Galaxies beyond ours

At a glance

Key points

Andromeda is the nearest large spiral. On the largest scales, galaxies line filaments around voids in the cosmic web.

• Galaxy collisions reshape arms and trigger bursts of star formation
• Dark matter outweighs ordinary matter about five to one
• Quasars are feeding supermassive black holes

How we learned they are separate

Until the 1920s many astronomers thought spiral nebulae were clouds inside the Milky Way. In 1923 Edwin Hubble, working with the 100-inch telescope on Mount Wilson, found a Cepheid variable star in Andromeda that brightened and faded every 31.4 days. A Cepheid's period reveals its true brightness, so its faintness gave a distance far beyond the edge of our galaxy. His first figure was too small; Andromeda is now placed about 2.5 million light-years away.

Kinds of galaxy

Hubble also sorted galaxies by shape. Spirals such as ours have flat discs and arms where stars form. Ellipticals are smooth and mostly old, and the giants among them, like M87, hold trillions of stars. Lenticulars have discs without arms, and irregulars, such as the Magellanic Clouds, have no tidy shape at all. By number, small, faint dwarf galaxies are by far the most common type.

A sense of scale

A large spiral spans about 100,000 light-years. Galaxies cluster into groups like our Local Group and into clusters of hundreds to thousands, and those gather into superclusters such as Laniakea, about 520 million light-years across. A survey at the depth of Hubble's Ultra Deep Field would count a couple of hundred billion galaxies across the sky; one study that adds the faintest dwarfs puts the total near two trillion.

Dark matter and collisions

Stars in the outer parts of spiral galaxies orbit faster than their visible matter can explain, as Vera Rubin and colleagues showed from the 1970s onward. Lensing and the motions of galaxies in clusters tell the same story: something unseen outweighs ordinary matter about five to one, and what it is remains unknown. When galaxies collide, their stars almost never hit one another because the gaps are so vast; it is the gas clouds that crash together and ignite new stars.

Related entries

• Andromeda GalaxyM31 · the nearest large spiral
• Messier 87Giant elliptical with the first imaged black hole
• MoM-z14 and JADES-GS-z14-0The most distant galaxies confirmed
• The cosmic webFilaments, voids and Laniakea

Sources

1. Rubin & Ford 1970, ApJ 159:379 (opens in a new tab)
2. NASA Science: Hubble Messier catalog: M31 (opens in a new tab)
3. NASA Science: Webb pushes boundaries of the observable universe (opens in a new tab)
4. Tully et al. 2014, Nature (arXiv) (opens in a new tab)
5. NASA Science: Hubble views the star that changed the universe (opens in a new tab)
6. Conselice et al. 2016, ApJ 830:83 (arXiv) (opens in a new tab)

Last reviewed 10 Oct 2026 against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.