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Author: Jeffrey Grube, Senior Lecturer in Physics Education, King's College London
Original article: https://theconversation.com/its-100-years-since-we-learned-the-milky-way-is-not-the-only-galaxy-242952
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 centre 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.
Edwin Powell Hubble Papers / The Huntington, Author provided (no reuse)
Measuring distances to stars
In 1893, a young Leavitt was hired as a “computer” to analyse 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 realised 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.
Mount Wilson Observatory, Author provided (no reuse)
But, in order for astronomers to be sure about distances within our galaxy, they needed a more direct way to measure distances to Cepheids. The stellar parallax method is another way to measure cosmic distances, but it only works for nearby stars. As the Earth orbits the Sun, a nearby star appears to move relative to more distant background stars. This apparent motion is known as stellar parallax. Through the angle of this parallax, astronomers can work out a star’s distance from Earth.
The Danish researcher Ejnar Hertzsprung used stellar parallax to obtain the distances to a handful of nearby Cepheid stars, helping calibrate Leavitt’s work.
The New York Times article emphasised the “great” telescopes at the Mount Wilson Observatory near Los Angeles, where Hubble was working. Telescope size is generally assessed by the diameter of the primary mirror. With a 100-inch (2.5-metre) diameter mirror for collecting light, the Hooker telescope at Mount Wilson was the largest telescope at the time.
Jeff Grube, Author provided (no reuse)
Large telescopes are not only more sensitive to resolving galaxies, but also create sharper images. Edwin Hubble was therefore well placed to make his discovery. When Hubble compared his photographic plates taken using the 100 inch telescope with those taken on previous nights by other astronomers, he was thrilled to see one bright star appear to change in brightness over time, as expected for a Cepheid.
Using Leavitt’s calculations, Hubble found that the distance to his Cepheid exceeded Shapley’s size for the Milky Way. Over subsequent months, Hubble examined other spiral nebulae as he searched for more Cepheids with which to measure distances. Word of Hubble’s observations was spreading among astronomers. At Harvard, Shapley received a letter from Hubble detailing the discovery. He handed it to fellow astronomer Cecilia Payne-Gaposchkin, remarking: “Here is the letter that has destroyed my universe”.
Expansion of the Universe
Besides estimating the distance to a galaxy, telescopes can also measure the speed at which a galaxy moves towards or away from Earth. In order to do this, astronomers measure a galaxy’s spectrum: the different wavelengths of light coming from it. They also calculate an effect known as the Doppler shift and apply it to that spectrum.
The Doppler shift occurs for both light and sound waves; it is responsible for the pitch of a siren increasing as an emergency vehicle approaches, then decreasing as it passes you. When a galaxy is moving away from Earth, features of the spectrum known as absorption lines have longer measured wavelengths than they would if they were not moving. This is due to the Doppler shift, and we say that these galaxies have been “redshifted”.
Edwin Powell Hubble papers, The Huntington Library, San Marino, California, Author provided (no reuse)
Beginning in 1904, the American astronomer Vesto Slipher used the Doppler technique with a 24-inch telescope at the Lowell Observatory in Flagstaff, Arizona. He found that nebulae, including Andromeda, were all redshifted. Slipher found they were moving away from Earth at speeds as high as a thousand kilometres a second.
Hubble combined Slipher’s measurements with his distance estimates for each galaxy and discovered a relationship: the further a galaxy is from us, the faster it is moving away from us. This can be explained by the expansion of the Universe from a common origin, which would become known derisively as the Big Bang.
The announcement 100 years ago cemented Hubble’s place in the history of astronomy. His name would later be used for one of the most powerful scientific instruments ever created: the Hubble space telescope. It seems incredible how, over the course of just five years, our understanding of the Universe was brought into focus.