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In 1923, Edwin Hubble found evidence that the Andromeda Nebula lay beyond the Milky Way. That finding changed the known universe from a single-galaxy system into a cosmos containing other galaxies. The evidence was a Cepheid variable star on a photographic plate—not Hubble’s later work on cosmic expansion.
What did Hubble discover 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 that label out and wrote “VAR!” when he recognized that its brightness changed. The plate is now known as the VAR! plate; Carnegie Science identifies the object as the first Cepheid variable discovered in the Andromeda Galaxy (M31). Carnegie Science’s plate archive records the exposure and annotation.
A Cepheid variable is a star whose brightness changes in a regular pattern. The key was that this star’s changing brightness could help establish its distance, rather than merely show that a new or changing object existed.
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How did the star show that Andromeda was outside the Milky Way?
Henrietta Swan Leavitt had established a relationship between a Cepheid’s pulsation period and its intrinsic luminosity. Astronomers could use the period to estimate how bright the star really was, then compare that estimate with how bright it appeared from Earth. The difference indicated its distance. Hubble’s inference relied on Leavitt’s foundational work, as Carnegie Science’s account of the discovery explains.
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Applying this method to the Andromeda Cepheid placed it beyond the Milky Way. Andromeda was not a nearby structure inside our galaxy; it was a separate galaxy in its own right.
What changed in our picture of the universe?
Before this evidence, astronomers disputed whether spiral nebulae were part of the Milky Way or independent galaxies. The distance to the Cepheid settled the central question for Andromeda: it lay outside our galaxy. The Milky Way was therefore not the whole universe; it was one galaxy among others.
The discovery’s immediate significance was a change in scale. It established that the universe extended beyond the Milky Way and contained other galaxies. It did not, by itself, explain the universe’s full structure, age, or history.
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No. The 1923 plate provided evidence that Andromeda was a separate galaxy beyond the Milky Way. Hubble’s work published in 1929 showed that the universe was expanding—a distinct finding that contributed to modern cosmology. The two milestones are related to Hubble’s career, but the VAR! plate demonstrated the existence of galaxies beyond our own, not cosmic expansion. Carnegie Science’s historical overview treats these as separate discoveries.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Why is the VAR! plate historically important?
The plate preserves the moment Hubble reinterpreted what he had seen: a presumed nova became a variable star whose known relationship between period and luminosity could serve as a distance indicator. The Hooker telescope at Mount Wilson made the observation possible; Carnegie records its first light in 1917 and says it was the world’s largest telescope from 1917 to 1949. Carnegie Science’s observatory history provides that context.
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