The astronomer who revealed the secrets of the universe with a simple scientific law
Humans have long believed that the universe contains nothing but our Milky Way galaxy, but the American astronomer Henrietta Swan Leavitt changed this belief forever, based on a simple scientific law, and without even needing a telescope.
A century ago, the universe seemed much smaller than it does now. Many astronomers were convinced that the universe consisted of nothing more than our galaxy known as the “Milky Way,” a perception that changed in 1923, when the American astronomer Edwin Hubble built an observatory on the top of Mount Wilson in California, and directed the observatory’s telescope toward a spiral vortex. Mysterious and sparkling, it appears from afar in the dark sky, and it was called - at the time - the Andromeda Nebula.
At that time, Hubble noticed that this vortex was far from being part of the Milky Way, and that it was not just a nebula, but rather composed of a trillion stars, and was 2.5 million light-years away from Earth. What is noteworthy is that this man would not have been able to make this wonderful observation, had it not been for the research efforts that had been undertaken in the previous years by the American astronomer Henrietta Swan Leavitt, without her needing even a single telescope.
The irony is that Levitt made her discoveries in this regard by directing her magnifying lenses downward, not upward, specifically at a thin glass plate mounted on a wooden frame, and covered on one side with a sensitive photographic surface, on which dots representing thousands of stars are scattered. These stars, which appeared black on a white background, formed a mirror image of the sky scene that we see at night with the naked eye. This was how Levitt learned about the universe and its details.
At that time, specifically the late nineteenth century and the beginning of the twentieth century, this woman was part of an all-female group working at the American Harvard College Observatory. These women were known as the group of “electronic minds,” and they succeeded in discovering dozens of asteroids, nebulae, and novae, as well as thousands of “variable stars,” which are known to vary in brightness between brightness and dimness. It may not seem surprising that Levitt is described as “the consummate expert on variable stars,” given that she has discovered more than 2,000 of them on her own. Her relentless pursuit of these stars led to her winning a bigger prize, which was to find a way to calculate distances across space.
This method was represented in a mathematical relationship that was known in the past as “Nouraniya - Time Period.” This relationship links the brightness of pulsating variable stars to the time period during which their pulsations last. Thanks to this method, which was developed by Levitt, astronomers were able to calculate the distances between galaxies, and Hubble had the equation he needed to observe objects and stars located outside our galaxy, and beyond in the vast universe.
“Our entire perception of the universe has changed as a result of what this woman discovered,” says Wendy Friedman, an astronomer and astrophysicist at the University of Chicago.
Astronomer Edward Pickering with "Electronic Brains" at Harvard College
After astronomy gained Levitt's interest in her final year of study, this young woman joined the Harvard College Observatory as an intern in 1895, at the age of twenty-seven. Levitt was no exception in this regard. At that time, departments and entities related to astronomy greatly celebrated the use of women to complete the major scientific projects that were taking place in their corridors, and which required the employment of a large number of poorly paid workers. Their need for this type of labor coincided with Levitt and an increasing number of her peers finishing their studies with high grades, as said by historian Margaret Rossiter, who discussed in her research the role of women in scientific fields at the beginning of the twentieth century. Despite their excellence, these women were excluded at that time from teaching in universities or holding leadership positions.
Levitt was appointed to the job by Edward Pickering, director of the Harvard Observatory, who led a multi-decade project to map the entire night sky, then classify the chains of stars within it and list them as objects with similar characteristics.
In keeping with the way jobs were divided between the sexes in the nineteenth century, women were seen as ideal for doing these classifications, as they required the ability to be patient and pay attention to detail, qualities that women were thought to have innately. As for men, the prevailing belief was that they were more inclined to assume leadership positions and engage in intellectual work centered around monitoring, observation, and developing theories.
Thus, women who held jobs similar to those held by Levitt were usually poorly paid and had almost no opportunities for advancement. In the jobs that were given to these women, they were able to benefit from their “feminine” skills, without posing a threat to their male colleagues, whether in terms of wages or job status.
Levitt (right) with Annie Jump Cannon, who was one of a very limited number of women allowed to use the Harvard Observatory Telescope
In fact, Pickering's decision to rely mainly on women in making up his staff was consistent with what he himself wrote in the Harvard Observatory Annual Report of 1898, that “to achieve the greatest possible efficiency, it is necessary not to require the skilled observer of the stars to He spends his time doing what an assistant could do for a lot less salary.” In support of this, this man was giving the “electronic mind” of this group of women 25 cents per hour, which amounts to $1,500 annually. As for the men working at the observatory, they were paid at least $2,500 a year.
This group of "electronic minds" was prohibited from using the observatory's telescope, except for Levitt and Annie Jump Cannon, who joined the work there a year later. Modifying the equipment used and taking photographs of the stars was limited to men only. As for the photographic glass panels, they were delivered to the “Electronic Minds” room, located on the second floor of the eastern wing of the observatory building, for analysis and special calculations, with stars scattered on their surface.
In that room, the women spent their days focusing on the boards that were handed to them, classifying the stars, identifying the degree of brightness of each of them, and making lists of them. This work was intense, exhausting and stereotyped as well.
This plate shows the Small Magellanic Cloud, and on it we see notes made by Levitt based on her studies of so-called Cepheid variable stars.
Levitt's task was to determine the brightness of each of the "variable stars" above the North Pole, based solely on the analysis of photographs placed on glass plates. One of the reasons why the light emanating from these stars fluctuates is because they “pulse,” or rather expand and contract, as a result of them compressing gases and releasing them in regular cycles. In order to determine whether the star is “variable” or not; Levitt would place two panes of glass showing the same patch of sky side by side; One of them shows this area in its natural form, and the other shows it in a negative way. In this case, the brightness of the stars on the two panels is assumed to be equal, unless they are “variable stars” whose brightness varies from time to time. At that time, Levitt did not realize the importance of this method for calculating distances between space objects, given that the ability to calculate the distance of a star from Earth lies in understanding the relationship between the pattern of its brightness variation and its total size.
Thus, from one star to another, and from one photographic plate to another, this astronomer spent her days at the observatory, busy determining the brightness level of each of these stars, and comparing them to the known size of other stars, before recording the results in her notebooks.
In 1896, a year after beginning these research efforts, Levitt left the Harvard Observatory to wander through Europe and work in art at a college in Wisconsin, near her family home. But the stars eventually called her again.
Although this woman left no diaries or journals, and her correspondence revealed only limited personal details about her, she was clearly devoted to the work of the observatory, to the point that one of the astronomers who worked there at that time said that she “was immersed in "It works, to an unusual degree." Her love for this work was also evident in what was stated in a letter she addressed to Pickering in May 1902, that is, six years after she left the observatory, stating that she missed what she was doing there, and that she felt regret beyond what she could express, because “the work that... I undertook it with such joy, and brought it to a point with so much pleasure, that it had to be left unfinished.”
In light of Levitt's great desire at that time to return to the field of astronomy, she asked Pickering if he could help her rejoin the observatory staff, or find her a job as a school astronomy teacher. But her opportunities to enter the teaching profession were limited. Years before that, she had begun to suffer from an increasing decline in her ability to hear. Therefore, the ear doctor did not allow her to watch the sky on cold nights, believing that this would exacerbate her suffering from deafness. At the time, Pickering was at a loss as to how to fulfill Leavitt's demands, given that working in astronomy was believed to increase her health problems, and his inability to find an observatory in a warm climate that would be willing to operate it. Other. So she was offered to work at the Harvard Observatory again, for 30 cents an hour, five cents more than the other “electronic minds” were receiving. Leavitt accepted the offer and joined the observatory again, full-time, in 1903.
Henrietta Swan Leavitt in a photograph taken in 1910 or so
After that, Levitt returned to work on variable stars, coinciding with Pickering's interest in the same subject, especially the search for this type of star in nebula regions. In 1903, the man received a grant from the Carnegie Institute to fund his research efforts in this context. But the institute did not renew the grant. The following year, the observatory granted that grant, forcing Williamina Fleming, one of the prominent members of the “Electronic Minds” group, who was responsible for preserving astronomical photographs at the observatory, to dispense with all the women in the group except Leavitt, who at that time became solely responsible. , about dealing with nebulous regions. The first task assigned to her in this field was to examine a nebula known as the “Orion Nebula,” in a constellation that also bears the name “Orion.”
After examining astronomical photographic glass plates that monitor the conditions of this constellation over a period of 10 years, Levitt discovered 77 “variable planets” there. This woman then took over observing the nebulae in the constellation Sagittarius, and in an astronomical region known as the “Little Magellanic Cloud.” In the summer of 1905, Scientific American reported that Miss Levitt had found 1,300 new variable planets since she had taken up the work of observing nebula regions alone in February 1904.
Throughout 1908, Levitt continued her efforts to hunt more variable stars in the Small Magellanic Cloud and the Large Magellanic Cloud. At that time, astronomers did not know that these two nebulae formed two dwarf galaxies revolving around the Milky Way galaxy. Levitt found 1,777 new variable stars in the two galaxies, and was able to identify the highest and lowest brightness levels for each of them.
At this particular stage, the American astronomer observed a unique astronomical pattern. After I classified 17 of the new “variable stars” and made lists of objects similar to them in certain characteristics, I noticed that the brighter the star, the longer the period between it reaching peak brightness and reaching the lowest degree of dimming.
The Large Magellanic Cloud as seen through a telescope lens, not on a glass plate
It will later become clear that this observation will have far-reaching implications. But Levitt didn't focus on that much, or consider what it might mean. After submitting the tables she had prepared based on her observations in this regard, she took a leave of absence from work at the observatory due to her illness, from which it took a full year to recover. Levitt did not address the observation she made in this context, except through a few lines in a report she prepared, in which she said: “It is noteworthy that the brighter the variable star is, the longer the periods that separate it from reaching its peak brightness and reaching its lowest degree of dimming.” .
In the following period, Levitt moved between times of illness and periods during which she carried out other tasks at the observatory. But that did not prevent her from continuing to think about the variable stars she observed in the Large and Small Magellanic Clouds. So, three years after she prepared her initial report on their children, she returned to examine the objects in them.
Although this woman did not know at the time the exact distance between Earth and the stars on her photographic glass plates, she did know that, as part of the Small Magellanic Cloud, these stars were all located at approximately equal distances from our planet.
This represented an important conclusion; What he meant was that the differences between these stars in levels of brightness and dimness have nothing to do with how far they are from Earth. The brightest stars were not because they were closer to the Earth, but because they were intrinsically brighter. To verify her suspicions, Levitt determined the brightness and dimming cycles of eight new variable stars, and here she discovered that the deduction she had made three years earlier had not changed; The brighter the star, the longer it takes for it to go from being extremely bright to almost completely dim.
This time, Pickering shared her observation. A few months later, on March 3, 1912, the man published Levitt's observation regarding the “noteworthy relationship” between the level of brightness and the length of the cycle between it and the dimming. Hence, what initially seemed like a “promising trend” from a scientific perspective, turned into a “law” centered around a mathematical relationship, which was called at the time “an enlightenment relationship - a period of time.”
Unfortunately, Leavitt died before she knew how important the law she established was to astronomy
An expanding universe
Although Levitt did not formulate any theory regarding her new scientific law at that time; At least as written records from that period indicate, many other scholars would do so in the following years. Astronomers gradually realized that this law allows them to determine the distance between Earth and stars whose distance from our planet was in the past impossible to determine, due to the vast distances between them.
Decades ago, there were problems that prevented us from being able to determine the distance between Earth and space objects. Among these problems are: The brightness of each star is different from the other, which prevents us from identifying how close or far it is from our planet, by monitoring its brightness. In addition, the method of "stellar perspective" or "stellar displacement", which astronomers used to determine distances by comparing the relative motion of stars, could only be followed with stars located at a certain distance from the Earth, and could not be applied to existing objects. , at greater distances than that.
Levitt has shown that astronomers can determine the true brightness of variable stars, called Cepheid variables, during their cycle between brightness and dimness, regardless of how bright they appear to those looking at them from the surface of the Earth. This means that the presence of one of these stars, one of which is now known as the “standard candle,” in a distant galaxy, enables astronomers to calculate the distance between Earth and this galaxy.
Astronomers do this through what is called the “cosmic distance scale” or “extragalactic distance scale.” This measurement process begins by determining the distance between the Earth and a “standard candle” close to it, sufficient to calculate its “stellar perspective” and determine its brightness. Accordingly, the distance between our planet and a more distant “standard candle” can be calculated based on its level of brightness, assuming that these two “standard candles” have the same brightness pattern.
A decade after Levitt published her scientific law; Hubble pointed his telescope at Andromeda and found there a "standard candle" - the one the astronomer had been discovering - shining amidst the surrounding hazy nebula. When the man observed a “Cepheid variable” in that spot in the sky, he had the opportunity to recognize the distance between “Andromeda” and the Earth, according to Levitt’s law. Accordingly, Hubble proved that this nebula is located very far from our Milky Way galaxy. Using the same method, the same scientist was able, shortly thereafter, to find 23 other galaxies and determine how far they were from Earth. Some of them were as much as 20 million light-years away from our planet. In 1929, Hubble also discovered that the universe is expanding and expanding, and Levitt's law was the basis on which this discovery was built.
Astronomer and astrophysicist Wendy Friedman says that Levitt's law has become "the basis by which we have measured distances (and dimensions between) galaxies for more than a hundred years. At first, one only had a two-dimensional picture in this regard. But This law provided - for the first time - a three-dimensional image so that you can measure this third dimension, regardless of how far away the stars are, which seeks to know the distance between them and the Earth.
Friedman herself represents a clear example in this regard, as she relies heavily in her work on the “Cepheid variables” discovered by Levitt. Before the launch of the Hubble Space Observatory in 1990, astronomers knew that the universe was expanding, but they did not know its true size or age. Here Levitt's discoveries provided the solution once again. Friedman led a project in which this advanced observatory was used to calculate distances in space using Cepheid variables, which led to achieving this goal with much greater accuracy. The ability of Friedman and her research team to identify the distances between Earth and “variable stars” with such accuracy helped resolve decades of controversy regarding the age of the universe, which this scientist later said she discovered was 13.7 billion years old.
“The brighter the mass, the longer its period of luminosity,” is Levitt's elegant yet simple law, which forever changed the way astronomers look at the universe. Unfortunately, Levitt died of stomach cancer before realizing the full significance of her discovery. Friedman comments on this by saying: “One of the saddest things is that Levitt died in 1921, (two years before) Hubble made his discovery of the Cepheid variables in 1923, and then discovered that the universe is expanding and expanding, which is what was achieved.” The year 1929. Unfortunately, she had no idea of the enormous implications of what she had accomplished.”
Although Levitt did not survive to see how her discoveries changed astronomy, she was not forgotten by the scientists who came after her. Among them, the British-American scientist Cecilia Helena Payne, who continued to work on the topic of "variable stars" and discovered the structure of the Sun. This astronomer, who had never met Levitt, worked at the old desk that the woman used at Harvard, and said of her: “I think she was the most intelligent of all the women who worked” at the observatory of this American college.
Until this moment, the “standard candles” discovered by Levitt still illuminate astronomers’ path through the ever-expanding universe. To show appreciation and respect for this, astronomers - including Friedman - voted in 2008 to change what was previously known as “the law of the light-period relationship” to “Leavitt’s law.”
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