What was the title of Hubble's 1926 paper on galaxy classification?

Edwin Hubble’s 1926 paper on galaxy classification was titled “Extra-Galactic Nebulae.” Published in The Astrophysical Journal, it examined the visible forms and statistical properties of hundreds of objects then commonly called nebulae.

The paper presented Hubble’s early classification system, grouping galaxies into elliptical, spiral, barred-spiral, and irregular forms. Its sequence later became known as the Hubble sequence, often illustrated as the “Hubble tuning fork.” The categories were based primarily on observed structure, not on a proven evolutionary path.

The historical title reflects the terminology of the time. Astronomers had not yet universally adopted “galaxy” for these distant stellar systems, so Hubble called them extra-galactic nebulae. The Realm of the Nebulae was a different work: Hubble’s 1936 book, which expanded on his views about galaxies and cosmology. The 1926 paper studied roughly 400 extra-galactic nebulae and helped make morphological classification a standard tool in astronomy.