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"
Zodiacal Light Panorama
5 April 2012
Sweeping from the eastern to western horizon, this 360 degree panorama follows the band of zodiacal light along the solar system's ecliptic plane. Dust scattering sunlight produces the faint zodiacal glow that spans this fundamental coordinate plane of the celestial sphere, corresponding to the apparent yearly path of the Sun through the sky and the plane of Earth's orbit. The fascinating panorama is a mosaic of images taken from dusk to dawn over the course of a single night at two different locations on Mauna Kea. The lights of Hilo, Hawaii are on the eastern (left) horizon, with the Subaru and twin Keck telescope structures near the western horizon. On that well chosen moonless night, Venus was shining as the morning star just above the eastern horizon, and Saturn was close to opposition. In fact, Saturn is seen immersed in a brightening of the zodiacal band known as the gegenschein. The gegenschein also lies near 180 degrees in elongation or angular distance from the Sun along the ecliptic. In the mosaic projection, the plane of our Milky Way Galaxy runs at an angle, crossing the horizontal band of zodiacal light above the two horizons. Nebulae, stars, and dust clouds of the bulging galactic center are rising in the east.