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

Try Our AI Features
Explore what Daily8 AI can do for you:
Comments
No comments yet...
Related Articles


San Francisco Chronicle
8 hours ago
- San Francisco Chronicle
Solstices brought Mayan communities together, using monuments shaped by science and religion – and kingly ambitions, too
Eds: This story was supplied by The Conversation for AP customers. The Associated Press does not guarantee the content. Gerardo Aldana, University of California, Santa Barbara (THE CONVERSATION) K'ahk' Uti' Witz' K'awiil knew his history. For 11 generations, the Mayan ruler's dynasty had ruled Copan, a city-state near today's border between Honduras and Guatemala. From the fifth century C.E. into the seventh century, scribes painted his ancestors' genealogies into manuscripts and carved them in stone monuments throughout the city. Around 650, one particular piece of architectural history appears to have caught his eye. Centuries before, village masons built special structures for public ceremonies to view the Sun – ceremonies that were temporally anchored to the solstices, like the one that will occur June 20, 2025. Building these types of architectural complexes, which archaeologists call 'E-Groups,' had largely fallen out of fashion by K'ahk' Uti' Witz' K'awiil's time. But aiming to realize his ambitious plans for his city, he seems to have found inspiration in these astronomical public spaces, as I've written about in my research on ancient Mayan hieroglyphically recorded astronomy. K'ahk' Uti' Witz' K'awiil's innovations are a reminder that science changes through discovery or invention – but also occasionally for personal or political purposes, particularly in the ancient world. E-Groups were first constructed in the Mayan region as early as 1000 B.C.E. The site of Ceibal, on the banks of the Pasión River in central Guatemala, is one such example. There, residents built a long, plastered platform bordering the eastern edge of a large plaza. Three structures were arranged along a north-south axis atop this platform, with roofs tall enough to rise above the rainforest floral canopy. Within the center of the plaza, to the west of the platform, they built a radially symmetric pyramid. From there, observers could follow sunrise behind and between the structures on the platform over the course of the year. At one level, the earliest E-Group complexes served very practical purposes. In Preclassic villages where these complexes have been found, like Ceibal, populations of several hundred to a few thousand lived on 'milpa' or 'slash-and-burn' farming techniques practices still maintained in pueblos throughout Mesoamerica today. Farmers chop down brush vegetation, then burn it to fertilize the soil. This requires careful attention to the rainy season, which was tracked in ancient times by following the position of the rising Sun at the horizon. Most of the sites in the Classic Mayan heartland, however, are located in flat, forested landscapes with few notable features along the horizon. Only a green sea of the floral canopy meets the eye of an observer standing on a tall pyramid. By punctuating the horizon, the eastern structures of E-Group complexes could be used to mark the solar extremes. Sunrise behind the northernmost structure of the eastern platform would be observed on the summer solstice. Sunrise behind the southernmost structure marked the winter solstice. The equinoxes could be marked halfway between, when the Sun rose due east. Scholars are still debating key factors of these complexes, but their religious significance is well attested. Caches of finely worked jade and ritual pottery reflect a cosmology oriented around the four cardinal directions, which may have coordinated with the E-Group's division of the year. Fading knowledge K'ahk' Uti' Witz' K'awiil's citizenry, however, would have been less attuned to direct celestial observations than their ancestors. By the seventh century, Mayan political organization had changed significantly. Copan had grown to as many as 25,000 residents, and agricultural technologies also changed to keep up. Cities of the Classic period practiced multiple forms of intensive agriculture that relied on sophisticated water management strategies, buffering the need to meticulously follow the horizon movement of the Sun. E-Group complexes continued to be built into the Classic period, but they were no longer oriented to sunrise, and they served political or stylistic purposes rather than celestial views. Such a development, I think, resonates today. People pay attention to the changing of the seasons, and they know when the summer solstice occurs thanks to a calendar app on their phones. But they probably don't remember the science: how the tilt of the Earth and its path around the Sun make it appear as though the Sun itself travels north or south along the eastern horizon. United through ritual During the mid-seventh century, K'ahk' Uti' Witz' K'awiil had developed ambitious plans for his city – and astronomy provided one opportunity to help achieve them. He is known today for his extravagant burial chamber, exemplifying the success he eventually achieved. This tomb is located in the heart of a magnificent structure, fronted by the 'Hieroglyphic Stairway ': a record of his dynasty's history that is one of the largest single inscriptions in ancient history. Eying opportunities to transform Copan into a regional power, K'ahk' Uti' Witz' K'awiil looked for alliances beyond his local nobility, and he reached out to nearby villages. Over the past century, several scholars, including me, have investigated the astronomical component to his plan. It appears that K'ahk' Uti' Witz' K'awiil commissioned a set of stone monuments or 'stelae,' positioned within the city and in the foothills of the Copan Valley, which tracked the Sun along the horizon. Like E-Group complexes, these monuments engaged the public in solar observations. Taken together, the stelae created a countdown to an important calendric event, orchestrated by the Sun. Back in the 1920s, archaeologist Sylvanus Morley noted that from Stela 12, to the east of the city, one could witness the Sun set behind Stela 10, on a foothill to the west, twice each year. Half a century later, archaeoastronomer Anthony Aveni recognized that these two sunsets defined 20-day intervals relative to the equinoxes and the zenith passage of the Sun, when shadows of vertical objects disappear. Twenty days is an important interval in the Mayan calendar and corresponds to the length of a 'month' in the solar year. My own research showed that the dates on several stelae also commemorate some of these 20-day interval events. In addition, they all lead up to a once-every-20-year event called a 'katun end.' K'ahk' Uti' Witz' K'awiil celebrated this katun end, setting his plans for regional hegemony in motion at Quirigua, a growing, influential city some 30 miles away. A round altar there carries an image of him, commemorating his arrival. The hieroglyphic text tells us that K'ahk' Uti' Witz' K'awiil 'danced' at Quirigua, cementing an alliance between the two cities. In other words, K'ahk' Uti' Witz' K'awiil's 'solar stelae' did more than track the Sun. The monuments brought communities together to witness astronomical events for shared cultural and religious experiences, reaching across generations. Coming together to appreciate the natural cycles that make life on Earth possible is something that – I hope – will never fade with fashion.


San Francisco Chronicle
8 hours ago
- San Francisco Chronicle
What is the summer solstice? An astronomer explains
(The Conversation is an independent and nonprofit source of news, analysis and commentary from academic experts.) Stephen Schneider, UMass Amherst (THE CONVERSATION) The summer solstice marks the official start of summer. It brings the longest day and shortest night of the year for the 88% of Earth's people who live in the Northern Hemisphere. People around the world traditionally observe the change of seasons with bonfires and festivals and Fête de la Musique celebrations. Astronomers can calculate an exact moment for the solstice, when Earth reaches the point in its orbit where the North Pole is angled closest to the Sun. That moment will be at 10:42 p.m. Eastern Time on June 20 this year. In Europe, Africa and points eastward, the moment of the equinox falls on June 21 locally, making that the day of the solstice. From Earth, the Sun will appear farthest north relative to the stars. People living on the Tropic of Cancer, 23.5 degrees north of the Equator, will see the Sun pass straight overhead at noon. Six months from now the Sun will reach its southern extreme and pass overhead for people on the Tropic of Capricorn, and northerners will experience their shortest days of the year, at the winter solstice. The Sun's angle relative to Earth's equator changes so gradually close to the solstices that, without instruments, the shift is difficult to perceive for about 10 days. This is the origin of the word solstice, which means 'solar standstill.' This slow shift means that daylight on June 20 is only about 2 seconds longer than on June 21, at mid-northern latitudes in the United States. It will be about a week before there's more than a minute change to the calculated amount of daylight. Even that's an approximation — Earth's atmosphere bends light over the horizon by different amounts depending on weather, which can introduce changes of more than a minute to sunrise and sunset times. Monuments at Stonehenge in England, Karnak in Egypt, and Chankillo in Peru reveal that people around the world have taken note of the Sun's northern and southern travels for more than 5,000 years. From Stonehenge's circle of standing stones, the Sun will rise directly over an ancient avenue leading away to the northeast on the solstice. We know little about the people who built Stonehenge, or why they went to such great effort to construct it — moving multi-ton stones from rock outcrops as far as 140 miles away. All this to mark the spot on the horizon where the Sun returns each year to rest for a while before moving south again. Perhaps they, like us, celebrated this signal of the coming change of seasons. Leer en español.


San Francisco Chronicle
8 hours ago
- San Francisco Chronicle
How Trump's ‘gold standard' politicizes federal science
(The Conversation is an independent and nonprofit source of news, analysis and commentary from academic experts.) H. Christopher Frey, North Carolina State University (THE CONVERSATION) The first time Donald Trump was president, the head of the U.S. Environmental Protection Agency developed a regulation known as the ' science transparency ' rule. The administration liked to call it the ' secret science' rule. 'Transparency' sounds positive, but this rule instead prevented the EPA from using some of the best available science to protect human health. For example, it required the EPA to ignore or downplay studies that established links between exposure to chemicals and health damage if those studies were based on confidential patient information that could not be released to the public. The problem: Many health studies, including those underpinning many U.S. pollution rules, rely on confidential patient information. A U.S. District Court struck down the rule on procedural grounds a few weeks after it was issued. But now, the idea is back. Trump's so-called Restoring Gold Standard Science executive order of May 23, 2025, resurrects many features of the EPA's vacated rule, but it applies them to all federal agencies. To many readers, the executive order might sound reasonable. It mentions 'transparency,' 'reproducibility' and 'uncertainty.' However, the devil is in the details. What's wrong with transparency and reproducibility? ' Transparency ' implies that scientists should adequately explain all elements of their work, including hypotheses, methods, results and conclusions in a way that helps others see how those conclusions were reached. ' Data transparency ' is an expectation that scientists should share all data used in the study so other scientists can recalculate the results. This is also known as ' reproducibility.' Trump's executive order focuses on reproducibility. However, if there are errors in the data or methods of the original study, being able to reproduce its results may only ensure consistency but not scientific rigor. More important to scientific rigor is ' replicability.' Replicability means different scientists, working with different data and different methods, can arrive at consistent findings. For example, studies of human exposure to a set of pollutants at different locations, and with different populations, that consistently find relationships to health effects, such as illness and premature death, can increase confidence in the findings. The science transparency rule in the first Trump administration was intended to limit the EPA's ability to consider epidemiologic studies like those that established the health harms from exposure to secondhand smoke and to PM2.5, fine particles often from pollution. Many large-scale studies that assess how exposure to pollution can harm human health are based on personal data collected according to strict protocols to ensure privacy. Preventing policymakers from considering those findings means they are left to make important decisions about pollution and chemicals without crucial evidence about the health risks. These attempts to create barriers to using valid science echoed tactics used by the tobacco industry from the 1960s well into the 1990s to deny that tobacco use harmed human health. Trump's new executive order also emphasizes 'uncertainty.' In the first Trump administration, the EPA administrator and his hand-picked science advisers, none of whom were epidemiologists, focused on 'uncertainty' in epidemiological studies used to inform decisions on air quality standards. The EPA's scientific integrity policy requires that policymakers 'shall not knowingly misrepresent, exaggerate, or downplay areas of scientific uncertainty associated with policy decisions.' That sounds reasonable. However, in the final 2020 rule for the nation's PM2.5 air quality standard, the EPA administrator, Andrew Wheeler, stated that 'limitations in the science lead to considerable uncertainty' to justify not lowering the standard, the level considered unhealthy. PM2.5 comes largely from fossil fuel combustion in cars, power plants and factories. In contrast, an independent external group of scientific experts, which I was part of as an environmental engineer and former EPA adviser, reviewed the same evidence and came to a very different conclusion. We found clear scientific evidence supporting a more stringent standard for PM2.5. The executive order also requires that science be conducted in a manner that is ' skeptical of its findings and assumptions.' A true skeptic can be swayed to change an inference based on evidence, whereas a denialist holds a fixed view irrespective of evidence. Denialists tend to cherry-pick evidence, set impossible levels of evidence and engage in logical fallacies. The first Trump administration stacked the EPA Clean Air Scientific Advisory Committee, which advises EPA on setting health-protective air quality standards, with opponents of environmental regulation, including people connected to industries the EPA regulates. The committee then amplified uncertainties. It also shifted the burden of proof in ways inconsistent with the statutory requirement to protect public health with an adequate margin of safety. The current administration has been dismantling science advisory committees in various agencies again and purging key EPA committees of independent experts. Who decides when politics trumps science According to Trump, ' violations ' of his executive order will be determined by a 'senior appointee designated by the agency head.' This means a political appointee accountable to the White House. Thus, science in each federal agency will be politicized. The political appointee is required to 'correct scientific information.' Anyone can file a 'request for correction ' regarding a published agency report. During the first Trump administration, chemical companies or their representatives repeatedly filed requests for changes to final EPA toxicity assessments on ethylene oxide and chloroprene. The administration delayed health-protective actions, which were finally addressed during the Biden administration for both chemicals. The request for correction process is intended to correct errors, not to bias assessments to be more favorable to industry and to delay protective actions. The bottom line on Trump's 'gold standard' While the language of the executive order may seem innocuous based on a casual reading, it risks undermining unbiased science in all federal agencies, subject to political whims. Setting impossible bars for 'transparency' can mean regulators ignore relevant and valid scientific studies. Overemphasizing uncertainties can be used to raise doubt and unduly undermine confidence in robust findings. A politicized process also has the potential to punish federal employees and to ignore external peer reviewers who have the temerity to advance evidence-based findings contrary to White House ideology. Thus, this executive order could be used to deprive the American public of accurate and unbiased information regarding chemicals in the environment. That would prevent the development of effective evidence-based policies necessary for the protection of human health, rather than advancing the best available science.