Hubble's Galaxy Classification (1926): The Tuning Fork Begins

Edwin Hubble’s 1926 paper “Extra-galactic nebulae” introduced the classification of galaxies still taught today. Studying 400 of them on photographic plates, he sorted galaxies by shape into ellipticals, normal spirals, barred spirals and irregulars. The scheme grew into the famous “tuning fork” diagram.

The question

Until the mid-1920s nobody knew for sure what the faint, fuzzy “nebulae” scattered across the sky were. In 1925 Hubble had shown, using Cepheid variable stars, that the Andromeda nebula lies far outside the Milky Way. That meant many nebulae were galaxies in their own right, island universes like ours. A whole new class of object needed a map: what kinds of galaxies are there, and how common is each?

What the paper found

• Hubble studied 400 extragalactic nebulae, which we now simply call galaxies.
• He sorted them by the shapes of their photographic images into four classes: ellipticals, normal spirals, barred spirals and irregulars.
• About 97% were “regular”, with rotational symmetry around a bright central nucleus; only about 3% were irregular.
• He also estimated the luminosities, masses and number of galaxies per volume of space, giving an early estimate of the average density of matter in the universe.

How he did it

The classification was purely visual and deliberately simple. Hubble used only the appearance of each galaxy on photographic plates: smooth, featureless ellipticals; spirals, with arms winding out from a bright center; barred spirals, whose arms start from the ends of a straight bar; and the leftovers, the irregulars. Within the regular types he arranged galaxies in sequences of changing shape, the seed of the later tuning-fork diagram. For the statistical part, he combined counts of galaxies with estimates of their brightness to work out how much matter they contain on average.

Why it mattered

Hubble gave astronomers a common language for galaxies. Before it, every nebula was a special case; after it, a galaxy could be described in a word, and the question became why galaxies come in these types and how one type might turn into another. That question shaped the study of galaxies for the next century, from Toomre and Toomre’s 1972 simulations of colliding galaxies to the faint, distorted shapes of young galaxies in the Hubble Deep Field of 1996.

What we know now

The tuning fork has been refined many times, but its main branches are still in use. Projects from Sloan Digital Sky Survey citizen science to the Euclid space telescope still sort galaxies by shape: Euclid’s first quick data release in 2025 used AI plus 9,976 Galaxy Zoo volunteers to classify more than 380,000 galaxies. We now know shape is linked to history: many ellipticals are thought to be the products of mergers. Our galaxies guide covers the types of galaxies in detail.

Who wrote it

Edwin Hubble worked at the Mount Wilson Observatory in California, now part of the Carnegie Science Observatories, whose 100-inch Hooker telescope he used to show that the universe extends far beyond the Milky Way. His distance measurements relied on Henrietta Leavitt’s 1912 Cepheid law. Three years after this paper he published the discovery that the universe is expanding.

Read the original

Hubble, E. (1926). “Extra-galactic nebulae.” The Astrophysical Journal 64, 321. doi:10.1086/143018

Free to read: a scanned PDF on NASA ADS.