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"
Downtown Auriga
13 February 2014
Rich in star clusters and nebulae, the ancient constellation of Auriga, the Charioteer, rides high in northern winter night skies. Spanning nearly 24 full moons (12 degrees) on the sky, this deep telescopic mosaic view recorded in January shows off some of Auriga's most popular sights for cosmic tourists. The crowded field sweeps along the plane of our Milky Way galaxy in the direction opposite the galactic center. Need directions? Near the bottom of the frame, at the Charioteer's boundary with Taurus the Bull, the bright bluish star Elnath is known as both Beta Tauri and Gamma Aurigae. On the far left and almost 3000 light-years away, the busy, looping filaments of supernova remnant Simeis 147 cover about 150 light-years. Look toward the right to find emission nebula IC 410, significantly more distant, some 12,000 light-years away. Star forming IC 410 is famous for its embedded young star cluster, NGC 1893, and tadpole-shaped clouds of dust and gas. The Flaming Star Nebula, IC 405, is just a little farther along. Its red, convoluted clouds of glowing hydrogen gas are energized by hot O-type star AE Aurigae. Two of our galaxy's open star clusters, Charles Messier's M36 and M38 line up in the starfield above, familiar to many binocular-equipped skygazers.