IN CONTEXT
Cosmology
1543 Nicolaus Copernicus concludes that Earth is not the centre of the Universe.
17th century The changing view of stars offered by Earth’s orbit around the Sun gives rise to the parallax method for measuring stellar distances.
19th century Improvements to telescopes pave the way for the study of starlight and the rise of astrophysics.
1927 Georges Lemaître proposes that the Universe can be traced back to a single point of origin.
1990s Astronomers discover that the expansion of the Universe is accelerating, driven by a force known as dark energy.
By the early 20th century, ideas about the scale of the Universe divided astronomers into two schools of thought – those who believed that the Milky Way galaxy was, broadly speaking, its entire extent, and those who thought that the Milky Way could be just one galaxy among countless others. Edwin Hubble was to solve the puzzle, and show that the Universe is much larger than anyone imagined.
Key to the debate was the nature of “spiral nebulae”. Today, a nebula is the term used for an interstellar cloud of dust and gas, but at the time of this debate, it was the name used for any amorphous cloud of light, including objects that were later found to be galaxies beyond the Milky Way.
As telescopes improved dramatically during the 19th century, some of the objects catalogued as nebulae began to reveal distinctive spiral features. At the same time, the development of spectroscopy (the study of the interaction between matter and radiated energy) suggested that these spirals were in fact made up of countless individual stars, blending seamlessly together.
The distribution of these nebulae was interesting too – unlike other objects that clustered together in the plane of the Milky Way, they were more common in the dark skies away from the plane. As a result, some astronomers adopted an idea from the German philosopher Immanuel Kant, who in 1755 suggested that nebulae were “island universes” – systems similar to the Milky Way but vastly more distant, and only visible where the distribution of material in our galaxy permits clear views into what we now call intergalactic space. Those who continued to believe that the Universe was far more limited in extent argued that the spirals might be suns or solar systems in the process of formation, in orbit around the Milky Way.
"There is a simple relation between the brightness of the variables and their periods."
Henrietta Leavitt
Stars with a pulse
The answers to this long-standing puzzle came in several stages, but perhaps the most important was the establishment of an accurate means of measuring the distance to stars. The breakthrough came with the work of Henrietta Swan Leavitt, one of the team of female astronomers at Harvard University who were analysing the properties of starlight.
Leavitt was intrigued by the behaviour of variable stars. These were stars whose brightness appeared to fluctuate, or pulse, because they periodically expanded and contracted as they neared the end of their lives. She began to study photographic plates of the Magellanic Clouds, two small patches of light visible from the southern sky that look like isolated “clumps” of the Milky Way. Each of the clouds, she found, contained huge numbers of variable stars, and by comparing them across many different plates, she not only saw that their light was varying in a regular cycle, she could also work out the period of the cycle.
By concentrating on these small, faint, isolated star clouds, Leavitt could safely assume that the stars within them were all at more or less the same distance from Earth. Though she could not know the distance itself, this was still enough to assume that differences in the “apparent magnitude” (observed brightness) of the stars were an indication of differences in their “absolute magnitude” (actual brightness). Publishing her first results in 1908, Leavitt noted in passing that some stars seemed to show a relationship between their variability period and their absolute magnitude, but it took another four years for her to work out what this relationship was. It turned out that, for a certain type of variable star known as a Cepheid variable, stars with greater luminosity have longer variability periods.
Leavitt’s “period-luminosity” law would prove the key to unlocking the scale of the Universe – if you could work out the star’s absolute magnitude from its variability period, then the star’s distance from Earth could be calculated from its apparent magnitude. The first step in working this out was to calibrate the scale, which was done in 1913 by Swedish astronomer Ejnar Hertzsprung. He worked out the distances to 13 relatively nearby Cepheids using the parallax method. Cepheids were immensely bright – thousands of times more luminous than our Sun (in modern terminology they are “yellow supergiants”). In theory, then, they were an ideal “standard candle” – stars whose brightness could be used to measure huge cosmic distances. But despite the best efforts of astronomers, Cepheids within the spiral nebulae remained stubbornly elusive.
Henrietta Leavitt received little recognition in her lifetime, but her discoveries relating to Cepheid variable stars were the key that allowed astronomers to measure the distance from Earth to faraway galaxies.
"We are reaching into space, farther and farther, until, with the faintest nebulae that can be detected…we arrive at the frontier of the known Universe."
Edwin Hubble
The Great Debate
In 1920, the Smithsonian Museum in Washington DC hosted a debate between the two rival cosmological schools, hoping to settle the issue of the scale of the Universe once and for all. Respected Princeton astronomer Harlow Shapley spoke for the “small Universe” side. He had been the first to use Leavitt’s work on Cepheids to measure the distance to globular clusters (dense star clusters in orbit around the Milky Way), and discovered that they were typically several thousand light years away. In 1918, he had used RR Lyrae stars (fainter stars that behave like Cepheids) to estimate the size of the Milky Way and show that the Sun was nowhere near its centre. His arguments appealed to public scepticism towards notions of an enormous Universe with many galaxies, but also cited specific evidence (later to be proved inaccurate), such as reports that over many years some astronomers had actually observed the spiral nebulae rotating. For this to be true without parts of the nebula exceeding the speed of light, they must be relatively small.
The “island Universe” supporters were represented by Heber D Curtis of the University of Pittsburgh’s Allegheny Observatory. He based his arguments on comparisons between the rates of bright “nova” explosions in distant spirals and in our own Milky Way. Novae are very bright star explosions that can serve as distance indicators.
Curtis also cited the evidence of another, crucial factor – the high redshift exhibited by many spiral nebulae. This phenomenon had been discovered by Vesto Slipher of the Flagstaff Observatory, Arizona, in 1912 – apparent through distinctive shifts in the pattern of a nebula’s spectral lines towards the red end of the spectrum. Slipher, Curtis, and many others believed that they were caused by the Doppler effect (a change in the wavelength of light due to relative motion between source and observer), and therefore indicated that the nebulae were moving away from us at very high speeds – far too fast for the Milky Way’s gravity to keep hold of them.
By measuring the light from Cepheid variable stars in the Andromeda nebula, Hubble established that Andromeda was 2.5 million light years way – and was a galaxy in its own right.
Measuring the Universe
By 1922–23, Edwin Hubble and Milton Humason of California’s Mount Wilson Observatory were in a position to end the mystery once and for all. Using the observatory’s new 2.5m (100in) Hooker Telescope (the largest in the world at that time), they set out to identify Cepheid variables shining within the spiral nebulae, and this time they were successful in finding Cepheids in many of the largest and brightest nebulae.
Hubble then plotted their periods of variability and therefore their absolute magnitude. From this, a simple comparison to a star’s apparent magnitude revealed its distance, producing figures that were typically millions of light years. This proved conclusively that the spiral nebulae were really huge, independent star systems, far beyond the Milky Way and rivalling it in size. Spiral nebulae are now correctly called spiral galaxies. As if this revolution in the way we see the Universe were not enough, Hubble then went on to look at how galaxy distances related to the redshifts already discovered by Slipher – and here he found a remarkable relationship. By plotting the distances for more than 40 galaxies against their redshifts, he showed a roughly linear pattern: the further away a galaxy is, on average, the greater its redshift and therefore the faster it is receding from Earth. Hubble immediately realized that this could not be because our galaxy is uniquely unpopular, but must be the result of a general cosmic expansion – in other words, space itself is expanding and carrying every single galaxy with it. The wider the separation between two galaxies, the faster the space between them will expand. The rate of expansion of space soon became known as the “Hubble Constant”. It was conclusively measured in 2001 by the space telescope bearing Hubble’s name.
Long before then, Hubble’s discovery of the expanding Universe had given rise to one of the most famous ideas in the history of science – the Big Bang theory.
"Equipped with his five senses, man explores the universe around him and calls the adventure science."
Edwin Hubble
In 1842, Christian Doppler showed that if a light source is moving towards us or away from us, the light waves arrive at different rates. If the light source is moving towards us, we see a bluer colour as waves bunch together at the blue end of the light spectrum; if it is moving away, we see a redder colour. Hubble guessed that sodium light was the same colour in far galaxies as it is on Earth, but the Doppler effect meant that it would be blueshifted or redshifted if moving towards or away from us.
EDWIN HUBBLE
Born in Marshfield, Missouri, in 1889, Edwin Powell Hubble had a fiercely competitive nature that manifested itself in his youth as a gifted athlete. Despite his interest in astronomy, he followed his father’s wishes and studied law, but at the age of 25, after his father’s death, he resolved to follow his early passion. His studies were interrupted by service in World War I, but after his return to the United States he won a position at the Mount Wilson Observatory. There he did his most important work, publishing his study on “extragalactic nebulae” in 1924–25, and his proof of cosmic expansion in 1929. In later years, he campaigned for astronomy to be recognized by the Nobel Prize Committee. The rules were only changed after his death in 1953 and so he was never awarded the prize himself.
Key works
1925 Cepheid Variables in Spiral Nebulae
1929 A Relation Between Distance and Radial Velocity among Extra-galactic Nebulae
See also: Nicolaus Copernicus • Christian Doppler • Georges Lemaître