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"
The Milky Way Over Mauna Kea
27 January 2009
Have you ever seen the band of our Milky Way Galaxy? In a clear sky from a dark location at the right time, a faint band of light becomes visible across the sky. Soon after your eyes become dark adapted, you might spot the band for the first time. It may then become obvious. Then spectacular. One reason for a growing astonishment might be the realization that this fuzzy swath contains billions of stars and is the disk of our very own spiral galaxy. Since we are inside this disk, the band appears to encircle the Earth. Visible in the above image, high above in the night sky, the band of the Milky Way Galaxy arcs. The bright spot just below the band is the planet Jupiter. In the foreground lies the moonlit caldera of the volcano Haleakala, located on the island of Maui in Hawaii, USA. A close look near the horizon will reveal light clouds and the dark but enormous Mauna Kea volcano on the Big Island of Hawaii. If you have never seen the Milky Way band or recognized the planet Jupiter, this year may be your chance. Because 2009 is the International Year of Astronomy, an opportunity to look through a window that peers deep into the universe may be coming to a location near you.