A galaxy is a huge system of stars, gas, dust and dark matter held together by gravity. In the 1920s Edwin Hubble showed that many fuzzy “nebulae” were galaxies far beyond the Milky Way, and in 1926 he sorted them by shape. His “tuning fork” scheme, refined since, is still used.
Spiral galaxies
A flat, rotating disk with spiral arms around a central bulge. The arms are full of gas, dust and young blue stars, while the bulge holds older stars. Examples: the Andromeda Galaxy (M31) and the Whirlpool Galaxy (M51).
Barred spiral galaxies
Spirals with a straight bar of stars across the center, from which the arms unwind. Roughly two thirds of spirals have a bar. The Milky Way is a barred spiral, see the Milky Way’s place in the universe.
Elliptical galaxies
Smooth, featureless ovals or spheres of mostly old, reddish stars, with little gas and few new stars forming. They range from dwarf ellipticals to giants containing trillions of stars, often at the centers of galaxy clusters. Example: M87, whose central black hole was the first ever imaged, see black holes.
Lenticular galaxies
A transition type: a disk and bulge like a spiral, but without arms and with little gas, so star formation has mostly stopped. Example: the Spindle Galaxy (NGC 5866).
Irregular galaxies
No clear shape, often because of gravitational interactions. Many are small and rich in gas, forming stars actively. Examples: the Large and Small Magellanic Clouds, satellites of the Milky Way visible to the naked eye from the Southern Hemisphere.
Other categories
- Dwarf galaxies: the most numerous type, often orbiting larger galaxies.
- Peculiar and interacting galaxies: distorted by collisions, like the Antennae.
- Active galaxies: galaxies with a bright, feeding central black hole, including quasars, see quasars.
Shape tells history
Hubble once thought galaxies evolved along his diagram, but that idea was abandoned. Shapes reflect a galaxy’s past: mergers tend to turn spirals into ellipticals, while gas supply and environment decide whether stars keep forming, see how galaxies form and evolve.
