Edwin Hubble

Edwin Hubble
Institutions	Mount Wilson Observatory; Carnegie Observatories
Known for	Expansion of the universe; Hubble's law; galaxy classification
Occupation	Astronomer
Notable works	The Realm of the Nebulae
Alma mater	University of Chicago; University of Oxford
Field	Extragalactic astronomy; observational cosmology
Wikidata	Q43027

Edwin Powell Hubble (1889–1953) was an American astronomer whose observations transformed our understanding of the cosmos. In the early 20th century he showed that the universe contains countless galaxies beyond the Milky Way and provided strong evidence that the universe is expanding. His work led to the formulation of Hubble’s Law, the linear relation between a galaxy’s distance and its recession speed, and established the field of extragalactic astronomy. He also devised the famous “tuning-fork” classification of galaxies by shape. Hubble’s discoveries laid the foundations of modern cosmology and helped shift the prevailing view from a static to a dynamic, evolving universe.

Early Life and Education

Edwin Hubble was born on November 20, 1889, in Marshfield, Missouri. His father, John Hubble, was a businessman, and his mother, Virginia Lee James, ran the household. As a youth, Hubble was athletic and an avid reader, but he was also deeply curious about science and the outdoors. In 1906 he entered the University of Chicago, where he studied mathematics and astronomy. While an undergraduate he worked in the laboratory of future Nobel laureate Robert Millikan, and he graduated in 1910 at age 21.

After college, Hubble won a Rhodes Scholarship to Oxford University in England, partly to study law (his father wanted him to have a conventional career). At Oxford (1910–1913), he earned a B.A. in jurisprudence, but he spent much of his free time studying astronomy and philosophy. By the end of his Oxford years, Hubble decided to pursue science rather than law. Returning to the United States, he briefly taught high school before enrolling in graduate school in astronomy at the University of Chicago.

Hubble completed his Ph.D. in astronomy at Chicago in 1917, just as World War I was raging. His doctoral work involved photographing and cataloging faint nebulae (then the term for objects like galaxies). Immediately after earning his degree, Hubble enlisted in the U.S. Army and served in France during the war, although he did not see combat. His former mentor George Ellery Hale, director of the Mount Wilson Observatory in California, held a position open for Hubble until the war ended. In 1919 Hubble joined the Mount Wilson Observatory staff, which housed the world’s largest telescopes.

Major Works and Ideas

At Mount Wilson, Edwin Hubble used the powerful 60-inch and later 100-inch Hooker telescopes to study the so-called “spiral nebulae.” These were fuzzy, cloud-like patches of light observed in the sky. In Hubble’s day it was debated whether these spiral nebulae were part of our own Milky Way Galaxy or were separate “island universes” (what we now call galaxies). Hubble’s critical contribution was to settle this debate with direct distance measurements.

In 1923–1924 Hubble detected Cepheid variable stars in the Andromeda Nebula (Messier 31) and other spiral nebulae. Cepheid variables are stars whose intrinsic brightness varies with a fixed period; the longer the period, the brighter the star. Because of this period–luminosity relation (discovered earlier by Henrietta Leavitt), Cepheids serve as “standard candles” for measuring astronomical distances. Hubble carefully measured the periods of Cepheids in Andromeda and used the known period–luminosity relation to calculate their true luminosity and distance. He found that Andromeda lay about 900,000 light-years away (later refined to about 2.5 million light-years), far outside the Milky Way’s ~300,000 light-year size. This demonstrated that Andromeda was its own galaxy, not a nebula within our galaxy. Within a few years Hubble’s observations of other spiral nebulae similarly confirmed that many distant star systems exist beyond the Milky Way. By the mid-1920s the debate over the “island universe” was effectively over: the universe contains a myriad of galaxies, each filled with billions of stars.

In parallel with establishing the extragalactic scale, Hubble worked on classifying these newly recognized galaxies by their shapes. In the 1920s he developed what became known as the Hubble Classification or Hubble Sequence. He sorted galaxies into a few broad categories based on their appearance in photographs. The main classes are elliptical galaxies (smooth, ellipsoidal collections of stars), spiral galaxies (flat disks with spiral arms), and lenticular galaxies (disk-shaped without prominent arms). Spiral galaxies were further divided into “normal” spirals and “barred” spirals (the latter have a straight bar of stars through the center). Hubble arranged these classes in a branching “tuning-fork” diagram, with ellipticals on one end and the two kinds of spirals on the other branches. Although later research has shown that galaxy evolution does not follow this sequence exactly, Hubble’s classification remains a fundamental way to organize galaxy types. In 1926 he published The Hubble Atlas of Galaxies, an influential work illustrating his classification scheme.

Hubble’s most famous discovery came in 1929 when he studied the velocities of galaxies. Hubble built on measurements of galaxy spectra (showing Doppler shifts) made by earlier astronomers Vesto Slipher and Milton Humason at Mount Wilson. These observations showed that the spectral lines of virtually all distant galaxies were shifted toward the red end of the spectrum, indicating they were moving away from us. Hubble combined this radial-velocity data with his own distance measurements (from Cepheid stars and other methods) for the same galaxies. He plotted recessional velocity versus distance and found a clear linear relationship: on average, the farther away a galaxy is, the faster it appears to be receding. This is the famous Hubble’s Law.

The linear velocity-distance relation implied that space itself is expanding: galaxies are carried apart by an expanding universe. In modern terms, the slope of Hubble’s law is called the Hubble constant (H₀), which sets the universe’s expansion rate. Hubble’s original value was much higher than current estimates (due to calibration errors), but the basic idea remains valid. The discovery that the universe is expanding had profound implications: it meant that the universe had a history and must have been smaller in the past, which underlies the Big Bang theory of cosmology. Although Hubble himself was cautious about drawing strong conclusions about cosmic origins, his observational evidence for expansion was a key turning point in astronomy.

Beyond these headline discoveries, Hubble made additional contributions. He and collaborators studied the distribution and brightness of galaxies, and Hubble found empirical laws describing how galaxy brightness falls off with radius (useful for later theoretical models). In the 1930s he coauthored papers examining whether the expansion implied cosmological evolution, often with mathematician Richard Tolman. In 1936 Hubble published a comprehensive book, The Realm of the Nebulae, summarizing his work on galaxies and the history of cosmology up to that point. By the late 1930s, Hubble had set down much of the groundwork of extragalactic astronomy for future researchers, and he remained at Mount Wilson for the rest of his career.

Method

Hubble’s breakthroughs were made possible by meticulous observation and measurement. He used the best telescopes of his day, especially the 100-inch Hooker reflector at Mount Wilson, which was the world’s largest telescope in the 1910s–30s. That large aperture allowed him to resolve individual stars in distant galaxies. Hubble took photographic plates and studied their images carefully under the microscope.

A key tool in his method was the Cepheid variable period–luminosity relation. In practice, Hubble identified Cepheid stars in galaxies like Andromeda by looking for stars whose brightness varied regularly over days to weeks. Once he measured the variation period, he applied the known relation (discovered by Henrietta Leavitt at Harvard) to infer the star’s absolute luminosity. Comparing the absolute luminosity with the apparent brightness on the photographic plate gave the distance to the star and hence to the galaxy. This technique of “standard candles” was novel at extragalactic distances.

For galaxy velocities, Hubble relied on spectroscopy. By analyzing the spectra of galaxies taken with a spectrograph, he measured the shift of known spectral lines. A shift toward the red (longer wavelengths) indicated motion away from Earth due to the Doppler effect. Although Vesto Slipher had measured many galaxy redshifts earlier, it was Hubble and his colleague Milton Humason who extended these measurements to fainter (more distant) galaxies. Hubble then plotted redshift-derived velocities against his distance estimates. The straight-line fit he found — that velocity is proportional to distance — came directly from these observational data.

Hubble’s work also involved statistical surveys of galaxies. He and collaborators like Astronomer G. P. Hume cataloged thousands of galaxies by brightness and apparent size to infer how galaxies are distributed in space. In analyzing such surveys, Hubble sometimes used mathematical tools (for example collaborating with theorist Richard Tolman) to compare the data with models of a static versus expanding universe. Throughout, Hubble was careful to account for observational uncertainties; he often withheld final interpretation until data were convincing. Even so, he published the key relation in 1929 once the pattern was clear, saying in the title of his paper: “A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae.”

Influence

Edwin Hubble’s discoveries had an immediate and lasting impact on astronomy and cosmology. By establishing that the universe extends far beyond the Milky Way, he opened up the vast field of extragalactic astronomy. The simple act of showing that other galaxies exist compelled astronomers to rethink the scale of the universe itself. Within a few years, cosmic structures beyond our galaxy became a standard part of astronomy textbooks.

Hubble’s Law and the expanding universe paradigm reshaped cosmology. It provided the first strong observational evidence that Albert Einstein’s equations of general relativity could describe a dynamic universe. Soon after, Georges Lemaître and others developed the idea of an expanding, evolving cosmos that would come to be known as the Big Bang theory. Hubble himself was not the first to propose an expanding model, but his data transformed the idea from speculation to widely accepted fact. The concept that all distant galaxies recede in proportion to their distance remains a cornerstone of cosmology: observatories around the world use Hubble’s Law (with updated calibrations) to measure the expansion rate and age of the universe.

Hubble’s influence extended beyond research papers. His classification of galaxies provided astronomers with a common language. The terms “spiral” and “elliptical” galaxies, and even the idea of barred spiral with a central bar, trace directly to Hubble’s scheme. Contemporary sky surveys and galaxy catalogs still organize objects partly by Hubble type. His 1936 book The Realm of the Nebulae was a landmark in summarizing extragalactic astronomy up to that point, and educated a generation of scientists.

In recognition of Hubble’s importance, many honors have carried his name. The most famous is the Hubble Space Telescope, launched in 1990 by NASA and the European Space Agency. This powerful space observatory was named after Edwin Hubble to honor his legacy of exploring the universe. (Some astronomers argue that Hubble himself might have balked at this honor — he was focused on data, not spacecraft names — but the telescope’s name ensures his contributions remain visible to the public.) Hundreds of astronomical institutions, awards, and even geographical features bear his name. For example, the “Hubble constant” is the standard term for the universe’s expansion rate, the “Hubble time” is a measure of cosmic age, and maps of the sky mark a region called the “Hubble deep field” in tribute to a famous image series taken by the Hubble Telescope.

Hubble’s work also inspired further developments in observation. The idea of looking deeper into space (longer exposures, larger telescopes) owes much to his trailblazing efforts with the Hooker telescope. Today’s massive surveys of millions of galaxies (such as the Sloan Digital Sky Survey or upcoming Vera Rubin Observatory) directly build on Hubble’s legacy of cataloging and classifying extragalactic objects. In short, Hubble helped pivot astronomy from a single-galaxy viewpoint to a true universal vista, and modern science still follows the paths he started exploring.

Critiques and Controversies

While Hubble is widely celebrated, historians note that his story is more nuanced than the simple legend of a lone genius. Some credit for discoveries attributed to Hubble rightly belongs to others. For example, American astronomer Vesto Slipher had already reported that most spiral nebulae had redshifted spectra (indicating recession) as early as 1912. In the 1920s, Georges Lemaître (a Belgian priest and physicist) derived mathematically that an expanding universe should show a linear velocity–distance relation and estimated it from observations in 1927. Other astronomers like Knut Lundmark and Carl Wirtz measured nebular distances and hints of expansion before Hubble. Hubble’s 1929 paper on the velocity–distance law famously presented the clear linear plot, but he acknowledged earlier attempts had been “not very convincing.” In hindsight, many historians say that a broad scientific effort, not a single person, established cosmic expansion — though Hubble’s work was the definitive demonstration.

Similarly, Hubble’s confirmation of galaxies owes something to earlier observers. The “Island Universe” debate had been lively for decades, with astronomers like Heber Curtis and Ernst Öpik arguing one side or the other. Some astronomers including Knut Lundmark published distance estimates for spiral nebulae in the early 1920s that hinted at them being too far to belong to the Milky Way. However, Hubble’s Cepheid measurement in Andromeda was the most direct piece of evidence. Historians note that Hubble’s success lay partly in having access to the best telescope and focusing persistently on finding Cepheids, whereas others had more speculative estimates. Still, credit for proving galaxies exist is seen as a collective achievement.

Hubble’s scientific methods also had limitations. His distance scale was based on calibrations of Cepheid variables that were later found to be too bright; as a result, his distances were underestimated. This error led him to infer a very large value of the primitive expansion rate (Hubble constant) on the order of 500 kilometers per second per megaparsec, far above the modern value (~70 km/s/Mpc). The overestimate implied a universe only about 2 billion years old, which conflicted with even geological estimates of Earth’s age. By the mid-20th century, astronomers had to adjust the distance scale and revise the Hubble constant downward by a factor of seven. Some criticism of Hubble’s legacy points to these systematic errors. On the other hand, it is now understood that Hubble was using the best information available then; his mistake was scientific, not methodological.

Hubble himself did not fully embrace all implications of his findings, and that opened him to criticism. He remained somewhat agnostic about cosmic origins through his life. He originally sought but did not find evidence for a declining galaxy brightness that might signal cosmological evolution; he wrote skeptically about “the obvious consequences” of expansion for a long time. It was only later, after he had published the velocity–distance relation, that he accepted a new interpretation of the universe’s structure. Some critics have argued that Hubble was overly cautious or that he did not publicize the expanding-universe idea as vigorously as he could. (However, many historians think he simply let the facts speak for themselves in data form, without grand claims.)

In the realm of galaxy classification, Hubble’s original interpretation is also questioned. He had speculated that ellipticals might evolve into spirals as part of a cosmic sequence, but today this is not accepted: galaxy morphology is shaped by many factors, including mergers and environment, rather than a single evolution track. The Hubble tuning-fork remains a useful scheme, but only as a way to sort appearance, not direct evolutionary states. This reflects a normal progression: early ideas are refined by later science.

Overall, while we now recognize that Hubble stood on the shoulders of earlier astronomers, no one disputes that he played a pivotal role. As the historian Allan Sandage put it, Hubble’s ability to cut directly to the key observations made him “the leading astronomer in the 1920s concerned with problems of the nebulae.” The critiques of Hubble’s legacy mainly refine the historical record: they remind us that science is cumulative. Even if credit is shared, Hubble’s name remains attached to fundamental discoveries of distance and expansion, and his leadership in observing the universe is secure.

Legacy

Edwin Hubble’s legacy looms large in both professional astronomy and popular culture. In science, his work anchored the modern view that the cosmos is vast and dynamic. Thanks to Hubble, the idea that space is stretching is a basic assumption in cosmology. The expansion rate he discovered is encoded in the Hubble constant, which still appears in every introductory discussion of the universe’s age and fate. Ongoing research into “dark energy” and accelerating expansion builds directly on Hubble’s early 20th-century insights.

Technologically, Hubble’s influence is immortalized in the Hubble Space Telescope (HST). Named in 1983 in anticipation of its 1990 launch, the HST is one of the most famous instruments in history, delivering stunning images from deep space. By placing a telescope outside Earth’s atmosphere, astronomers realized a dream that Hubble himself first articulated: a space-based observatory. The HST is explicitly intended to continue Hubble’s mission of charting the universe, from measuring distant galaxies to discovering new ones. For instance, the Hubble Deep Fields — extremely long exposures of tiny sky patches — have revealed galaxies at cosmic distances, echoing Hubble’s own deep-sky surveys. Hubble’s name on the satellite reminds students and the public of the man who showed us how to measure cosmic distance.

In education and culture, Hubble is often cited as one of the greatest astronomers of all time. Popular books and articles describe him as the discoverer of the expanding universe (sometimes omitting mention of predecessors like Lemaître or Slipher), which underscores how strongly his name is associated with these ideas. The concept of an expanding universe is now widely known outside the scientific community, partly through Hubble’s popular legacy. The “Hubble tuning fork” diagram is still printed in many astronomy textbooks. Even phrases like “Hubble-time” appear in discussions of cosmological models (Hubble time is the characteristic time given by 1/Hubble constant).

After his death, Hubble’s memory was preserved by scholarships and memorials. The American Astronomical Society and others marked his centenary (1989) with conferences. Places like the Mount Wilson Observatory itself and institutions such as the California Institute of Technology (which took over the Mount Wilson telescopes) keep his name and work in their histories. In recent years, the “Hubble Legacy Archive” has made his original data and correspondence available to the public and researchers, preserving his papers.

Overall, Edwin Hubble is remembered as the founder of modern extragalactic astronomy. By proving that the universe is teeming with galaxies and that these galaxies are flying apart, he forever changed our cosmic perspective. His methods and discoveries continue to influence astronomy; his name remains attached to concepts and tools that guide scientists even today. In Hubble’s own words, “Equipped with his five senses, man explores the universe around him and calls the adventure Science.” Decades after his passing, astronomers still follow Hubble’s lead in exploring the universe on the grandest scales.

Timeline

1889: Born on November 20 in Marshfield (later renamed Wheaton), Missouri.
1910: Graduates from the University of Chicago (A.B. in math and astronomy).
1910–1913: Rhodes Scholar at Oxford University (B.A. in jurisprudence).
1914: Receives Ph.D. in astronomy from University of Chicago; teaching and research at Yerkes Observatory.
1917–1919: World War I service in U.S. Army (major). Mount Wilson Observatory position held open for him.
1919: Joins Mount Wilson Observatory staff, soon using 100-inch Hooker telescope.
1923–1925: Measures Cepheid variables in Andromeda (M31) and other galaxies, proving extragalactic nature. Publication in 1925.
1926: Publishes “Extragalactic Nebulae” and creates the Hubble galaxy classification scheme (spiral, elliptical, etc.).
1929: Publishes the velocity–distance relation (Hubble’s Law) in Proceedings of the National Academy of Sciences.
1931: Collaborates with Milton Humason on extending redshift and distance data, strengthening evidence for expansion.
1936: Publishes The Realm of the Nebulae (book summarizing extragalactic research) and The Hubble Atlas of Galaxies.
1949: Palomar Observatory’s 200-inch Hale Telescope begins operation at Mt. Palomar; by then Hubble had largely completed his pioneering work.
1953: Dies on September 28 in San Marino, California.
1990: NASA’s Hubble Space Telescope (named in his honor) is launched, continuing exploration of the universe.

Selected Works

Hubble, E. P. “Cepheids in Spiral Nebulae.” Proceedings of the National Academy of Sciences, vol. 9 (1923), pp. 282–284. (Measurements of Cepheid variable stars in Andromeda, determining its distance.)
Hubble, E. P. “Extragalactic Nebulae.” Astrophysical Journal, vol. 64 (1926), pp. 321–369. (Hubble’s galaxy classification scheme and survey of nearby galaxies.)
Hubble, E. P. “A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae.” Proceedings of the National Academy of Sciences, vol. 15 (1929), pp. 168–173. (First statement of Hubble’s Law for cosmic expansion.)
Hubble, E. P., and Humason, M. L. “The Velocity-Distance Relation among Extra-Galactic Nebulae.” Astrophysical Journal, vol. 74 (1931), pp. 43–74. (Expanded dataset of galaxy redshifts and distances.)
Hubble, E. P. The Realm of the Nebulae. Yale University Press, 1936. (Monograph summarizing the state of extragalactic astronomy.)
Hubble, E. P. The Hubble Atlas of Galaxies. Carnegie Institution of Washington, 1926. (Atlas of photographic images of galaxies illustrating Hubble’s classification.)