(C) Daily Kos This story was originally published by Daily Kos and is unaltered. . . . . . . . . . . "VAR!" 100 years ago tonight, Edwin Hubble found a star that revealed the vastness of the Universe [1] ['This Content Is Not Subject To Review Daily Kos Staff Prior To Publication.'] Date: 2023-10-04 How big is the Universe? Humanity made it all the way into the 1920s having no idea . Many people at that time thought that the Milky Way was the entire Universe. Yes, big, but not crazy big. Others thought that maybe those spirals and smudges we could see were in fact other galaxies altogether, which would mean the Universe is unfathomably large. On the night of October 5-6, 1923 — 100 years ago tonight — Edwin Hubble made an observation that would settle the question once and for all. He took a 45-minute exposure of Andromeda, one of the largest and thus apparently nearest “spiral nebulae”, and within it he spotted a single compelling star that we now call Hubble V1. We here on Earth will forever have a very special relationship with that star, because Hubble used its characteristics to show conclusively that Andromeda was not a “spiral nebula” within the Milky Way at all, but rather an entirely separate galaxy which he later estimated to be about 1 million (now understood to be closer to 2.5 million) light-years away. The Andromeda Galaxy. This image won first prize overall in the Insight Investment Astronomy Photographer of the Year 2020 competition. It uses focusing techniques to depict the Andromeda Galaxy as a cute little object we can reach out and touch In Hubble’s time, many other “spiral nebulae” were known that appeared much smaller than Andromeda and were therefore presumably quite a bit farther away. Because of his observation a hundred years ago tonight, Hubble realized that even Andromeda, close enough to be seen with the naked eye, is on the order of 10 quintillion (10,000,000,000,000,000,000) miles away from Earth. And that must mean that the Universe is absurdly, almost comically, huge. Imagine what Edwin Hubble felt like sitting there at Mt. Wilson Observatory, probably by himself late one evening, when he became the first person ever to know that for sure. His use of red ink and an exclamation point in the main figure of this diary illustrates that he knew darn well the significance of what he had found the minute he realized what it was. But how could he know that by looking at just one dot on a single photographic plate? He could, because he knew about the work of two astronomers that had come before him: Henrietta Swan Leavitt and Ejnar Hertzsprung. Let’s see what they found, so that when we go back to Edwin Hubble at Mt. Wilson, we’ll realize along with him the profound implications of what he had just seen. Leavitt had deciphered a kind of secret code among a special group of stars. It had been known for awhile that certain large and luminous stars, called (Class I) Cepheid variables, get brighter and dimmer very regularly. The Cepheid class is named after its archetype, a star called Delta Cephei, discovered in 1784 by John Goodricke. You can see how predictable its variation in brightness is: Delta Cephei’s period of brightness oscillation is 5.366 days If you monitor photographic plates of the same area of the sky over time, Cepheids are definitely findable. Leavitt had been doing just that, and she found 1,777 of them altogether in the Large and Small Magellanic Clouds (LMC and SMC). She published a catalog of them in 1908, and in it she pointed out a very interesting feature of these Cepheids that she had noticed: the brighter they appeared, the more slowly their brightness varied. The Small Magellanic Cloud (SMC), a dwarf galaxy which we now know lies about 200,000 light-years from the Milky Way, making it among our closest galactic neighbors She wanted to turn this into more than just an anecdotal relationship, so she started focusing on the SMC. No one knew how far away it was, or truthfully what it was, but she assumed that all the stars within it are about the same distance from Earth, being all bunched up like that. So if, from our vantage point, one star within the SMC appears twice as bright as another, then it really is about twice as bright. She had enough photographic data to write a followup 1912 paper, where she looked at 25 Cepheids in the SMC whose cycle times she could nail down pretty well. She could then say with confidence that there is a strong relationship between relative brightness and cycle time, just as she suspected: The smaller the magnitude number, the brighter the star. Each decrease in magnitude by 1 unit means the star is about 2.5 times brighter. The limit for naked-eye viewing in absolutely dark sky is about magnitude 6 or 7. This strong relationship meant that you could now know the relative distances of Cepheids. Let’s say you found two Cepheids with the same cycle time but one looked only ¼ as bright as the other. You’d know right away that the dimmer Cepheid was twice as far away as the brighter one: But no one knew the distance to any Cepheid, so this was frustrating. If only we knew the distance to one Cepheid and we used Leavitt’s relationship, we could find the distance to any Cepheid. Were there any Cepheids out there whose distance from us we could measure? This is where Ejnar Hertzsprung comes in. He realized in 1913 that there were 13 Cepheids that were close enough to Earth that their distances, with recently improved methods, could be estimated by parallax measurements. As the Earth goes around the Sun, it moves relative to the stars. The closer a star is to us, the more it seems to move over the course of a year So that’s what he undertook to do. He acknowledged that there could be errors associated with the fact that stars do actually move relative to us, not only sideways but also toward us or away from us, and you had to try and account for all of that and hope you had complete enough data to get around it. But he did the best he could, and he came up with an average distance to these stars of about 100 light-years, so they were reasonably close. The parallax of the SMC is really small, so he knew he couldn’t directly measure its distance from us that way. But then he said, if Miss Leavitt’s assumptions are right, and all Cepheids with the same cycle time are indeed equally bright, then I can take one of the nearby Cepheids I just observed with a cycle time of say, 6 days, and it should have the same absolute brightness as one she measured with that same 6-day cycle time. Hers will appear a lot dimmer than mine from Earth, of course, but if I use the inverse square law, I can figure out how much farther away her star is than mine, and so I can figure out an approximate distance to the SMC. If I do that, I can fill in the only number that’s missing from her work, and that’ll give us an absolute measuring stick. From then on, when we look at any Cepheid, we can measure its cycle time and how bright it appears to us, and we’ll know how far away it is! Hertzsprung came up with a distance of 33,000 light-years to the SMC, which we now know is not a terrific estimate (in part because of all those sources of error he mentioned), but it was still of a magnitude that would make it generally useful. To Hertzsprung’s credit, he did note in his 1913 paper that he thought the SMC was just far enough away to be considered “outside the Milky Way”, and he turned out to be right about that. Aren’t you glad you know how big the Universe is? You can thank, among others, Henrietta Swan Leavitt and Ejnar Hertzsprung So somebody got right on the hunt for distant Cepheids, right? Well, no. Trouble was, at that time, you couldn’t resolve individual stars very well any farther out than the Magellanic Clouds, because the telescopes just weren’t powerful enough. That all changed in 1917 with the completion of the giant 100-inch Hooker Telescope at Mt. Wilson Observatory in Los Angeles. Finally, we could get a closer look at all those mysterious “smudges” and try to figure out what they were. A staffer named Edwin Hubble who had arrived as a doctoral student in 1919 had started getting time on the huge telescope, and by the summer of 1923 he was resolving individual bright stars on the fringes of the Andromeda “nebula” and tracking them to see how they would behave. If they were stars just like any others, he ought to start seeing starlike behaviors, and indeed he began to track three apparent novae. He called them novae because they brightened and then started to dim. Hubble took a 45-minute exposure in the wee hours of October 5-6 to keep on tracking his novae, and he marked each of them on the photographic plate with an “N”. But on the evening of October 10 , he had noted the apparent brightness of the “novae” on this plate and compared them to previous observations. One of the “novae” had begun to brighten again, to the point where it convinced Hubble that it was not a nova at all, but a Cepheid variable. Hubble knew very well the relationship between brightness and cycle time of Cepheid variables, and it was also obvious to him that the one he was observing was much, much dimmer from Earth than the ones Leavitt and Hertzsprung had observed. It had to be at least 10 times as far away as the SMC, which meant the big Andromeda “smudge” must be a galaxy in its own right, entirely separate from the Milky Way. And that almost certainly meant that all the other smaller “smudges” people had seen were also galaxies in their own right, but much, much farther away still. Sure, there were calculations to be done, but Edwin Hubble must have known right away that they were only a formality. He could do them tomorrow. Hubble broke out the red marker and enthusiastically crossed out one ostensible nova’s “N” and called it by its rightful name. “VAR!” What must he have felt there at the observatory after he put the red marker down, being at that moment the only person in the world to know the answer to one of humanity’s biggest questions? Edwin Hubble Earlier that afternoon, Casey Stengel of the Giants hit a game-winning inside-the-park home run in the ninth inning, in the first-ever World Series game at Yankee Stadium. Edwin Hubble had been a star athlete in high school and college, including at baseball, so he might have been looking forward to reading all about that game in the next morning’s paper. But as it so happened, a few hours later in Los Angeles, he got a little distracted, by discovering that the Universe is far more vast than anyone ever knew. 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