By the time Albert Einstein’s ground-breaking theory of relativity was published in 1905, astronomy had made considerable progress in several other important areas. The development of new techniques and more powerful telescopes was beginning to enable astronomers to measure the distances of far-away stars and galaxies more accurately, indicating that the universe was much larger than anyone had previously imagined, and was expanding and evolving.
America’s contribution to astronomy was minimal until the period after their civil war. The stability that followed the American Civil War (1861–5) allowed the US economy and the universities to expand rapidly, and more money became available for research. The contribution of the Americans then became very significant.
Henrietta Leavitt (1868–1921) was born in Lancaster, Massachusetts. From 1886 to 1888 she attended the Society for the Collegiate Instruction of Women, later known as Radcliffe College, where she graduated in 1892. In 1895 she became a volunteer assistant at Harvard Observatory and in 1902 she was given a permanent staff appointment. The observatory’s great astronomical project, started by Edward Pickering (1846–1919), was to measure the brightness of all the known stars as accurately as possible. Leavitt was appointed head of the photographic stellar photometry department, and with rapidly improving photographic techniques the observatory was soon measuring stellar magnitudes to a greater accuracy than anyone had achieved before.
A new phase of the work began at Harvard in 1907 when Edward Pickering set up an ambitious plan to ascertain the stellar magnitudes photographically. Unlike earlier measurements by eye, which could be subjective, the photographic plates gave a truer reading of the colors of the stars. Henrietta Leavitt began by studying a sequence of 46 stars in the vicinity of the North Celestial Pole. With the new methods of analysis now available she determined all the magnitudes, and then followed up her work with a much larger sample in the same region of the sky. She extended the scale of standard brightness from the 3rd magnitude right down to the 21st magnitude, with her work being published progressively between 1912 and 1917. By the time of her death, Henrietta Leavitt had determined the magnitudes for stars in 108 areas of the sky. She had also discovered four new stars and 2400 variable stars. In fact, during the 1920s, she had discovered more than half of all the known variable stars in the sky.
Leavitt’s most valuable contribution to astronomy, however, was her discovery in 1912 of the properties of a certain class of stars known as the Cepheid variables. Cepheids are like lanterns flashing in the sky—the brightness of a Cepheid is not constant but oscillates over a period of time that varies from a few days to a few months. Leavitt’s breakthrough came when she compared the relative brightness of Cepheid variable stars in the Small Magellanic Cloud, all of which could be assumed to lie at approximately the same distance from Earth. She discovered that the maximum absolute brightness of this class of star was closely related to the period of the luminosity variation (the regular changes from rapid brightening to gradual fading). Thus she was able to derive a simple formula to calculate the distance of any Cepheid by measuring its period and maximum luminosity. This was a major advance in our understanding of the scale of the universe, and it provided an alternative technique to the method of parallax for determining distances. More importantly, it enabled the distance to a Cepheid variable to be measured at much greater distances than had previously been possible by the parallax method.
Observational astronomers were trying hard to understand the nature of the faint fuzzy clouds visible in the sky—such as the “nebulae” that had been cataloged by Messier and Herschel. Some were clearly stellar in nature, such as the globular clusters. Others appeared as hazy clouds containing a few bright stars, which were eventually identified as star-formation regions within the galaxy. The most puzzling, however, were the small nebulae that showed a spiral structure, but without clearly resolved individual stars.
At the Lowell Observatory in Flagstaff, Arizona, Vesto Slipher (1874–1969) was using the 61-centimeter (24 in) refractor to make spectroscopic observations of the spiral-shaped nebulae. The nebulae were incredibly faint, and each measurement required very long exposures of many hours. The spectra showed a phenomenon known as “redshift.” The light reaching Slipher’s telescope from the nebulae invariably showed the familiar absorption lines, but shifted toward the red end of the spectrum.
There were two reasons for redshift to occur and both were fairly well understood. One possibility was that the light was created in a very strong gravitational field. (Einstein predicted this effect, although he applied it to individual stars rather than to a whole galaxy.) The other explanation for redshift was if the emitting object was moving away from the Earth at a very great speed, analogous to the “Doppler effect.” In 1842 Christian Doppler (1803–53) had shown how the frequency of sound changes if it is emitted by a source traveling either toward or away from an observer. This is the reason for the rise and drop in note of a racing car as it rushes past. The wavelength of light emitted by a moving object can similarly be shifted to appear either bluer or redder, according to the speed of the emitter relative to the observer. The shift in spectral features was already used in observational astronomy as a technique to assess the motions of nearby stars in the Milky Way.
In 1917 Slipher published the astonishing results of spectroscopy for a sample of 25 spiral nebulae. Only four of these were approaching the Earth, with the other 21 receding, giving an overall sense that the nebulae were mostly “scattering” away from us. If due to the Doppler effect, the velocities implied by the redshift were hundreds of kilometers per second—far greater than any speeds observed for other known galactic objects. Slipher continued to observe a further 20 spiral nebulae, which were all found to be receding. However, the cause of the dispersion of the nebulae was to remain, for the moment, a mystery.
The nature of “spiral nebulae” such as the Andromeda Galaxy was a subject of hot debate, with implications not only for the size and structure of the galaxy but also the size of our whole universe. Either the nebulae were intrinsically small structures contained within our Milky Way, which in itself comprised the entire universe, or they were intrinsically large objects that appeared small because they lay well outside our own galaxy.
The “Great Debate” was held on April 26, 1920 at the Smithsonian Museum of Natural History in Washington DC, between the two eminent American astronomers Harlow Shapley (1885–1972) and Heber Curtis (1872–1942). Shapley assumed that our galaxy was large, and therefore for Andromeda to be the same size it would need to lie at an immense distance, thus rendering the entire universe unimaginably big. He thus proposed the spiral nebulae were located within our galaxy, which comprised the whole universe. On the other hand, Curtis maintained that the spiral nebulae were independent “island universes” located outside our own galaxy. He favored a model in which the Sun was (incorrectly) placed at the center of a much smaller galaxy. In his support, Curtis pointed out that unlike known galactic objects, the spiral nebulae were not observed to lie in the band of the Milky Way, and they had discrepant redshifts. He also noted that they had dust lanes similar to those observed in our own galaxy, and that the Andromeda Nebula had produced many nova stars, which would be unusual if it were only part of our galaxy.
The issue was resolved only five years later, when Hubble published the first measurement of the distance to Andromeda, proving beyond doubt the extragalactic nature of the spiral nebulae. Despite his erroneous model of our galaxy, Curtis had been proved right.
In the 1920s California came to the forefront of astronomical research. The main reason for its prominence was the building of the Mount Wilson telescopes, situated high in the San Gabriel Mountains of southern California. These were two large telescopes which not only offered very clear views of the skies from their high altitude position but were also the largest telescopes in the world.
The Mount Wilson Observatory was founded in 1904 by astronomer George Ellery Hale (1868–1938). He became its first director, and was responsible for hiring both Harlow Shapley and Edwin Hubble. For many years the Mount Wilson Observatory and the neighboring Palomar Observatory, near San Diego, were operated jointly as the Hale Observatories by the Carnegie Institution of Washington and by the California Institute of Technology in Pasadena. This arrangement continued until 1980 when the observatories became separate units.
The Mount Wilson Observatory holds a number of optical telescopes. The most significant of these are the 1.5-meter (60 in) reflector and the 2.5-meter (100 in) reflector. In the early part of the 20th century the 2.5-meter (100 in) reflector was the world’s most powerful telescope, and with it Edwin Hubble and his associates made important discoveries about the distant galaxies, notably their extragalactic nature and motion away from the Earth. These results were the foundation of a new view of the universe, and of our galaxy’s place within it.
In 1914 Harlow Shapley (1885–1972) joined the staff of the Mount Wilson Observatory. He used the observatory’s 1.5-meter (60 in) reflecting telescope to study the distribution of globular star clusters in the Milky Way Galaxy. These clusters are dense groups of stars, each containing hundreds of thousands of members packed into a tight spherical ball. He found that of about 100 clusters known at that time, a third of them lay in the constellation of Sagittarius. In the star clusters Shapley was able to identify pulsating stars very like the Cepheid variables. In fact they were of a type now known as RR Lyrae variables. These have a shorter period than the Cepheids, but are the same class of star. Using Henrietta Leavitt’s period-luminosity relationship, Shapley could therefore calculate the distance to 93 globular star clusters. He constructed a three-dimensional map of the stars centered on a point in Sagittarius that he assumed was the middle of the Milky Way. From this conclusion and his other distance data Shapley deduced that the Sun lay at a distance of 65,000 light years from the center of the galaxy, although this distance was later modified to 30,000 light years. Before Shapley’s work the Sun was believed to lie near the center of the galaxy, but he not only proved this to be wrong, he also made the first realistic estimate for the actual size of the galaxy. It was another milestone in galactic astronomy.
Edwin Hubble gained a degree in mathematics and astronomy at the University of Chicago. While there, he was inspired by the astronomer George E. Hale (1868–1938). Hubble was also known for his athletic prowess, and gained a reputation as a boxer. After graduating, he decided on a change of career path, and traveled to England as a Rhodes scholar to study law at Oxford University. In 1913, on his return to America, he joined the Kentucky bar but found himself bored with law and dissolved his practice soon afterward. He returned to the University of Chicago and to the Yerkes Observatory at Wisconsin; while there he gained a PhD in astronomy. Hubble served in World War I (1914–18), and after the war he settled down to work at the Mount Wilson Observatory in California where he began to make his discoveries concerning the distant galaxies.
Hubble left Mount Wilson in 1942 hoping to join the armed forces again and to serve in World War II (1939–45), but he found that he had more to offer as a scientist. After the war he went back to Mount Wilson and was able to convince his employers that they should build an even greater telescope than the 2.5-meter (100 in) reflector. He was instrumental in the design of the Hale Telescope at Mount Palomar Observatory. Hubble died in 1953. In recognition of his work the first large orbiting space telescope, launched in 1990, was named after him. Today the Hubble Telescope is almost synonymous with spectacular photographic images of distant stars and galaxies.
It was while he was at the Mount Wilson Observatory that Hubble observed Cepheid variable stars contained within the Andromeda Nebula. Using Leavitt’s relation between the period of variability and the brightness of Cepheids, he derived a distance of some 900,000 light years to the nebula, so large that Andromeda clearly could not be part of our own galaxy. Similarly, his observations of the spiral object now known as the nearby Triangulum Galaxy showed that it, too, must lie beyond our Milky Way. The enormous distances also implied that the “nebulae” that the astronomers observed as faint and tiny, were in reality incredibly bright and large. When Hubble made his results public in 1925, astronomers accepted the conclusion that the spiral objects could only be other galaxies outside our own. The findings also demonstrated for the first time the insignificance of our Sun, being one amidst thousands of billions of stars in a galaxy which in its turn is just one of billions.
Having established both the nature and mind-boggling distances to the other galaxies, Hubble set out to discover their properties, and to use them to find out more about the universe. He soon realized that galaxies had a variety of different shapes, and characterized these shapes in a classification scheme that spanned the range of elliptical, spiral, barred spiral and irregular galaxies. Within each class there were subdivisions based on the detailed structure: the degree of ellipticity in an elliptical galaxy; how tightly the spiral arms were wrapped, and the relative brightness of the bulge and disc in the spiral systems. The irregulars were more difficult to classify, and we now suspect they form in the gravitational chaos when two or more galaxies collide with each other. Hubble published this scheme as a “tuning fork” diagram, which he thought represented an evolutionary sequence (an idea which has subsequently been disproved). He also estimated the mass of each type of galaxy, and made the first measurements of the matter density of the universe.
But Hubble had another major discovery to make. He continued to assemble measurements of the distances to the galaxies, but was curious about Slipher’s early results showing their almost uniform recession. This was completely contrary to the expectations of a static universe, which predicted a random but balanced distribution of galaxy motions both toward and away from the Earth. Working with his assistant Milton Humason (1891–1972), he first confirmed Slipher’s spectroscopic results. It was when he combined these velocities with his distance estimates to the galaxies that an astounding result emerged. The galaxies described a clear proportionality between distance and velocity—galaxies five times as far away were moving five times as fast, and galaxies ten times further away moving ten times as fast.
In 1929 Hubble first published this result for the 20 galaxies for which he was most confident of the data, and two years later confirmed the relationship also held for galaxies beyond a distance of 100 million light years and at velocities up to 12,427 miles per second (20,000 km/s). This showed that light from the furthest galaxies had taken many million light years to reach the Earth. In other words, he was looking at such galaxies as they were 100 million years ago!
Hubble’s findings posed many questions about the nature of the universe. The direct relationship between velocity and distance revealed a pattern in the motions of the galaxies that is a natural consequence of a systematic and constant expansion. This was the first experimental suggestion for an expanding universe; the few nearby galaxies showing a discrepant blueshift could be explained as those that were responding to the local gravitational attraction of the Milky Way. The distances to all other galaxies are increasing with time, and the overall mass density of the universe steadily decreasing. The observational evidence for an expansion paved the way for the concept of the Big Bang, an explosion of immense energy responsible for the creation of all of space, matter and energy at the beginning of time. If the galaxies were now expanding away from each other, then in the past they were closer together. Go back sufficiently far in time, and the galaxies would all be compressed into a much smaller region. The Milky Way does not have a “special” place representing the center of the expansion—all the galaxies are receding from one another, and Hubble’s law could have been derived from observations of any galaxy.
The velocity–distance relation became known as “Hubble’s law,” with the scaling between the two properties known as “Hubble’s constant,” which is measured in units of km per second per Megaparsec (Mpc). (A Megaparsec is one million parsecs; a parsec is a distance equivalent to 3.26 light years.) Hubble’s constant gives a measure of the present rate of expansion of the universe, and offers an estimate for its lifetime. For any galaxy traveling at constant speed over a known distance, the ratio of the two attributes allows an estimate of the time it has taken. Thus the inverse of Hubble’s constant yields an estimate of the age of the universe, assuming expansion has remained uniform. The first value for Hubble’s constant was 500 km/s/Mpc, from which the universe was deduced to have an age of two billion years. The accurate evaluation of Hubble’s constant was an important aim throughout 20th-century astronomy, and for many decades it was in error by over 50 percent. Its determination was one of the key projects guiding observations with the Hubble Space Telescope. It is only in the first years of the 21st century that measurements pinned down the Hubble constant to a value of 70 km/s/Mpc, revising the resulting estimate of the universe’s age closer to 14 billion years.
Hubble’s discovery did not just revolutionize how astronomers viewed the history of the universe, overturning the idea of a static universe in favor of one that was evolving with time, it also pushed them to consider its future over the next billions of years. If the universe were expanding, would it ever stop? There were three immediate possibilities, and which was correct would depend on the total mass density of the universe. The expansion of space was carrying the galaxies further and further from each other. If the total mass of the universe was high, then the combined gravity would eventually slow the expansion—at some instant the galaxies would cease moving, and then pull back toward each other in an inevitable “Big Crunch” marking the end of the universe. At the other extreme, if the universe were comparatively empty, there would be insufficient gravity ever to draw the galaxies back together, and the expansion would continue until the galaxies moved so far apart that they would no longer be visible to each other. The third option was the boundary point between these two eventualities, with a critical mass density of the universe that would be enough to decelerate but not reverse the expansion, with the galaxy motions coming to rest only at infinity.
Before Hubble’s work suggested the universe started with the Big Bang, a theory for a static universe had been put forward by Sir James Jeans (1877–1946) in about 1920. It was developed after World War II by Fred Hoyle (1915–2001) at Cambridge University as a rival to the Big Bang theory; Hoyle called it the steady-state theory. He proposed a universe where matter was created from nothing. The production of only a few atoms per year would be sufficient to cause the universe to expand as observed by Hubble. The idea of creating matter from nothing did not appeal to many astronomers, but the steady-state advocates pointed out that the Big Bang theory required a whole universe to be created out of nothing as well. The steady-state theory did at least encourage discussion about alternatives to the Big Bang as a way of explaining the origins of the universe. Hermann Bondi (1919–2005) and Thomas Gold (1920–2004) raised it again in a revised form in the 1980s, but by then the evidence in favor of the Big Bang was almost conclusive.