Robert McNees
Edwin Hubble announced #OTD in 1925 that Andromeda and other spiral nebulae were in fact separate galaxies outside the Milky Way, in a paper read to an AAS meeting by H.N. Russell.
The Universe was far larger than what many astronomers had allowed themselves to imagine. It was more than just our little island of stars.
Hubble built on observations by Vesto Slipher, collaborated with Milton Humason, and relied on Henrietta Swan Leavitt’s Cepheid variable work to reach this conclusion.
Robert McNees
It seems hard to believe, but there are people alive today who were born into what scientists thought was a much smaller universe.
Former US President Jimmy Carter was three months old when Hubble made his announcement.
Betty White, who passed away just a few years ago, was always my favorite example of this. She was about to turn three years old when we realized there are other galaxies.
Robert McNees
Hubble’s announcement — Other galaxies exist! Our Milky Way is just one in a Universe full of them! — 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 December 30th; I don’t know if Hubble waited for New Year’s Day to be dramatic.
For astronomers, the announcement probably wasn’t a sudden revelation. Hubble had been discussing this result with colleagues, and word had gotten around.
Robert McNees
It’s hard to imagine the pre-Hubble view of the Universe, though it was less than 100 years ago.
Just a year before Hubble’s announcement, many astronomers thought the collection of stars that make up our galaxy was Everything.
Astronomer Harlow Shapley was perhaps 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.
Images: Huntington Library
Robert McNees
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.
Robert McNees
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.
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.
Robert McNees
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.
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.
Robert McNees
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!”
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.
Image: Carnegie Observatories
Robert McNees
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, who had long argued that the Milky Way was the whole Universe, doubted Hubble at first. But he eventually relented in the face of growing evidence.
Upon receiving one of Hubble's letters, Shapley remarked to doctoral student Cecilia Payne-Gaposchkin “Here is the letter that destroyed my universe.”
Robert McNees
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.
But for those of us who reckon time by this system, today is the 99th 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!
Image: Richard Powell
David Colarusso
@mcnees I think about analogs of this from my own life all the time. “When I was a kid astronomy et al.” We didn’t know if black holes were really real or have evidence of exoplanets. Dinosaur extinction and space, how could there be a connection? That sort of thing.
STEMcoding Project
@mcnees Our star has above average metallicity, have a little respect
Neal
@mcnees happy new year!
nobody you care about
@mcnees everything is just dust in the wind and meaningless outside of our stupid little traditions and celebrations to break up the otherwise bleak monotony of existence. Life is pointless and we all die eventually. Everything you know and love will come to and end and be forgotten. But hey, there's dark matter somewhere that might be interesting. Thanks for this reminder!
panton41
@mcnees He was also a science and Spanish teacher at New Albany (Indiana) High School around 1910 and also coached basketball. He lived in the Highlands neighborhood of Louisville, Kentucky at the time.
(((_SjG_)))
@mcnees my astronomer father, in his 90s, likes to pretend he’s explaining general relativity and fast living when he says “the universe was seven billion years old when I was in elementary school, now it’s thirteen.“
Robert McNees
@mckra1g I’m putting this in my rotational motion lecture notes. @angst_ridden
Molly Cantrell-Kraig
@angst_ridden @mcnees is your father Bill Watterson?
Debbie Goldsmith 🏳️⚧️🏳️🌈
@mcnees We don't know if the many-worlds interpretation of quantum mechanics is correct—and may never know—but if it is, the universe is immensely larger still.
Robert McNees
@jeanoappleseed Happy new year!
jeanoappleseed
@mcnees A very good point. Happy New Year nonetheless!
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