EIn 1923, American astronomer Edwin Hubble crossed out an “N” and replaced it with the letters “VAR” in one of his astronomical photographs, forever revolutionizing our view of the universe. He discovered the secret of the star V1 and through it the true nature of the universe. V1 is the most important star in the Universe – at least when its true dimensions are understood.
“This letter destroyed my universe,” astronomer Harlow Shapley said after Hubble wrote to her to inform her of his discovery. At the beginning of XXE century, humans still do not know the nature of the universe in which they live. We know that it is massive and that it contains stars more distant than the Sun. But there are strange “nebulae” all over the sky: cloud-like objects that are definitely not like stars and cannot be scientifically explained.
Shapley and his followers believe that these clouds are no more distant than the stars, and are in fact only more or less gaseous accumulations of gas between them. The stars we see in the sky will correspond to all the stars that exist – beyond that, nothing. His colleague Heber D. Curtis believes that the universe is full of “space-islands”. Stars combine to form large clusters called galaxies, our sun, and all the individual stars we observe in the sky are just one star among the others. Nebulae, on the other hand, are actually distant galaxies, separated from the Milky Way by vast empty spaces.
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Looking for Nova
Known as the “Great Controversy,” this controversy dominated astronomy in the early 20th century.E century We owe it to Edwin Hubble for ending it. To this end he uses the large Hooker telescope at the Mount Wilson Observatory in California, thus locating each of the stars in the Andromeda Nebula – they still do not know their distances. Given the technical means of the time, accurate distance measurements were only possible for relatively close stars.
Hubble then looks for novae, stars that explode at the end of their lives. They should always go the same way, changing their brightness more or less the same way. If he compares the luminosity he just observed with the predicted theoretical luminosity of the nova, he can accurately calculate the distance to the Andromeda Nebula: the fainter the nova, the larger the cloud should be.
In a photograph taken on October 5, 1923, Hubble identified three stars that could represent a nova and labeled them “N.” But when he compared the picture with old photos, he found that one of them was behaving strangely. It sometimes shines dimly and sometimes brightly, which a nova does not; On the other hand, that is what characterizes a variable star.
Outside the Milky Way
So Hubble switches from “N” to “VAR” and takes a closer look at the star. He discovered that they were a very special type of variable star known as Cepheids. These stars change in brightness over a period of time, which depends on their brightness. Hubble easily measures the period and determines the star’s intrinsic luminosity from the measured luminosity. He compares this to the apparent brightness of the star in the sky and reduces its distance from Earth.
The following year, he discovered several other Cepheids in the Andromeda Nebula. All with the same result: the nebula is at a distance much greater than the size of the Milky Way (the modern value of this distance is 2.4 million light-years; the distance obtained by Hubble is slightly less). So the Andromeda Nebula cannot be a “nebula”; It must be a vast collection of billions of stars, so far away that we can only see it as an amorphous cloud.
Today we know that the Andromeda Nebula is actually the Andromeda Galaxy. Most other hypothesized nebulae are galaxies. With Hubble’s discovery, the Milky Way was suddenly just one of countless stellar islands in a universe that suddenly became much larger than previously imagined. In short, Hubble’s star V1 gave us our first glimpse of the expanding universe—and we’re in the process of understanding and exploring the boundaries of this gigantic universe.
This article« the stars A history of the universe with a hundred stars » D. Florian FreistetterFlammarion Editions, November 2020, 463 pages, €25
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