The Milky Way and the island-universe debate
The hazy band across the night sky is our own galaxy, the Milky Way, seen edge-on from inside. In 1610 Galileo's telescope showed the band to be countless stars. Yet as late as the early 1920s astronomers argued over whether "spiral nebulae" such as Andromeda were gas clouds within the Milky Way or separate "island universes" (the Shapley–Curtis Great Debate of 1920). In 1924 Edwin Hubble found Cepheid variable stars in Andromeda, measured its distance and proved it lay far outside our galaxy, enlarging the known universe at a stroke. The Milky Way is a barred spiral about 100,000 light-years across; the Sun sits in a spiral arm some 26,000 light-years from the centre and takes about 230 million years to complete one orbit.
Types of galaxy
In 1926 Hubble sorted galaxies by shape into ellipticals, spirals, barred spirals and irregulars; his "tuning-fork" scheme is still the basic framework. Galaxies gather into groups of dozens and clusters of thousands, which in turn trace a web-like large-scale structure. Hubble's law, the finding that galaxies recede at speeds proportional to their distance, became key evidence for the Big Bang theory.
| Type | Features | Example |
|---|---|---|
| Elliptical | Ball- or rugby-ball-shaped, mostly old stars | M87 |
| Spiral | Disc with spiral arms, active star formation | Andromeda Galaxy |
| Barred spiral | Central bar plus spiral arms | Milky Way |
| Irregular | No clear shape | Large Magellanic Cloud |
Black holes, dark matter and the future
Most large galaxies harbour a supermassive black hole at their centre. The Milky Way's, Sagittarius A*, is about four million solar masses, and in 2022 the Event Horizon Telescope imaged its shadow. In the 1970s Vera Rubin measured that stars in the outer parts of galaxies orbit faster than visible matter can explain, giving strong evidence that invisible dark matter makes up most of a galaxy's mass. The observable universe is estimated to contain around 200 billion galaxies, and possibly more. In roughly 4.5 billion years the Milky Way and Andromeda are expected to merge into a single giant elliptical galaxy, though stars are so far apart that direct collisions between them will be extremely rare.
FAQ
- Is the Milky Way the same as our galaxy?
- Yes, in everyday use. Strictly, "Milky Way" first meant the band of light in the sky, and by extension the whole galaxy the Solar System belongs to.
- Can you see another galaxy with the naked eye?
- Yes. From a dark site the Andromeda Galaxy, about 2.5 million light-years away, appears as a faint smudge, and from the Southern Hemisphere the Large and Small Magellanic Clouds are easy to see.
Sources reviewed: 2026-10-26
References
- Hubble, E. P. (1926). Extragalactic nebulae. The Astrophysical Journal, 64, 321–369.
- Event Horizon Telescope Collaboration. (2022). First Sagittarius A* Event Horizon Telescope results. I. The Astrophysical Journal Letters, 930, L12.
- Conselice, C. J., et al. (2016). The evolution of galaxy number density at z < 8 and its implications. The Astrophysical Journal, 830(2), 83.
- Binney, J., & Tremaine, S. (2008). Galactic Dynamics (2nd ed.). Princeton University Press.
- Kant, I. (1755). Allgemeine Naturgeschichte und Theorie des Himmels. Königsberg.
- Hubble, E. P. (1925). Cepheids in spiral nebulae. The Observatory, 48, 139–142.
- Hubble, E. (1936). The Realm of the Nebulae. Yale University Press.
- Rubin, V. C., & Ford, W. K. Jr. (1970). Rotation of the Andromeda Nebula from a spectroscopic survey of emission regions. The Astrophysical Journal, 159, 379–403.
- van der Marel, R. P., et al. (2012). The M31 velocity vector. III. Future Milky Way–M31–M33 orbital evolution, merging, and fate of the Sun. The Astrophysical Journal, 753(1), 9.