One large Milky Way galaxy or many galaxies? 100 years ago, a young Edwin Hubble settled astronomy's ‘Great Debate.'

Yahoo

17-02-2025

This article was originally featured on The Conversation.
A hundred years ago, astronomer Edwin Hubble dramatically expanded the size of the known universe. At a meeting of the American Astronomical Society in January 1925, a paper read by one of his colleagues on his behalf reported that the Andromeda nebula, also called M31, was nearly a million light years away – too remote to be a part of the Milky Way.
Hubble's work opened the door to the study of the universe beyond our galaxy. In the century since Hubble's pioneering work, astronomers like me have learned that the universe is vast and contains trillions of galaxies.
In 1610, astronomer Galileo Galilei used the newly invented telescope to show that the Milky Way was composed of a huge number of faint stars. For the next 300 years, astronomers assumed that the Milky Way was the entire universe.
As astronomers scanned the night sky with larger telescopes, they were intrigued by fuzzy patches of light called nebulae. Toward the end of the 18th century, astronomer William Herschel used star counts to map out the Milky Way. He cataloged a thousand new nebulae and clusters of stars. He believed that the nebulae were objects within the Milky Way.
Charles Messier also produced a catalog of over 100 prominent nebulae in 1781. Messier was interested in comets, so his list was a set of fuzzy objects that might be mistaken for comets. He intended for comet hunters to avoid them since they did not move across the sky.
As more data piled up, 19th century astronomers started to see that the nebulae were a mixed bag. Some were gaseous, star-forming regions, such as the Orion nebula, or M42 – the 42nd object in Messier's catalog – while others were star clusters such as the Pleiades, or M45.
A third category – nebulae with spiral structure – particularly intrigued astronomers. The Andromeda nebula, M31, was a prominent example. It's visible to the naked eye from a dark site.
The Andromeda galaxy, then known as the Andromeda nebula, is a bright spot in the sky that intrigued early astronomers.
Astronomers as far back as the mid-18th century had speculated that some nebulae might be remote systems of stars or 'island universes,' but there was no data to support this hypothesis. Island universes referred to the idea that there could be enormous stellar systems outside the Milky Way – but astronomers now just call these systems galaxies.
In 1920, astronomers Harlow Shapley and Heber Curtis held a Great Debate. Shapley argued that the spiral nebulae were small and in the Milky Way, while Curtis took a more radical position that they were independent galaxies, extremely large and distant.
At the time, the debate was inconclusive. Astronomers now know that galaxies are isolated systems of stars, much smaller than the space between them.
Edwin Hubble was young and ambitious. At the of age 30, he arrived at Mount Wilson Observatory in Southern California just in time to use the new Hooker 100-inch telescope, at the time the largest in the world.
He began taking photographic plates of the spiral nebulae. These glass plates recorded images of the night sky using a light-sensitive emulsion covering their surface. The telescope's size let it make images of very faint objects, and its high-quality mirror allowed it to distinguish individual stars in some of the nebulae.
Estimating distances in astronomy is challenging. Think of how hard it is to estimate the distance of someone pointing a flashlight at you on a dark night. Galaxies come in a very wide range of sizes and masses. Measuring a galaxy's brightness or apparent size is not a good guide to its distance.
Hubble leveraged a discovery made by Henrietta Swan Leavitt 10 years earlier. She worked at the Harvard College Observatory as a 'human computer,' laboriously measuring the positions and brightness of thousands of stars on photographic plates.
She was particularly interested in Cepheid variables, which are stars whose brightness pulses regularly, so they get brighter and dimmer with a particular period. She found a relationship between their variation period, or pulse, and their intrinsic brightness or luminosity.
Once you measure a Cepheid's period, you can calculate its distance from how bright it appears using the inverse square law. The more distant the star is, the fainter it appears.
Hubble worked hard, taking images of spiral nebulae every clear night and looking for the telltale variations of Cepheid variables. By the end of 1924, he had found 12 Cepheids in M31. He calculated M31's distance as a prodigious 900,000 light years away, though he underestimated its true distance – about 2.5 million light years – by not realizing there were two different types of Cepheid variables.
His measurements marked the end of the Great Debate about the Milky Way's size and the nature of the nebulae. Hubble wrote about his discovery to Harlow Shapley, who had argued that the Milky Way encompassed the entire universe.
'Here is the letter that destroyed my universe,' Shapley remarked.
Always eager for publicity, Hubble leaked his discovery to The New York Times five weeks before a colleague presented his paper at the astronomers' annual meeting in Washington, D.C.
But Hubble wasn't done. His second major discovery also transformed astronomers' understanding of the universe. As he dispersed the light from dozens of galaxies into a spectrum, which recorded the amount of light at each wavelength, he noticed that the light was always shifted to longer or redder wavelengths.
Light from the galaxy passes through a prism or reflects off a diffraction grating in a telescope, which captures the intensity of light from blue to red.
Astronomers call a shift to longer wavelengths a redshift.
It seemed that these redshifted galaxies were all moving away from the Milky Way.
Hubble's results suggested the farther away a galaxy was, the faster it was moving away from Earth. Hubble got the lion's share of the credit for this discovery, but Lowell Observatory astronomer Vesto Slipher, who noticed the same phenomenon but didn't publish his data, also anticipated that result.
Hubble referred to galaxies having recession velocities, or speeds of moving away from the Earth, but he never figured out that they were moving away from Earth because the universe is getting bigger.
Belgian cosmologist and Catholic priest Georges Lemaitre made that connection by realizing that the theory of general relativity described an expanding universe. He recognized that space expanding in between the galaxies could cause the redshifts, making it seem like they were moving farther away from each other and from Earth.
Lemaitre was the first to argue that the expansion must have begun during the big bang.
NASA named its flagship space observatory after Hubble, and it has been used to study galaxies for 35 years. Astronomers routinely observe galaxies that are thousands of times fainter and more distant than galaxies observed in the 1920s. The James Webb Space Telescope has pushed the envelope even farther.
The current record holder is a galaxy a staggering 34 billion light years away, seen just 200 million years after the big bang, when the universe was 20 times smaller than it is now. Edwin Hubble would be amazed to see such progress.