On the night between October 5 and 6, 1923, 100 years ago today, all the galaxies were born for humans. On that date, Edwin Hubble took a photo of what he called the Messier 31 nebula and we now know it more as Andromeda. Most astrophysicists don’t like the word photo, we prefer to talk about images of the sky, but Hubble really used a photographic plate. The plate with which he took the famous photograph measured about 10×13 cm², it took data for 45 minutes through the 100-inch Hooker telescope at the Mount Wilson Observatory, in what is now a place tremendously polluted by the light of Los Angeles. That photographic plate, called H335H—Hooker plate 335 taken by Hubble—can be conceived as the birth certificate that humans create for all galaxies, the first record.
It is not well known, but that photo should be iconic, including the red inscription that Hubble made on it: he wrote “VAR!”, also crossing out an “N” that was next to a star. Initially, he had identified a small object visible in the photo as a nova, a star that randomly brightened briefly, then faded and became much fainter again. But that night he discovered, with surprise, if we are guided by the exclamation point, that the star varied its brightness periodically.
That was exactly what I was looking for. On that autumn night in 1923, more than three years had passed since the one known as the Great Debate, in which it was discussed whether the Milky Way was the entire universe or whether there could be other places similar to the Milky Way, others… At that time there was no word for what we know today as galaxies! The options that were discussed in the Great Debate were two. One said that what were known as spiral nebulae, like that Messier 31 that we named in the first paragraph and that had been known by that name since Charles Messier published his work at the end of the 18th century, Catalog of nebulae and star clusters, were part of our Milky Way. The other option was that it was more like other similar objects, beyond the limits of our house. That debate, in which arguments were presented in favor of both possibilities, was won by the option that three years later would be proven wrong, that the universe was limited to the size of our Milky Way. Science sometimes takes steps backward to make leaps forward. Note that it has cost me a lot not to write galaxy in this paragraph, but the word did not exist as we know it today, and the Great Debate did not bode well for that word.
Hubble must not have been very convinced by what was concluded in the Great Debate, because he continued trying to answer a question so basic (but fundamental) that it seems asked by a child: how big is the universe? And for this he was looking for a type of stars that had been discovered, about 10 years after Messier built his catalog, in 1784, and that varied their brightness periodically. More than a century later, in 1908, science advanced slowly: astronomer Henrietta Swan Leavitt discovered that the period of variability of these stars depended on their luminosity. These stars are known as Cepheids, because the first (well, today we think it was the second) discovered was the fourth brightest star in the constellation of Cepheus, father of Andromeda (what a coincidence!). And hence its name, Delta Cephei, and the adjective for this type of star, the Cepheids.
Hubble knew that this great property of the Cepheids that the universe gave us could be used to determine the distance to distant objects. All you had to do was look for Cepheids, study their variability, determine their period (all with a lot of patience and the help of human computers), and with it calculate their intrinsic power (the energy they release per second), what astrophysicists call luminosity (because There is a lot of history in astrophysics and we resist using the proper physics word: power). By comparing that power with the light we received, the distance could be calculated. It is enough to use the physical-mathematical description of the very obvious property that headlights, no matter how bright and blinding they are when viewed up close, appear dimmer at increasingly greater distances.
With that great objective in mind, today we can say that on the night of October 5 to 6, 1923, Hubble discovered the immensity of the universe. Using observations from that night and several more in the following weeks, Hubble calculated the distance to Andromeda to be about 2 million light years. In the Great Debate, sizes of the Milky Way of between 30,000 and 300,000 light years were mentioned, so Hubble’s calculation left no doubt: that star, today known as V1 (and observed by telescope Hubble 80 years later), which surely belonged to the Andromeda nebula, was much further away than the confines of the Milky Way. What’s more, at that distance, and taking into account the size of the nebula in the sky, now using another physical-mathematical equation that expresses that distant objects appear smaller than they are (something that, in general, is a lie, but that’s another story) it could be calculated that Andromeda was similar in size to the Milky Way.
The many other nebulae known at the time were at distances as great or greater than those 2 million light years, something that Hubble and other astronomers measured in successive years. It matters little that Hubble’s distances were off by a factor of 2. The numbers were so large that it could only be concluded that there were other Milky Ways, other… Galaxies. A new term had been born, a new branch of science, a completely new conception of the universe. On one night 100 years ago we took the data to learn that the cosmos is gigantic, its size changed for us forever in the blink of a camera shutter (which was 45 minutes). We were, furthermore, on the verge of a paradigm shift in our conception of the cosmos, which shortly after would be discovered to be expanding, also thanks to that “VAR plate!” and that first Cepheid from M31 discovered by Hubble 100 years ago. The universe was not the same from that moment on.
Cosmic Void It is a section in which our knowledge about the universe is presented in a qualitative and quantitative way. It aims to explain the importance of understanding the cosmos not only from a scientific point of view but also from a philosophical, social and economic point of view. The name “cosmic vacuum” refers to the fact that the universe is and is, for the most part, empty, with less than one atom per cubic meter, despite the fact that in our environment, paradoxically, there are quintillions of atoms per meter cubic, which invites us to reflect on our existence and the presence of life in the universe. The section is made up Pablo G. Pérez Gonzálezresearcher at the Astrobiology Center, and Eva Villaverresearch professor at the Institute of Astrophysics of the Canary Islands.
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