“Where are we?” This is the question driving the passion of the astronomer Henrietta Leavitt in Lauren Gunderson’s play Silent Sky, to be presented
by the Los Alamos Little Theatre.
Copernicus taught us that the solar system is centered on the Sun, not our Earth. To understand our Solar System’s place in the larger world, requires measuring the distances to the stars. When Leavitt began her career at the Harvard College Observatory, the distances to only a few dozen stars had been measured through the technique of parallax, pioneered by Friedrich Bessel in 1838. The size and shape of the Milky Way that lay beyond those stars was still unknown, and the question of what, if anything, lay beyond the Milky Way was a topic of speculation.
The play tells part of Leavitt’s story. Assigned the task of measuring positions and brightnesses of stars recorded on the glass plates of Harvard College Observatory’s vast archive, her persistence and dedication led her to a discovery of the highest order. Patiently studying some 1800 variable stars in the Magellanic Clouds, she noticed that some of them, the Cepheids, followed a pattern: the brighter ones had longer periods.
The Danish astronomer Ejnar Hertzsprung recognized the implications of Leavitt’s work and published a calibration of her law in 1913. Unfortunately, Leavitt died too young to see the ultimate fruits of her labor.
Harlow Shapley used Leavitt’s law, also known as the Period-Luminosity Relationship for Cepheid Variables, to measure the size and shape of our galaxy. A few years later, in 1925, Edwin Hubble used it to show that the Andromeda “Nebula” (as it was then called) was a galaxy in its own right, far outside our own, confirming a famous guess by Immanuel Kant. The work of Leavitt, Shapley, and Hubble immediately extended the horizon of our vision to millions of lightyears. Remember that only a hundred years
before Hubble we didn’t know the distance to a single star beyond the sun.
Well past the Andromeda galaxy lie many other galaxies and clusters of galaxies. Measuring the distances to Cepheid variables in these galaxies led Hubble to the discovery of the law that bears his name, the universal cosmic expansion. We can now measure distances using Cepheids out to a few hundred million lightyears. Using other distance-comparing techniques, astronomers have assembled interlocking ladders that climb to billions of lightyears, to the very edge of space and the beginning of time.
The Copernican “revolution” is said to have begun modern astronomy, but we forget that it took over a hundred years before the heliocentric hypothesis was firmly established through the work of Galileo, Kepler, and Newton.
The Leavitt-Shapley-Hubble revolution exploded our horizon by a factor of a million in just 20 years, and we’ve gained another factor of a thousand in the century since. The revolution continues to this day; we are still harvesting the fruits of Henrietta Leavitt’s patient labors. Her work gave us the fundamental tool we still use to chart the size, age, and geometry of our Universe, to learn the answer to her probing question, “Where are we?”