Today, while exploring star periods 🔭 and the Cepheid variable stars, I learned about Leavitt's Law. I assumed it was a man, but to my surprise, it was Henrietta Swan Leavitt. I want to talk about her. I believe her work is very noteworthy. As a female scientist, stories and discoveries like this deserve to be in the spotlight.
Cepheids are stars that pulsate. They physically expand and contract in a repeating cycle driven by internal pressure waves. Smaller, less massive Cepheids pulsate faster because there is less mass to move. Bigger ones pulse more slowly, like a large bell ringing slower than a small one.
In the 1900s, we had telescopes to detect starlight, and we knew how to calculate periods of stars. One period being one complete cycle of behaviour, repeating over time. However, we had no way of saying: this star's light is this bright, so it must be this far. Distance and brightness didn't have a solid prior to go about separating them.
This is where Henrietta comes in. Harvard, 1908, Small Magellanic Cloud. Observing Cepheids in the same galaxy for years. Stars all sitting at roughly the same distance from Earth, so any brightness difference between them is real, not a distance artifact. Mapping out their periods, she noticed that slower pulsing stars were consistently brighter, and faster pulsing stars were consistently dimmer.
This was her job: 25 cents an hour, just to catalogue measurements from glass photographic plates by hand.
She mapped the brightness of each star across its full cycle, noticing they all traced the same sawtooth curve: a rapid rise to peak brightness, then a slow, gradual fall back down, and repeat. Today, we call this phase folding.
She did this for all the SMC Cepheids until they followed a linear log plot. (see image below) and confirmed the relationship. The slope of that line is fixed by physics, the same everywhere in the universe. But- the intercept shifts with distance. Point this law at Cepheids in any galaxy, measure their periods, compare predicted brightness to what your telescope sees. The gap tells you exactly how far that galaxy is.
Henrietta, just with her eyes and mind, made one of the most important scientific discoveries that became a ruler of the universe, to accurately measure distances. I thought this was beautiful.
I pulled data from the OGLE-IV survey: 2439 real Cepheid stars in the Large Magellanic Cloud, and wrote Python code to reproduce this result — the same relationship Henrietta found in the SMC, holding just as cleanly a galaxy away. For those of you interested, the code and notes are on my GitHub.
#astronomy #womeninscience #henriettaleavitt #cepheids #leavittslaw #python
https://github.com/saniasingh211/Research/tree/main/stars
Reproduced the Leavitt Law from 2439 real Cepheid stars using OGLE-IV survey data. Phase folded a single Cepheid 160,000 light years away. All notebooks and theory notes are in the repo.