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In 1923, Edwin Hubble found a Cepheid variable star in the Andromeda Nebula. Its distance showed that Andromeda lay beyond the Milky Way, changing the known universe from one galaxy into a cosmos that includes others. The discovery depended on work by Henrietta Swan Leavitt, and it was distinct from Hubble’s 1929 finding that the universe is expanding.
What Hubble discovered in 1923
On the night of October 5–6, 1923, Hubble exposed photographic plate H335H using the 100-inch Hooker telescope at Mount Wilson Observatory. He initially marked a point of light as a nova, then crossed out that label and wrote “VAR!” after recognizing that the star’s brightness changed. Carnegie Science identifies H335H as the plate on which Hubble discovered the first Cepheid variable in the Andromeda Galaxy, M31. Carnegie Science’s plate archive records the plate and its annotation.
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A Cepheid variable is a star that brightens and dims in a regular pattern. That repeated change is more than a curiosity: the length of the cycle, or period, can help reveal how luminous the star truly is.
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How the star showed Andromeda was beyond the Milky Way
Henrietta Swan Leavitt had established a relationship between a Cepheid’s pulsation period and its intrinsic luminosity. Astronomers could use that relationship to estimate a star’s actual brightness, then compare it with how bright it appeared from Earth to infer its distance. Hubble applied this method to the variable star in Andromeda. The result put Andromeda outside the Milky Way. Carnegie Science’s history of the discovery explains the role of the Cepheid distance method and Leavitt’s contribution.
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This evidence addressed a major question in astronomy: were spiral nebulae, including Andromeda, structures within the Milky Way, or independent galaxies far beyond it? The Cepheid distance settled the question for Andromeda. It was not part of our galaxy.
Why the discovery changed the scale of the universe
Before the finding, the Milky Way could be understood as the bounds of the known universe, with spiral nebulae potentially lying inside it. Showing that Andromeda was external meant the universe extended beyond our own galaxy and contained other galaxies. The change was not a complete account of the universe’s structure, age, or history; it was a decisive expansion of its known scale.
The instrument made the observation possible in a period of major astronomical advances. Carnegie Science says the Hooker 100-inch telescope saw first light in 1917 and remained the world’s largest telescope from 1917 to 1949. Carnegie Science’s observatory history provides that context.
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The two milestones answer different questions:
| Year | What the evidence established | Why it mattered |
|---|---|---|
| 1923 | A Cepheid variable in Andromeda showed that the galaxy lay beyond the Milky Way. | The known universe included galaxies beyond our own. |
| 1929 | Hubble’s later work showed that the universe is expanding. | It became a separate milestone in modern cosmology. |
The “VAR!” plate is evidence for the 1923 distance discovery, not the 1929 expansion result. Keeping the dates distinct prevents the two changes to our picture of the universe from being conflated. Carnegie Science discusses both milestones in its historical overview.
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