Standard candle
A star or other object whose true brightness is known, so how dim it looks reveals how far away it is.
A standard candle is an object whose real brightness is known. Since light fades with the square of distance, comparing how bright such an object really is with how bright it looks gives its distance: one that looks a quarter as bright as expected is twice as far away.
The first standard candles were found by Henrietta Swan Leavitt at Harvard College Observatory, where she worked measuring stars on photographic plates, eventually for 30 cents an hour. Studying variable stars in the Small Magellanic Cloud, a small galaxy near our own, she noticed in 1908 that the brighter ones took longer to pulse. All the stars in that cloud are at roughly the same distance from us, so the brighter ones had to be genuinely more luminous. In 1912 she published the relationship for 25 of these stars, now called Cepheid variables: the longer a Cepheid's period, the greater its true brightness.
That made Cepheids measuring sticks. Time a Cepheid's pulse to learn its true brightness, compare that with how bright it appears, and you have its distance. In 1923 and 1924 Edwin Hubble found Cepheids in the Andromeda Nebula and showed they were far too distant to belong to our galaxy: Andromeda was a separate galaxy, and the universe was vastly larger than many had believed.
The catch is that dust between us and a star dims it too, making it seem farther away, so astronomers have to correct for it.
Coming up in future stories
- The Star Cairo Saw Three Times the Size of Venus 1006 · Cairo, Fatimid Egypt
- The Woman Who Measured the Universe for 30 Cents an Hour 1908–1912 · Harvard College Observatory, Massachusetts