THE WOMAN WHO MEASURED THE UNIVERSE Sometimes the most extraordinary tasks are
achieved by the most unsuspected ones. Generally, people think that to be a famous physicist
it’s necessary to be a genius, the rest is only a simple consequence. Well, it’s
not exactly like that! Surely, it’s important to be brilliant and clever, but hard work
and persistence are crucial too. Making great discoveries isn’t so easy, and sometimes
researchers go on working for years on topics that turn to be useless. In some other cases,
they spend a lot of time on projects of which they won’t see the final result. For example,
Einstein never “saw” the image of a black hole.
There’s a story that shows all these aspects really well. The protagonist is probably the
most unexpected: a deaf humble American woman, daughter of a congregationist pastor, who
apparently didn’t deserve the “status” of astronomer and was appointed to the role
of human calculator. We’re talking about one of the most influential women in the history
of Modern physics: Henrietta Swan Leavitt, the woman who measured the Universe.
Why is she so famous? What did she study? And why was she considered a human calculator?
Did she really measure the Universe? In what sense? These and other questions will be answered
in this video, so, if you are interested, stick with me and prepare for this great journey
in the life of one of the most important women of the last century.
I’m sure you are pretty confused by this introduction so let
me make things clear with a little explanation of what I said before.
Henrietta Leavitt was an American woman working at the observatory of Harvard to measure important
parameters of hundreds of stars. Thanks to her calculations, it was possible to understand
that the luminosity of some celestial bodies, called cepheids, follows a particular rule.
As a consequence of this relation, in 1908 physicists discovered how to calculate the
distance (which is the most difficult thing to derive in Astronomy) of thousands of stars.
A pretty impressing result for a woman that even lost her ability to hear and who was
coming from a humble family! Given this resume, let’s tell the story
of Henrietta. Where did she come from? Born in Massachusetts, she was the daughter of
a congregationist pastor and she attended the Radcliff College (also known as Harvard’s
Society for the Collegiate Instruction of Women).
Following an interest aroused in her last year, she decided to join Harvard Observatory.
In 1898 she became a member of the Harvard staff, as their first "Curator of Astronomical
Photographs". Then, she made two different trips in Europe during which she discovered
to suffer from hearing problems, which successively made her deaf. In 1903, she returned in America
and received a permanent staff appointment, working for Edward C. Pickering who had the
great project of measuring the brightness of all existing stars.
But why was Harvard hiring women at the beginning of 20th century? At that time, it was not
so common seeing a woman at work. Even in the scientific community there was a general
negative prejudice against women, and they couldn’t be employed for a role of astronomer.
According to this point of view, and considering the lack of funds for research, Pickering
decided to hire women as they could be paid a lot less than men. As a consequence of that
more staff could be hired for the same budget: this was a crucial element in an era in which
the power to collect astronomical data was surpassing the ability to process them in
a short period. Together with Henrietta, Harvard appointed the older Williamina Fleming and
the more nearly contemporary Annie Jump Cannon. They were known as the “computers” because
their main task was to measure brightness, position, and color of hundreds of stars.
Because Leavitt had independent means, Pickering initially did not have to pay her. Later,
she received $0.30 an hour for her work, being paid only $10.50 per week. She was reportedly
"hard-working, serious-minded …, little given to frivolous pursuits and selflessly
devoted to her family, her church, and her career". Consider that although some of Pickering's
female staff were astronomy graduates, their wages were similar to those of unskilled workers.
They usually earned more than a factory worker but less than a clerical one. However, this
wasn’t a problem for women, that sometimes, in order to get into this field of research,
also approved to work for free as volunteers. Leavitt soon advanced from routine work to
a position as head of the photographic stellar photometry department. This meant that she had the task to measure
the intensity of the radiation emitted by the star.
To understand better the main achievements of this incredible woman we must define some
astronomical quantities. You’ll probably know that luminosity is
a very important element regarding stars and galaxies. In astronomy we define two types
of luminosity. The first one is absolute luminosity, which
is the radiation power of a light emitting object. Consider it as the measurement of
its brightness: a small lamp has a lower luminosity than a lighthouse. Moreover, we can define
another quantity that derives by the previous one and it is named apparent luminosity. It
is proportional to the inverse of square distance. Shortly, apparent luminosity is higher when
the star is closer and viceversa. A lighthouse is certainly brighter than a lamp, but if
it’s 10 or 20 times more distant the lamp will “seem” having a greater power. Apparent
luminosity is a sort of superficial density of power.
In parallel, physicists defined apparent magnitude which is greater when luminosity is lower
and smaller when luminosity is higher. Obviously, it is dependent on distance and can be considered
as an equivalent of apparent luminosity. In addiction, another quantity called absolute
magnitude has been introduced by scientists: it measures the magnitude that all stars would
have at a distance of 10 parsec. What is a parsec? A parsec, nearly around 1016 meters
or 3,26 light years, represents the distance that causes a change of one second of degree
in the sky over an entire year. Why are these quantities important? Because
if we know both of them we can derive the distance of stars, since they’re linked
to each other through distance. This is a pivotal factor in astronomical research,
knowing how far a star is, is very difficult. Usually astronomers try to calculate the absolute
magnitude of a celestial body and derive its distance by comparing it with the apparent
magnitude coming from observations. "Before finding out more about this, be sure
to like or dislike the video so that we can continue to improve and make these videos
better for you the viewer. Plus, be sure to subscribe to the channel clicking the bell
so that you don't miss ANY of our weekly videos." How is this linked to Henrietta’s work?
She’s been the first to discover a method to calculate the absolute magnitude of a star.
After working on photometry, Pickering asked Henrietta to study the characteristics of
variable cepheids, a type of star with a luminosity that grows and falls periodically. How did
they manage to discover these celestial bodies? Pickering and the computer women used to observe
photographs of a sky region in different times. For example, they studied two images taken
in November and February: one plate was normal with a dark sky filled with white stars, while
the other was negative with a white sky and dark stars. Then, the plates were overlapped
and if the luminosity of all the objects hadn’t changed in the two images, everything would
be neutral as white and black white cancel each other. Consequently, when Henrietta started
seeing some brighter or darker points on the plate, she realized that it was a sign that
those stars were variable cepheids which had a different luminosity in those two images.
By repeating this procedure, it was possible to plot the luminosity variation in time.
the members of the Harvard observatory were able to see that the period of these stars
was nearly constant. But there’s more, Henrietta noticed something
that none of their colleagues could see. In the Spring of 1904 She began to study the
Magellanic Clouds, two nebulous objects firstly seen from Europeans in the 16th century.
She realized that this celestial bodies were extremely filled with variable cepheids. As
a result of that, she wrote a report in 1908, called “1777 Variables in the Magellanic
Cloud”. She deeply catalogued all of them showing different plates used to “detect”
them. However, sometimes the most important results are hidden and difficult to notice
at first sight and this is the case. Near the end Henrietta added a simple and innocent
consideration (perfectly fitting her humble personality) that was successively elaborated
by other physicists to determine the distance of hundreds of celestial bodies. That phrase
is the reason why today we’re able to symbolically measure the Universe. Here it is: “It is
worthy of notice that (in Table 6) the brighter variables have the longer periods”.
In 1912 Henrietta, after some domestic problems, published a further study that showed the
evidence of this correlation between luminosity and period. What was the incredible result?
Assuming that all these variables were at the same distance because they all belonged
to the Magellanic Cloud, it was possible to determine the distance of the cloud itself.
By observing the “heartbeat” in the luminosity of variable cepheids it was possible to derive
the distance of stars and galaxies. This result was extraordinary. In fact, at
that time the only method used to measure gaps between Earth and stars was the parallax.
This geometric procedure was based on the angular displacement of the celestial body
in the sky, caused by the movement of Earth around the Sun. Therefore, the displacement
of the furthest objects in the sky was nearly null: this method had a size limit, it could
just provide us information about our “neighbours stars”.
To be more precise, we must underline that Henrietta’s discovery just furnished the
proportion between the distances of two different cepheids. Wait a moment, but why? If the two
stars have the same periodic variation, this meant that they have the same absolute magnitude,
but it’s not the same for apparent magnitude. If one seems to have a lower brightness, this
means that its distance is bigger. Even Henrietta admitted that it was necessary to measure
the parallax of one cepheid to derive its distance and, consequently, that of all the
others with the same period. Initially, variables seemed to not undergo
any angular displacement in the sky, but astronomers developed a different parallax, based on the
relative position between the star and the Sun which amplified the size limit of this
method. With this procedure it was possible to measure the distance of same cepheids:
Henrietta’s luminosity-period correlation could be finally applied.
What is the end of this story? The great researcher from Massachusetts died
from stomach cancer in 1921 at the age of only 53 years. Leavitt’s work was pivotal
to the development of astronomy, astrophysics, and cosmology. Magnus Gösta Mittag-Leffler
of the Swedish Academy of Sciences tried to nominate her for the 1926 Nobel Prize in physics,
but she was no longer alive and Nobel Prizes are not awarded posthumously.
The list of discoveries made by exploiting this law is practically infinite, but the
one I’m going to show is the most impressive. In 1921 Edwin Hubble used Leavitt’s relationship
between a Cepheid’s pulse rate and its brightness to measure the distances of Cepheids located
in a number of nebulae. He discovered that the Cepheids were so far distant that they
were beyond the Milky Way; the nebulae were actually other galaxies and the universe was
much bigger than anyone had realized: it was expanding. Later on in time (always thanks
to Henrietta’s work), physicists discovered that the Universe was expanding at an accelerated
rate. This result is so important that a Nobel Prize was awarded for this discovery.
"This video ends here! Thanks for watching everyone! Did you find this topic exciting?
Are you amazed by Henrietta Leavitt’s achievements? Did you already know about her calculations?
Do you want to know more about important women in the history of Astronomy? Have you ever
heard about Henrietta before? Let me know in the comments below, be sure to subscribe,
and I'll see you next time on the channel!"