The Milky Way isn’t the only galaxy: 100th anniversary
- Until 100 years ago, many astronomers believed the Milky Way was the only galaxy in the universe. Edwin Hubble – for whom the Hubble telescope was named – proved otherwise 100 years ago this month.
- Hubble measured distances to stars in other galaxies using a large telescope. He showed some star distances exceeded the measurement for the size of our Milky Way. So these stars must be located beyond our galaxy’s boundaries.
- Eventually, Hubble discovered that our universe is expanding. He did this by noting that the greater the distance to a galaxy, the faster it is moving away.
By Jeffrey Grube, King’s College London
The Milky Way is one galaxy of trillions
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 amused 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.
A universe of galaxies
These objects were actually the Andromeda and Messier 33 galaxies, the closest large galaxies to our Milky Way. Today, astronomers estimate up to several trillion galaxies 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, D.C., 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 his method was inherited from the work of a pioneering U.S. astronomer: Henrietta Swan Leavitt.
Measuring to stars outside the Milky Way
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 as 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: they could use the relationship 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.
Stellar parallax
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 emphasized 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-meter) diameter mirror for collecting light, the Hooker telescope at Mount Wilson was the largest telescope at the time.
Large telescopes are not only more sensitive to resolving galaxies, but also create sharper images. So Edwin Hubble was in a good location 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.
Distances to stars exceeding the size of the Milky Way
Using Leavitt’s calculations, Hubble found 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.
The Doppler shift and redshift
Besides estimating the distance to a galaxy, telescopes can also measure the speed at which a galaxy moves toward 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.
Expansion of the universe
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 were either redshifted (moving away) or blueshifted (traveling toward us). Slipher found some nebulae were moving away from Earth at speeds as high as a thousand kilometers a second.
Hubble combined Slipher’s measurements with his distance estimates for each galaxy and discovered a relationship: the farther 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.
Jeffrey Grube, Senior Lecturer in Physics Education, King’s College London
This article is republished from The Conversation under a Creative Commons license. Read the original article.
Bottom line: It was 100 years ago that Edwin Hubble showed the Milky Way is only one galaxy of many in our universe.
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