellite galaxy, disrupted by gravitational forces and destined to merge with NGC 4449. With relatively few stars, small galaxies are thought to possess extensive dark matter halos. But since dark matter interacts gravitationally, these observations offer a chance to examine the significant role of dark matter in galactic merger events. The interaction is likely responsible for NGC 4449's burst of star formation and offers a tantalizing insight into how even small galaxies are assembled over time.