Post B1rQJWSQx7MU71VDXM by mcnees@mastodon.social
(DIR) More posts by mcnees@mastodon.social
(DIR) Post #B1rQIL81aYf2gaOO5g by mcnees@mastodon.social
2026-01-02T02:14:20Z
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101 years ago, #OTD in 1925, Edwin Hubble announced that Andromeda and other spiral nebulae were definitely separate galaxies outside the Milky Way, in a paper read to an AAS meeting by H.N. Russell.There was no doubt that the Universe was more than just our little island of stars.
(DIR) Post #B1rQIQhIsoqrxRw8H2 by mcnees@mastodon.social
2026-01-02T02:19:27Z
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Astronomers were divided on the issue; many still subscribed to the idea that everything they saw was within our Milky Way.Hubble built on observations by Vesto Slipher, collaborated with Milton Humason, and relied on Henrietta Swan Leavitt’s Cepheid variable work to make his case. (2/n)
(DIR) Post #B1rQIVoDsWMDoMDEFk by mcnees@mastodon.social
2026-01-02T02:19:48Z
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It seems hard to believe, but there are people alive today who were born into what many scientists thought was a much smaller universe. (3/n)
(DIR) Post #B1rQIagbkGEkwBgiZs by mcnees@mastodon.social
2026-01-02T02:20:54Z
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Until recently, my favorite examples of this were Jimmy Carter and Betty White.President Carter was three months old when Hubble made his announcement. Betty White was about to turn three years old when we realized there are other galaxies. (4/n)
(DIR) Post #B1rQIfctNVEgG6zJyq by mcnees@mastodon.social
2026-01-02T02:22:09Z
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Hubble’s announcement — Other galaxies exist! Our Milky Way is just one of many! — was made on the third day of the 33rd Meeting of the American Astronomical Society, in a paper read by H.N. Russell. The meeting started on the 30th; maybe Hubble chose New Year’s Day for dramatic effect. (5/n)
(DIR) Post #B1rQIkkWKZJC9Daz44 by mcnees@mastodon.social
2026-01-02T02:22:52Z
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For astronomers, the result likely wasn’t a sudden revelation. Hubble had been discussing his evidence with colleagues, and word had gotten around.Many scientists were already sure of the result, and had been for years. They just needed irrefutable evidence. (6/n)
(DIR) Post #B1rQIpkhjJQVfEihXs by mcnees@mastodon.social
2026-01-02T02:23:25Z
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It’s hard to imagine the pre-Hubble view of the Universe, though it was just 100 years ago. Before Hubble’s announcement, it was still a popular view among astronomers that the collection of stars making up our galaxy was Everything. (7/n)
(DIR) Post #B1rQIuc1f0SIjlhLDE by mcnees@mastodon.social
2026-01-02T02:23:39Z
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Astronomer Harlow Shapley was, at the time, the leading proponent of the establishment view. He had long argued that spiral nebulae seen by astronomers — shapes any kid would now recognize as galaxies — were just dust clouds inside the Milky Way. (8/n)
(DIR) Post #B1rQJ0ejBsBLTt0aG0 by mcnees@mastodon.social
2026-01-02T02:23:52Z
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But there had always been astronomers who suspected that the Milky Way was just one of many galaxies. This view goes at least as far back as Immanuel Kant’s “Universal Natural History and Theory of the Heavens,” which he published anonymously in 1755. (9/n)
(DIR) Post #B1rQJ5oU11xLTabwem by mcnees@mastodon.social
2026-01-02T02:24:04Z
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Just five years before, in 1920, astronomer Heber Curtis argued against Shapley in the “Great Debate.” Curtis’s position, supported by a growing number of his colleagues, was that spiral nebulae were independent galaxies outside the Milky Way. (10/n)
(DIR) Post #B1rQJAaqFB10IjGcaW by mcnees@mastodon.social
2026-01-02T02:24:16Z
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It was the identification of Cepheid variable stars in M31 and other spiral nebulae that allowed Hubble to prove they were so far away that they must be outside the Milky Way. (11/n)
(DIR) Post #B1rQJFqchvbsb7gnNg by mcnees@mastodon.social
2026-01-02T02:24:28Z
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A Cepheid variable is a type of star whose brightness waxes and wanes over a period of time that tightly correlates with its maximum brightness. Measure that period and you know its absolute brightness. Compare that to how bright it appears, and you can estimate its distance. (12/n)
(DIR) Post #B1rQJLPu0BnhrzEXZ2 by mcnees@mastodon.social
2026-01-02T02:24:59Z
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This important property of Cepheid variable stars was discovered by astronomer Henrietta Swan Leavitt. It makes them “standard candles” (a term she coined) that we can reliably use for establishing distances to cosmologically nearby galaxies. (13/n)
(DIR) Post #B1rQJQmQ3tmcVGo5r6 by mcnees@mastodon.social
2026-01-02T02:25:10Z
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Henrietta Swan Leavitt’s period-luminosity relationship for Cepheids is one of the first rungs on the “cosmic distance ladder,” the collection of methods used by astronomers to measure extragalactic distances. (14/n)
(DIR) Post #B1rQJWSQx7MU71VDXM by mcnees@mastodon.social
2026-01-02T02:26:12Z
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In the early morning hours of October 6, 1923, Edwin Hubble took a photo plate of M31 showing a Cepheid variable star. He originally mistook it for a nova - you can see where he has crossed out an “N” on the plate and excitedly replaced it with “VAR!” (15/n) Image: Carnegie Observatories
(DIR) Post #B1rQJbTKJE2xfExV7g by mcnees@mastodon.social
2026-01-02T02:26:19Z
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Applying Henrietta Swan Leavitt’s period-luminosity relationship to subsequent observations, Hubble concluded that the distance to M31 was greater than reliable size estimates for the Milky Way. Therefore, it must be outside our galaxy. (16/n)
(DIR) Post #B1rQJgN85h3orT67eq by mcnees@mastodon.social
2026-01-02T02:26:32Z
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Hubble kept Shapley in the loop as he built the case for separate galaxies throughout 1924, alerting him to the discovery of Cepheid variables in M31, M33, and other spiral nebulae.Shapley doubted Hubble at first. But he eventually relented in the face of growing evidence. (17/n)
(DIR) Post #B1rQJlzFDPWIH7y8GG by mcnees@mastodon.social
2026-01-02T02:26:48Z
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Upon receiving one of Hubble's letters, Shapley remarked to doctoral student Cecilia Payne-Gaposchkin “Here is the letter that destroyed my universe.” (18/n)
(DIR) Post #B1rQJrQ11IuB7bX5BQ by mcnees@mastodon.social
2026-01-02T02:27:01Z
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Astronomically, January 1st is a meaningless date. It is an arbitrary reference point in a system based on the approximate orbital period of an otherwise unremarkable star that is perhaps only notable for having a life-bearing planet. (19/n)
(DIR) Post #B1rQJw3VmOsJVG2gIi by mcnees@mastodon.social
2026-01-02T02:27:57Z
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But for those of us who reckon time by this system, today is the 101st anniversary of humanity taking a monumental step towards understanding our place in a Universe that is vast and puzzling, but ultimately knowable. Happy New Year! (20/20)Plot: Richard PowellAnnotations: Me