Nature does not give up her secrets easily. But sometimes, with great skill and effort, someone makes a truly important discovery and reveals knowledge even the wisest philosophers once believed to be beyond the reach of mankind.
The composition of stars is one example. In 1835, the French scientist Auguste Comte declared the composition of stars to be an example of knowledge forever beyond human understanding. Just a few years after Comte’s death, 19th century astronomers carefully measured starlight with spectroscopes and discovered that stars are made of the same material found on Earth… hydrogen and carbon and oxygen, and other common elements.
The distance to the stars and nearby galaxies was another example of the “unknowable” becoming known to determined and patient observers. Until the mid-19th century, many believed the distance to the stars was another example of knowledge beyond reach. Then, careful measurement of the parallax of a few nearby stars as the Earth moved around the Sun revealed the distance to a few nearby stars. But the true scale of our galaxy was a complete mystery. No one knew for sure whether the Milky Way was all there was to the universe, and whether it was a hundred light years across, or a thousand, or a trillion.
This changed in 1912 when an obscure and underpaid astronomer named Henrietta Leavitt discovered a type of bright variable star pulsated with a period directly proportional to its true brightness. She studied these variable stars in the Large Magellanic Cloud, a small irregular galaxy associated with the Milky Way, and noticed that brighter stars had longer pulsation periods. Since all these stars were roughly the same distance from Earth, she was able to use the period of these variable stars to determine their true brightness by calibrating them with similar stars closer to our solar system. And the true brightness of these variable stars could be compared to their apparent brightness to figure out their true distance, as well as the distance to star clusters and galaxies to which these stars belonged
This was a revolutionary discovery. Edwin Hubble used Leavitt’s work to measure these variable stars in the Andromeda “nebula” and determined it was not a nebula at all, but a galaxy in its own right lying more than 2 million light years away, some 20 times the span of our Milky Way. This discovery exploded the size the known universe, and was one of the most stunning and famous scientific discoveries in history.
Hubble became famous, of course, and remains so to this day. For her effort, Henrietta Leavitt was paid just $10.50 a week. She died in obscurity in 1921 at the age of 53, nearly forgotten, once of dozens of women employed as human “calculators” in the late 19th and early 20th centuries to help astronomers make many key discoveries. To his credit, Hubble said Leavitt deserved the Nobel Prize for her work.
The stars discovered by Leavitt are called “Cepheid variables”, after the prototype star δ (delta) Cephei in the constellation Cepheus. There are more than 700 known Cepheid variables in our galaxy, and thousands more in most galaxies visible out to a distance of 100 million light years. You can see a few other Cepheid variables yourself. Bright stars such as η (eta) Aquilae and Polaris, the North Star, are Cepheid variables. These stars, along with δ Cephei, are easy targets for even the most casual stargazer armed with a modest pair of binoculars. If you’re keen, you can track for yourself the change in brightness of some of these stars.
The remarkable story of Henrietta Leavitt is told well by George Johnson in his short book, “Miss Leavitt’s Stars: The Untold Story of the Woman Who Discovered How to Measure the Universe”. It’s a delightful read.