Light and waves
1912
A ruler for the universe
Henrietta Swan Leavitt
How far away is a star you can never visit?
Studying photographs at Harvard, Leavitt found that certain pulsing stars, Cepheids, pulse more slowly the brighter they truly are. Time the pulse, know the true brightness, compare it with how bright it looks, and you know how far away it is.
First, make a guess
Move the screen twice as far from the lamp. Each patch gets…
- Squares the light covers
- 4.00
- Brightness of each
- 1/4.00 = 0.250
The same light spreads over a square whose sides grow with distance, so its area grows with distance squared. Twice as far: four squares share the light, each a quarter as bright. Leavitt’s pulsing stars tell you how bright a star truly is; how dim it looks then tells you how far away it is.
How they figured it out
Henrietta Leavitt worked as a “computer” at Harvard College Observatory, measuring the brightness of stars on photographic plates for about 30 cents an hour. She studied thousands of variable stars in the Small Magellanic Cloud, a distant cluster where all the stars are at roughly the same distance.
That meant brightness differences were real, not tricks of distance. She noticed that the brighter Cepheid stars pulsed more slowly, a clean rule. Published in 1912, it became the ruler for the universe. In 1925 a Swedish mathematician wrote to propose her for a Nobel Prize, not knowing she had died four years earlier.
The equation
Light spreads over a sphere, so a star twice as far looks four times dimmer.
See it in real life
Headlights at night
At night you judge how far away a car is from how bright its headlights look, because you know roughly how bright headlights really are. Astronomers call such objects standard candles. Leavitt found the first one in the sky.
Where engineers use it
Lighting design
Lux levels on a desk fall with the square of the lamp’s height.
Radio links
Signal strength falls with distance squared, setting how far a transmitter reaches.
What it led to
- The expanding universe 1922–1929
Built on
Leavitt’s pulsing stars gave Hubble his distances.
- The expansion speeds up 1998
Same technique
Both judge distance by how bright a star of known power appears.
Where the comparison stops: The 1998 teams used exploding stars (type Ia supernovae), visible billions of light-years away, far beyond Cepheids’ reach, and calibrated with Cepheids nearby.