On the night of October 5th to 6th, 1923, 100 years ago today, all galaxies were born for humans. That day, Edwin Hubble took a photo of the Messier 31 Nebula, which we now know better as Andromeda. Most astrophysicists don’t like the word photo, we prefer to talk about pictures of the sky, but Hubble actually used a photographic plate. The plate he used to take the famous photo was about 10 x 13 cm² and took 45 minutes of data through the 100-inch Hooker Telescope at Mount Wilson Observatory, at a location now visible by the light of Los Angeles is extremely polluted. This photographic plate called H335H – Hooker Plate 335, taken by Hubble – can be viewed as the birth certificate that humans created for all galaxies, the first record.
It’s not well known, but this photo is likely to be iconic, including the red inscription Hubble put on it: he wrote “VAR!” and also crossed out an “N” next to a star. First, he had identified a small object visible in the photo as a nova, a star that happened to briefly become brighter, then go out and become significantly fainter again. But that night, to his surprise, if we go by the exclamation point, he discovered that the star periodically changed its brightness.
More information
This was exactly what I was looking for. On that autumn night in 1923, more than three years had passed since the so-called Great Debate, in which it was debated whether the Milky Way was the entire universe or whether there could be other places similar to the Milky Way, others. .. Back then there was no word for what we know today as galaxies! Two options were discussed in the Great Debate. One said that so-called spiral nebulae, like Messier 31, which we mentioned in the first paragraph and which has been known by that name since Charles Messier published his catalog of nebulae and star clusters at the end of the 18th century, were part of our Milky Way. The other possibility was that it was more like other similar objects outside the confines of our home. This debate, in which arguments were made for both possibilities, was won by the option that would be proven false three years later, namely that the universe was limited to the size of our Milky Way. Science sometimes takes steps backwards in order to make progress. Note that it took a terrible time for me not to write “galaxy” in this paragraph, but the word did not exist in the form we know it today, and the Great Debate did not bode well for that word.
Photo by Edwin Hubble with the annotation “VAR!”. Carnegie Institution for Science
Hubble may not have been very convinced by the results of the Great Debate, because he continued to try to answer a question so basic (but fundamental) that it seemed like it was being asked by a child: how tall is the universe? And to do this, he looked for a type of star that had been discovered about ten years after Messier compiled his catalog in 1784 and whose brightness varied periodically. More than a century later, in 1908, science made slow progress: astronomer Henrietta Swan Leavitt discovered that the period of variability of these stars depends on their luminosity. These stars are known as the Cepheids because the first (we now believe it was the second) star discovered was the fourth brightest star in the constellation Cepheus, the 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 Cepheid property 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 therefore their intrinsic power (the energy they release per second). calculate what astrophysicists say they call luminosity (because astrophysics has a long history and we refuse to use the correct physics word: force). By comparing this 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 dazzling they are when viewed up close, appear dimmer at increasing distances.
With this great goal in mind, we can now say that Hubble discovered the immensity of the universe on the night of October 5th to 6th, 1923. Using observations from that night and several others in the following weeks, Hubble calculated the distance to Andromeda to be about 2 million light-years. The Great Debate mentioned sizes of the Milky Way between 30,000 and 300,000 light-years, so Hubble’s calculation left no doubt: this star, now known as V1 (and observed 80 years later by the Hubble Telescope), of which it certainly belonged For the Andromeda Nebula, it was much further away than the boundaries of the Milky Way. In addition, at this distance and taking into account the size of the nebula in the sky, we now use another physical-mathematical equation that expresses that distant objects appear smaller than they are (which is generally a lie, but it is so). Another story) it was calculated that Andromeda was similar in size to the Milky Way.
The many other nebulae known at the time were at distances of at least these 2 million light-years, which Hubble and other astronomers measured in successive years. It doesn’t matter that Hubble’s distances differ by a factor of 2. The numbers were so large that one could only conclude that there were other Milky Ways, other… galaxies. A new concept was born, a new branch of science, a completely new idea of the universe. One night 100 years ago, we learned from the data that the cosmos is gigantic and that its size changed forever for us in the blink of a camera shutter (which took 45 minutes). Furthermore, we were on the verge of a paradigm shift in our conception of the cosmos, which soon turned out to be expanding, thanks in part to this “VAR plate!” and the first Cepheid of M31 discovered by Hubble 100 years ago. From that moment on, the universe was never the same.
Cosmic emptiness It is a section in which our knowledge of the universe is presented qualitatively and quantitatively. The aim is to explain the importance of understanding the cosmos not only from a scientific perspective, but also from a philosophical, social and economic perspective. The name “cosmic vacuum” refers to the fact that the universe is mostly empty and there is less than one atom per cubic meter, although paradoxically in our environment there are trillions of atoms per meter cubic, which invites us to wonder about our existence and to reflect on the presence of life in the universe. The section is composed Pablo G. Pérez GonzálezResearchers at the Astrobiology Center, and Eva VillaverResearch Professor at the Institute of Astrophysics of the Canary Islands.
You can follow THEME on Facebook, X and Instagram, or sign up here to receive our weekly newsletter.