Mile Markers to the Galaxies
Posted by Brian Ventrudo
Sometimes, though not often, nature points the way to knowledge even the wisest philosophers believe to be beyond our comprehension.
The composition of stars is one example. In 1835, the French scientist Auguste Comte declared the composition of stars was an example of knowledge forever beyond human understanding. Just a few years after Comte’s death, 19th century astronomers carefully measured starlight with prisms and spectroscopes and discovered that stars are made of the same material found on Earth… hydrogen and carbon and oxygen, among other elements.
The distance to the stars and nearby galaxies is another example of the “unknowable” becoming known to determined and patient observers. Until the mid-19th century, no one knew the distance to the stars, and no one knew for sure if anyone would ever know how to find the distance to the stars. Then, careful measurement of the parallax of a few nearby stars as the Earth moved around the Sun revealed the distance to a few closeby 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. That’s when an obscure and underpaid astronomer named Henrietta Leavitt discovered a particular 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, 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. And their true brightness could be compared to their apparent brightness to figure out the true distance to these stars… and the other star clusters and galaxies to which these stars belonged.
This was a revolution. Edwin Hubble took Leavitt’s discovery 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 of the past 100 years.
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. Though to his credit, Hubble often said she deserved the Nobel Prize for her work.
The stars discovered by Leavitt are called “Cepheid variables”, after the prototype star delta Cephei in the constellation Cepheus, not far from the star clusters we looked at in our last article. 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 Cepheid variables yourself. Bright stars such as eta Aquilae and Polaris, the north star, are Cepheid variables, not to mention delta Cephei itself (see below). These three stars are ideal 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 location of delta (δ) Cephei (upper left), the namesake of Cepheid variable stars (click to enlarge)
The inspiring but melancholy 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.