Hubble's Cepheids (1925): Andromeda Is Another Galaxy

Edwin Hubble’s discovery that Andromeda is a separate galaxy was announced on 1 January 1925, when his short paper “Cepheids in Spiral Nebulae” was read at the American Astronomical Society meeting in Washington. Using Cepheid variable stars, Hubble placed the Andromeda and Triangulum nebulae far outside the Milky Way, showing the universe is full of galaxies.

What the paper found

• Cepheids in spiral nebulae. Hubble identified Cepheid variable stars in two spirals, Andromeda (M31) and Triangulum (M33).
• A huge distance. Using the Cepheid period-luminosity relation, he derived a distance of about 285,000 parsecs, roughly 900,000 light-years, for both.
• Outside our galaxy. That distance was far larger than any estimate of the Milky Way’s size, so the spirals had to be independent star systems.

How Hubble did it

Hubble photographed Andromeda with the 100-inch Hooker telescope at Mount Wilson Observatory, then the most powerful telescope in the world, run by the Carnegie Institution (today Carnegie Science). On a plate taken on the night of 5 to 6 October 1923 he marked three new spots of light with an “N” for nova. Comparing with older plates, he realized one of them brightened and faded on a regular cycle. He crossed out the “N” and wrote “VAR!”: it was a Cepheid, now known as V1.

Cepheids pulse with a period that reveals their true brightness, a relation discovered by Henrietta Leavitt in 1912. Comparing V1’s true brightness with how faint it looks gives its distance. Hubble found more Cepheids in Andromeda and M33 over the following months, and the distances agreed.

Hubble did not attend the meeting. Henry Norris Russell of Princeton read the paper on his behalf on 1 January 1925. It was printed the same year in The Observatory and in Popular Astronomy.

Why it mattered

In April 1920 Harlow Shapley and Heber Curtis had argued in public, in what became known as the Great Debate, over whether spiral nebulae were clouds inside the Milky Way or “island universes” of their own. Hubble’s Cepheids settled it in Curtis’s favor on that point: the spirals are galaxies in their own right. The Milky Way became one galaxy among a vast number.

The result led straight to Hubble’s next projects: his 1926 classification of galaxies and the 1929 velocity-distance relation that revealed the expanding universe, both built on distances measured this way.

What we know now

Hubble’s distances were too small. The Cepheid calibration of the 1920s was off, and modern measurements put Andromeda more than twice as far away as his figure. The method itself survived and was refined: Cepheids measured with the Hubble Space Telescope and JWST anchor today’s local measurement of the expansion rate, 73.04 km/s per megaparsec from the SH0ES team, which disagrees with the early-universe prediction and sits at the center of the Hubble tension.

Who wrote it

Edwin Hubble was a staff astronomer at Mount Wilson. He wrote this report alone. In a 2011 account, NASA notes that Shapley, on reading Hubble’s letter about V1, reportedly told a colleague it was the letter that had destroyed his universe.

Read the original

Hubble, E. P. (1925). “Cepheids in Spiral Nebulae.” The Observatory 48: 139-142; also printed in Popular Astronomy 33: 252-255. Both are free to read as scans on NASA ADS: The Observatory and Popular Astronomy. The paper has no DOI.