Structures · Universe scale
Galaxy
A vast system of stars, gas, dust, and dark matter, held together by gravity. Galaxies range from dwarf irregulars with a few hundred million stars to giant ellipticals with hundreds of trillions. The observable universe contains an estimated 2 trillion galaxies.
Our galaxy
The Milky Way is a barred spiral galaxy ~100,000 light-years across, containing 100–400 billion stars. The Sun is located ~26,000 light-years from the centre.
Source: HubbleSite Galaxies
Galaxies — islands of stars in the universe
Until the 1920s, astronomers debated whether "spiral nebulae" were clouds within the Milky Way or distant galaxies ("island universes") in their own right. In 1923, Edwin Hubble resolved Cepheid variable stars in the Andromeda Nebula and proved it lay far beyond the Milky Way — the discovery that instantly expanded our concept of the universe by an almost incomprehensible factor.
Galaxies are not uniformly distributed. They cluster into groups (the Milky Way belongs to the Local Group — about 54 galaxies including Andromeda and the Magellanic Clouds), which aggregate into clusters, and clusters into superclusters. On the largest scales, galaxies trace a cosmic web — filaments and walls of galaxies surrounding vast, nearly empty voids.
For more on how galaxies' distances are measured, see our cosmic distances guide. The distribution of dark matter within galaxies is covered in our dark matter guide.
Frequently asked questions
- Galaxies are classified by shape. Spiral galaxies — like the Milky Way and Andromeda — have a flat disc with curved arms of stars, gas, and young blue stars winding outward from a central bulge. Barred spiral galaxies (like the Milky Way) have a bar-shaped structure across the centre from which the spiral arms emerge. Elliptical galaxies range from nearly spherical to elongated football shapes; they are mostly older, redder stars with little gas or active star formation, often the result of galaxy mergers. Lenticular galaxies are disc-shaped like spirals but lack prominent spiral arms. Irregular galaxies have no regular shape — often the result of gravitational disturbances. The Hubble sequence (tuning-fork diagram) organises these classes.
- The Milky Way is a barred spiral galaxy about 100,000 light-years (30 kpc) in diameter, with a disc roughly 1,000 light-years thick. It contains an estimated 100–400 billion stars and at least as many planets. The Sun lies in the Orion Arm (Orion Spur) — a minor spiral arm — about 26,000 light-years (8 kpc) from the galactic centre. The Sun orbits the galactic centre at about 220 km/s, taking roughly 225–250 million years for one complete orbit — one "galactic year". At the galactic centre lies Sagittarius A* (Sgr A*), a supermassive black hole of about 4 million solar masses, imaged by the Event Horizon Telescope in 2022.
- Based on a 2016 analysis of Hubble Space Telescope deep-field images, astronomers estimated approximately 2 trillion (2 × 10¹²) galaxies in the observable universe — about 10 times more than previous estimates. However, most are small, faint dwarf galaxies undetectable at cosmological distances. The number of stars in the observable universe is consequently around 10²⁴ — more than the grains of sand on all Earth's beaches combined. The James Webb Space Telescope is pushing the frontier further, discovering galaxies at redshift z > 13, formed less than 300 million years after the Big Bang.
- The Andromeda Galaxy (M31) is approaching the Milky Way at about 110 km/s and will merge with it in approximately 4.5 billion years. Despite the enormous number of stars involved, stellar collisions are extremely unlikely because the distances between individual stars are so vast — even within a galaxy, stars are separated by light-years. What will happen: the two discs will pass through each other multiple times over billions of years, gravitational interactions will distort both galaxies' structures, and intense bursts of new star formation will occur as gas clouds collide. The final merged galaxy — sometimes called Milkomeda — will likely be a large elliptical galaxy. See our cosmic distances guide for how these distances are measured.
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