The theory of the Period-Luminosity law
Henrietta Swan Leavitt's discovery may seem complex, but it is based on a simple and revolutionary principle. To understand it, we must first understand what Cepheids are and how they helped us measure the universe.
Cepheids: The lighthouses of space
A variable Cepheid star pulsating in space
Cepheids are stars whose brightness varies regularly, like a lighthouse blinking in the night. Some shine, then dim, then shine again, following a very precise rhythm. Before Leavitt, astronomers observed these stars but did not understand the link between their rhythm and their true luminosity.
The enlightenment: The slower it is, the brighter it is
By analyzing thousands of Cepheids, Leavitt discovered a remarkable relationship: the longer the pulsation period of a Cepheid, the brighter its absolute luminosity.
This means that by simply measuring the rhythm at which a Cepheid blinks, we can deduce how bright it really is. And by comparing this real brightness to its apparent brightness as we see it from Earth, we can calculate its distance.
"The slower the star blinks, the brighter it is. This is the rule that allowed us to finally measure the universe."
From luminosity to distance
This law transformed Cepheids into standard candles, reliable tools for measuring cosmic distances. Thanks to Leavitt, astronomers were finally able to determine the size of our galaxy, then discover that there were other galaxies far beyond the Milky Way. It was a revolution that completely changed our understanding of the cosmos.