“Obviously Bold.” A Feminist Generation Keeps Marching
Podcast transcripts
Welcome to Math! Science! History! I’m Gabrielle Birchak, your host.
For Women’s History Month, I wanted to feature one brilliant thing, one clean win, and one woman whose work still quietly runs the world, even if most of us do not realize it.
Today’s “one brilliant thing” was a sorting system. A classification scheme. A way to take the universe, which is a chaotic glitter hurricane, and file it into something you could actually study.
The woman was Annie Jump Cannon.
If you have ever heard the sequence O, B, A, F, G, K, M, you have met her legacy. Astronomers still used that spectral classification system to describe stars. It was basically a cosmic organizing spell, and it worked. That sequence is part of the Harvard spectral classification system, which Cannon helped refine into the streamlined form widely used today.
Now, that string of letters looked like nonsense at first glance, so people invented silly mnemonics to remember it. The classic one went, “Oh Be A Fine Girl, Kiss Me.” It was very early-1900s, very cringe, very memorable. But I’m going to go with a more feminist vibe, and go with Obviously Bold, A Feminist Generation Keeps Marching.
I translate OBAFGKM into a sensory map: letters become temperatures, and temperatures become a color ladder. The letters are just names, but the ladder gives them meaning.
Think of OBAFGKM as turning a single dial, temperature. Turn it all the way up and O stars announce themselves with helium. Step down to B and helium is still strong, while hydrogen starts to show its hand. At A, hydrogen takes center stage. Cool further into F and G, and hydrogen softens while metals become clearer in the spectrum. By K, those metal signatures are bold. And at M, the air is cool enough for molecules, so titanium oxide paints the spectrum with wide, dark bands.
So the stars in our night sky have colors. And they correlate to that acronym. OBAFGKM.
O is Oxford blue, these are the hottest stars in our night sky. And they are between 30,000 and 50,000 Kelvin. They are rare. That dark blue Shows that the star is composed of ionized and neutral helium.
Moving on to B, bluish white, these stars are also very hot but not as hot as the dark blue ones. They’re between 10,000 and 30,000 Kelvin. And that color shows that they are also neutral helium but they’re also strongly composed of hydrogen.
Moving on to A, alabaster, white, those white stars that you see in the sky are between 7500 and 10,000 Kelvin. Still very very hot and composed strongly of hydrogen and ionized metals.
For the letter F, F represents the color fawn or flax, which is a yellowish white. These stars are between 6000 and 7500 Kelvin which shows that they have weaker hydrogen and more ionized metals.
For the letter G, gold, yellowish. These stars are composed of weaker amounts of hydrogen and are ionized and have neutral metals. They are between 5200 and 6000 Kelvin. So for example our sun is Listed in this classification as G2.
Moving on to K, those stars that you see in the K classification are kumquat, or orange. But to call them kumquats helps to remind me that K equals orange. These stars that you see have weaker hydrogen and neutral metals. And they are approximately 3700 to 5200 kelvins.
Finally the classification M, maroon, reddish. Those stars have little or no hydrogen at all they are neutral metals and have molecular bands like titanium oxide. And those stars are about 2400 to 3700 Kelvin.
But the real point is not the rhyme. The real point was that Cannon helped turn starlight into data.
Here is the setup.
At the end of the 1800s and the start of the 1900s, astronomy got a new superpower: By the late 1800s, astronomers were using spectroscopy, spreading starlight into a spectrum and recording it on photographic plates so the patterns could be compared and cataloged.
Instead of only looking at where a star was, astronomers could spread a star’s light into a spectrum, like a rainbow barcode. Those dark and bright lines in the spectrum told you what the star was made of, and crucially, what its physical conditions were.
At Harvard College Observatory, thousands of photographic glass plates captured those spectra. Each plate held a crowd of stars, each star leaving behind its own thin streak of light.
Someone had to look at those plates, identify patterns, and classify the stars consistently.
That “someone” was not a single person. At Harvard College Observatory, a large effort grew around analyzing those plates, and a group of women became known as the Harvard Computers for their work processing and cataloging astronomical data. Cannon was one of those astronomers, and she became especially known for her speed and consistency in classifying stellar spectra.
And Annie Jump Cannon became the one who took stellar classification and made it fast, consistent, and usable at a massive scale.
Cannon did not begin with a blank slate. Harvard already had earlier classification schemes. They were complicated. They used lots of letter categories, and they reflected an era when astronomers were still figuring out what the spectrum lines meant.
Cannon’s genius was partly scientific and partly practical. She simplified. She standardized. She kept what worked and threw the rest into the recycling bin of history.
Early spectral categories used letters that were not originally arranged by temperature, and Cannon’s major contribution was to simplify and reorder the system into the sequence OBAFGKM that is strongly associated with a star’s surface temperature in modern astronomy.
She ultimately emphasized the sequence OBAFGKM, which corresponded to a temperature sequence, from the hottest stars to cooler ones. It was not “alphabetical.” It was physical.
In other words, Cannon helped shift classification from “these spectra look kind of similar” to “these stars were actually different kinds of objects.”
Then she did the part that makes my brain short-circuit in admiration.
She classified stars. A lot of stars.
Then she helped produce one of the biggest “data products” in early astrophysics.
Working from those photographic plates, Cannon and her colleagues produced the Henry Draper Catalogue, which published spectroscopic classifications for 225,300 stars in the main catalog volumes, released between 1918 and 1924. It was 9 volumes published in the annals of Harvard college observatory.
After Edward C. Pickering’s death in 1919, Cannon oversaw completion of the remaining volumes and continued extensive classification work in the Henry Draper Extension publications.
Depending on which count you use, her lifetime total was often summarized as hundreds of thousands of stars, frequently cited around 350,000.
Let’s translate that into human terms.
That was not “she wrote a paper.” That was “she performed a census of the sky.”
That is why she was called the “census taker of the sky.” And the most delightful part was how direct the work was. It was not abstract. It was not a metaphor. It was literally someone sitting with glass plates, examining spectral lines, and making careful judgments, over and over, for years.)
To some, classification can sound boring. (sounds like heaven to me!) A shared classification system lets astronomers compare huge numbers of stars consistently, which helped astronomy scale into astrophysics.
There is a modern instinct to imagine this as mindless, mechanical labor. It was not.
Classification is where science stops being a scrapbook and becomes a machine. A classification system decides what questions you can ask next. It tells you what counts as the same, what counts as different, and what counts as weird enough to deserve a closer look.
Once a star had a spectral type, astronomers could connect it to other properties. Over time, spectral classification became a key piece of how scientists understood stellar temperatures, stellar evolution, and the structure of our galaxy.
So yes, the letters looked like a weird alphabet soup. But they became a shared language. They let astronomers talk to each other with precision.
Cannon’s system also did something else that was sneaky and powerful. It made astronomy scalable.
If you could classify stars consistently, you could compare thousands of them. Then tens of thousands. Then hundreds of thousands. You could start to ask statistical questions. You could look for patterns across populations of stars, instead of treating every star like a one-off curiosity.
That is the bridge from “beautiful objects in the sky” to “astrophysics.”
And Cannon, quietly, built that bridge.
There was also a human side to this story that mattered in Women’s History Month, and I will keep it light but real.
Cannon worked in an era when women’s scientific labor was often treated as support work, even when it was foundational. The Harvard Computers produced core results that underpinned the field, and it took a long time for the world to fully credit what they had done.
Cannon did receive major recognition during her lifetime, including major honors in astronomy and the creation of an award in her name that supported women in the field.
But the deeper recognition was simpler: her work endured. Her system stayed. Her classifications remained useful. When a scientific idea survives, it is usually because it is doing real work.
So, for this week, while the universe continued doing its chaotic glitter-hurricane thing, I wanted to pause and salute the woman who looked at starlight and said, “I can organize that.”
Because she did.
Three takeaways
First, classification is not boring. Classification is power. It shaped what a science could become.
Second, Annie Jump Cannon helped turn the night sky into a dataset, one spectrum at a time.
Third, the next time you heard “OBAFGKM,” you could remember this: that sequence was not just a mnemonic. It was a woman’s method made permanent.
I’m Gabrielle Birchak, and this has been Math! Science! History! Carpe Diem.