On the night of October 5–6, 1923, Edwin Hubble photographed a star in Andromeda that helped show the galaxy lies beyond the Milky Way. The decisive clue was a variable star—not the discovery of Andromeda itself, but evidence that it was a separate galaxy.
What did Edwin Hubble discover in 1923?
Hubble established that the Andromeda Galaxy, also known as M31, was outside the Milky Way. At the time, astronomers were debating whether spiral nebulae such as Andromeda were nearby objects within our galaxy or distant “island universes.” Showing that Andromeda lay beyond the Milky Way meant our galaxy was not the whole universe.
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The observation took place at Mount Wilson Observatory with its Hooker 100-inch telescope. Carnegie Science’s archive identifies the photographic plate as H335H, “Hooker plate 335 by Hubble.” The archive dates the exposure to the night of October 5–6, rather than specifying one calendar-day instant. Carnegie Science’s H335H plate archive records the telescope, target and annotation.
How did Hubble prove Andromeda was outside the Milky Way?
On the plate, Hubble first marked one star as a possible nova, a star that appears to flare dramatically. He later crossed out the “N” and wrote “VAR!” after recognizing that the star varied in brightness like a Cepheid. The inscription is on the plate; it was not a spoken remark.
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Cepheids brighten and dim in a regular cycle. Earlier work by Henrietta Swan Leavitt had shown that a Cepheid’s pulsation period is related to its intrinsic luminosity. By comparing the star’s inferred luminosity with how bright it appeared from Earth, astronomers could estimate its distance. Applying that method to the Cepheid in M31 placed Andromeda far beyond the Milky Way. Hubble used Leavitt’s established relationship; he did not invent the method. Carnegie Science’s account of the discovery explains the distance inference and Leavitt’s contribution.
What is Hubble’s “VAR!” plate?
“VAR!” is Hubble’s handwritten reclassification of the star on plate H335H as a variable, rather than a nova. The plate is significant because it preserves the clue that enabled the distance estimate: a Cepheid variable in Andromeda. Carnegie Science’s historical account of Hubble’s discovery places the plate in the debate over whether spiral nebulae belonged to the Milky Way.
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When did Hubble announce the result?
The exposure dates to October 1923, but Hubble did not report the result that night. Carnegie Science says he presented it to the American Astronomical Society in January 1925, more than a year later. Keeping the observation and announcement separate avoids confusing the date on the plate with the date the finding was reported.
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