Annie Jump Cannon: Ordering the Stars

Patterns can be discovered, behind the chaos and noise.

Black and white illustrated portrait of Annie Jump Cannon, surrounded by a starry halo
Annie Jump Cannon by Sachi Weerasooriya
A dark night sky with the milky way from top left to bottom right and orange light in the distance beyond a black mountains
The Milky Way

When you look at a picture of our galaxy, it might seem like there are more stars in the Milky Way than we can ever possibly study. After all, every tiny pinprick of light is a unique object with its own size, brightness, and temperature.

But astronomers have managed to catalog almost a hundred million stars in our galaxy, using a classification method developed more than a hundred years ago by a scientist named Annie Jump Cannon.

The stars in the night sky may seem uncountable, but thanks to Cannon, we have catalogued millions of them

Learn more by watching the video or reading the blog below!


Studying the Stars

A cartoon of a women and child (Annie and her mother) sitting on a roof facing away toward a starry purple sky

Annie Jump Cannon was born in the mid 1800s. As a young girl, she had a fascination with stargazing. After dark, she and her mother would crawl out from the attic onto the roof of their home and look up at the stars together. Using an old textbook, Annie would identify the constellations she could see above her, watching them from her roof as they changed with the seasons.

In her youth, Annie Jump Cannon got very sick, which caused her to lose her hearing completely, so to communicate, she learned how to read lips.

Cannon’s love of the stars inspired her to go to school to study astronomy. She took classes at Wellesley and Radcliffe College, two schools for women near Boston. At Radcliffe, professors from the neighboring Harvard University would come and repeat their lectures to the female students, since women were not even allowed into Harvard classrooms. Cannon continued to take special advanced courses until she earned a master’s degree, and eventually became a teacher at Wellesley College.


The First Computers

In 1896, Annie Jump Cannon was hired at the Harvard Observatory to analyze pictures of stars. Before we had modern computers, these pictures. Taken through telescopes, had to be poured over by people with magnifying glasses!

These scientists, called “computers”, were all women, many with degrees in astronomy like Cannon. However, because of their gender, they didn’t have many jobs that were open to them, and they weren’t paid much-- only around 50 cents a day, which was just a little more than what they could have made working in a factory.

Eight women in long dresses sit at desks with notebooks and magnifying glasses and a bookshelf in the back left of the room
Harvard Computers analyzing pictures of stars
Two rows of women in long dresses and one man in a suit stand in front of a brick building with a wood and glass door
The Harvard Computers

This group of women was responsible for many important scientific discoveries. The Harvard computers categorized hundreds of thousands of stars, developed methods to determine stellar size, and discovered a new way to calculate distances in space.

While working as a computer, Annie Jump Cannon developed her new system for classifying stars, the same system we still use today!


Spectral Classification

Cannon’s method for categorizing stars is based on something called spectral classification. Scientists had noticed that when they looked at the light coming from stars, there were always dark lines of color missing from the rainbows that they emit.

This is because atoms and molecules can absorb light, leaving behind dark bands. And because different atoms and molecules have different sizes and structures, they actually absorb different colors, or wavelengths, of light.

This can be seen in the diagram below, where the incoming light shows the full spectrum of the rainbow initially. Then it enters the hydrogen gas cloud in the middle, where the atoms and molecules in the gas absorb different colors of the light. Then once the light comes out the other side you can see the dark bands in the rainbow.

A yellow line on right connected to a rainbow above bends in a white cloud and then connects to a rainbow with black lines

We call these dark bands missing from the rainbow spectral lines, and the pattern they produce can act as a kind of fingerprint that identifies which atoms are present in the star.

Cartoon of a spectroprah in lower right facing stars in the upper right with a rainbow for the absorption spectrum to the left

Cannon looked at the spectral lines of stars, and noticed that the fingerprint for hydrogen, which we call the Balmer lines, was stronger for some stars and weaker for others. Depending on the strength of the lines, she could then classify the stars with a letter- O, B, A, F, G, K, and M.

We now know that the reason that the lines look different is because the stars are different temperatures! O stars are bright furnaces that burn very hot, while M stars are dwarf stars that are much cooler. Our Sun is a G type star, not too hot and not too cold.

Seven circles in a line with each increasing in brightness and size with the small ones labeled as cool and the bright as hot

What Stars Are Made Of

Cannon’s classification system led to many important discoveries. For example, another astronomer named Cecilia Payne worked with Cannon and used her system to discover something very exciting about stellar composition. At the time, scientists thought that stars were made up of the same stuff as Earth.

Cecilia Payne in a long sleeved shirt smiling at camera sits and rests her hand at a desk in an office with a notebook open
Cecilia Payne

But Cecilia Payne studied Cannon’s spectral lines, and realized that stars aren’t made of heavy, metallic elements, but instead are composed of the lightest gases- hydrogen and helium.

This conclusion was so radical that at first it was rejected. It wasn’t until years later that other scientists realized that Payne was right. Thanks to Cecilia Payne and Annie Jump Cannon, we now know what stars are made of.


A Stellar Career

Annie Jump Cannon worked very hard to catalogue the stars in the sky with her new system of spectral classification. Throughout her career, she classified almost half a million stars, many of which she discovered herself. She got so good at identifying stars that by the end of her career, she could classify 200 stars an hour, with just her eyes and a little help from a magnifying glass.

So the next time you look up at the night sky and wonder just how many stars are out there, you can thank Annie Jump Cannon. Thank you for dedicating your life to figuring out the answer.

A black and white photo of an older Annie Jump Cannon, seated at a desk with papers and a photographic plate on the desk.
Annie Jump Cannon at the Harvard Observatory


Photos courtesy of Harvard-Smithsonian Center for Astrophysics and Smithsonian Institution Archives

Written by Caroline Martin

Edited by Ella King, Taylor Contreras

Original Illustrations by Sachi Weerasooriya

Primary source and additional reading:

Annie Jump Cannon (1863-1941) by Whitin Observatory, Wellesley College


Learn more about Annie Jump Cannon's research with these activities!

Investigate (30-45 minutes): Discover why Hydrogen's spectral lines appear when light from a nearby star passes through a cloud of Hydrogen gas.

Deepen (45-60 minutes): Learn more about the lifetime of stars using this Star in a Box simulation.

Observe (10-20 minutes): This post talks about the spectral lines of hydrogen, but all molecules have their own unique spectral "fingerprints". Check out this Molecules and Light simulator to see how some molecules interact with different wavelengths of light.

Challenge (2-3 hours): Build your own spectrometer to explore how light can be split into all the colors of the rainbow and give us clues about the makeup of stars.