It’s been 100 years since we learned the Milky Way is not the only galaxy
On Sunday November 23, 1924, 100 years ago this month, readers perusing page six of the New York Times would have found an intriguing article, amid several large adverts for fur coats. The headline read: Finds Spiral Nebulae are Stellar Systems: “Dr. Hubbell Confirms View That They Are ‘Island Universes’; Similar to Our Own.”
The American astronomer at the center of the article, Dr. Edwin Powell Hubble, was probably bemused by the misspelling of his name. But the story detailed a groundbreaking discovery: Hubble had found that two spiral-shaped nebulae, objects made up of gas and stars, which were previously thought to reside within our Milky Way galaxy, were located outside it.
These objects were actually the Andromeda and Messier 33 galaxies, the closest large galaxies to our Milky Way. Today, up to several trillion galaxies are estimated to fill the universe, based on observations of tens of millions of galaxies.
Four years before Hubble’s announcement, an event called “the great debate” had taken place in Washington DC between the American astronomers Harlow Shapley and Heber Curtis. Shapley had recently shown the Milky Way to be larger than previously measured. Shapley argued that it could accommodate spiral nebulae within it. Curtis, on the other hand, advocated for the existence of galaxies beyond the Milky Way.
In hindsight, and ignoring certain details, Curtis won the debate. However, the method Shapley used to measure distances across the Milky Way was critical to Hubble’s discovery, and was inherited from the work of a pioneering US astronomer: Henrietta Swan Leavitt.
Measuring distances to stars
In 1893, a young Leavitt was hired as a “computer” to analyze images from telescope observations at Harvard College Observatory, Massachusetts. Leavitt studied photographic plates from telescope observations of another galaxy called the Small Magellanic Cloud carried out by other observatory researchers.
Leavitt was searching for stars whose brightness changed over time. From over a thousand variable (changing) stars, she identified 25 were of a type known as Cepheids, publishing the results in 1912.
The brightness of Cepheid stars changes with time, so they appear to pulse. Leavitt found a consistent relationship: Cepheids that pulsed more slowly were intrinsically brighter (more luminous) than those pulsing more quickly. This was dubbed the “period-luminosity relationship.”
Other astronomers realized the significance of Leavitt’s work: the relationship could be used to work out distances to stars. While a student at Princeton University, Shapley used the period-luminosity relationship to estimate distances to other Cepheids across the Milky Way. This is how Shapley reached his estimate for our galaxy’s size.
