One hundred years ago, on the night of October 5-6, 1923, humanity first learned of the existence of galaxies. That was when Edwin Hubble took a photo of what he called the Messier 31 Nebula. Today it is better known as Andromeda. Most astrophysicists don’t like the word photography, preferring to talk about images of the sky, but Hubble actually used photographic plates. The plate on which he took this famous photo is about 10 by 13 square centimeters, and he acquired the data for 45 minutes through the 100-inch Hooker Telescope, now located at Mount Wilson Observatory in a heavily light-polluted area of Los Angeles. That photographic plate called H335H (Hooker Plate 335 taken by Hubble) can be considered the birth certificate that we humans have created for all galaxies, the first record of their existence .
Although this image is not well known, it must be symbolic, as Mr. Hubble’s red inscription reads “VAR!” Cross out the “N” next to the star. Initially, he identified the small objects in his photos as novae, stars that randomly and briefly increase in brightness, then disappear and become considerably fainter again. But that night, Hubble discovered to his surprise (judging by his exclamation mark) that the star’s brightness changed periodically.
It was exactly what he was looking for. On an autumn night in 1923, more than three years had already passed since the so-called Great Debate, in which he debated whether the Milky Way was the entire universe or whether other places similar to the Milky Way existed. At that time, there was no word to describe what we know today as a galaxy. In the grand debate he discussed two options. Some say that what is known as a spiral nebula, such as Messier 31 (mentioned in the first paragraph of this article), has been under that name since the late 18th century, when Charles Messier published his Catalog of Nebulae and Star Clusters. Said it was known. It was part of our Milky Way. Another possibility was that they were other similar objects, beyond the scope of our home. The debate advanced both possibilities, but the option that the universe was limited to the size of the Milky Way, which was proven wrong three years later, prevailed at the time. Science sometimes takes a step back in order to move forward. Please note that I did not write galaxy in this paragraph. However, the word did not exist as we know it today, and the great controversy did not bode well for the word.
Hubble must have been less than satisfied with the conclusion of the great debate. Because Hubble was trying to answer a very basic (but fundamental) question, as if a child had asked it: “How big is the universe?” And to find the answer, he was looking for a type of star whose brightness changes periodically, discovered in his 1784 year, about 10 years after Messier cataloged it. More than a century later, in his 1908, science progressed slowly. Astronomer Henrietta Swan and his Leavitt discovered that the period of fluctuation of these stars depends on their brightness. These stars are known as Cepheids. Because the first star discovered (today it is believed to be his second) is his fourth brightest star in the constellation Cepheus, named after Andromeda’s father (what a coincidence! Of course!). Hence its name is Delta Cephi, a qualifier of such a star, the Cepheids.
Hubble knew that we could use this wonderful property of Cepheids that the universe has given us to measure distances to distant celestial objects. All we need to do is look for Cepheids, study their variability, determine their periodicity (all of which requires a lot of patience and the help of human computers), and use it to You can then calculate the Cepheid’s specific force (the energy it releases per second). . We astrophysicists call it luminosity (because astrophysics has a long history and we don’t want to use the proper physical term power). By comparing its power to the light we receive, we can calculate distance. It suffices to explain in physical-mathematical terms the very obvious property that lighthouses, no matter how bright and dazzling they are when seen up close, become darker as they move away.
Today, we can say that on the night of October 5-6, 1923, Hubble discovered the vastness of the universe with his great goal in mind. From observations that night and several others in the weeks that followed, Hubble calculated the distance to Andromeda to be about 2 million light-years. Hubble’s calculations were impeccable, as the Great Debate argued that the Milky Way’s size was between 30,000 and 300,000 light years. The star, known today as V1 (observed 80 years later with the Hubble Telescope), definitely belonged to the Andromeda Nebula. It was much further away than the Milky Way. Additionally, it takes into account the size of nebulae in the sky using another physics-mathematics equation that says that at that distance, distant objects appear smaller than they actually are (which is generally a lie, but that’s another story) ), we can calculate that the size of Andromeda is the same as the size of the Milky Way.
Many other nebulae known at the time were as far away as or more than the 2 million light-years that Hubble and other astronomers had successively measured. It hardly matters that Hubble’s distance was twice as wrong as his. The number was so large that we had no choice but to conclude that there are other Milky Ways, other… galaxies. It was the birth of a new term, a new field of science, and an entirely new concept of the universe. On that night 100 years ago, we acquired data that showed us that the universe is huge, and that its size can be changed forever with the blink of a camera shutter (45 minutes). We were also on the cusp of a paradigm shift in the way we view the universe. We can quickly see that the universe is expanding. This was also thanks to that “VAR!” Plate” and the first Cepheid of M31 discovered by Hubble 100 years ago. From that moment on, the universe was never the same.
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