Once a month, Duty challenges history enthusiasts to decipher a current theme based on a comparison with a historical event or character.
In 1924, the American astronomer Edwin Powell Hubble (1889-1953) established the distance of the “great nebula” of Andromeda, clearly placing it outside the limits of our Milky Way. Suddenly, the observable universe had just expanded by at least a million times.
During the beautiful evenings of late summer and autumn, you can observe in the constellation Andromeda what appears through binoculars as a small diffuse object. From a place free of light pollution, you can even see this strange object with the naked eye.
The ancients had noted this. In its magnificent Book of Fixed Stars published in 964, the Persian astronomer Abd al-Rahman al-Sūfī (903-986) from Isfahan (Iran) identified it as the Small Cloud. In 1612, the German mathematician Simon Marius became the first to observe the “nebula” using a telescope; he describes it as “the light of a candle through a horn”. The Little Cloud is well known today, it is the Andromeda galaxy.
French astronomer Charles Messier (1730-1817) was a comet hunter. He had listed the Little Cloud in his catalog with the number 31, warning that if we were looking for new comets, we should not confuse Messier 31 with a comet.
At 2.5 million light years, it is the most distant object that can be seen with the naked eye. The light that reaches our eyes today was emitted by the stars of the Andromeda galaxy while our hominin ancestor Homo habilis roamed the savannahs of South Africa.
Gordian knot
How did Edwin Hubble measure the distance to the Andromeda Galaxy? It is from the recognition and observation of giant stars of variable luminosity of the Cepheid type — these evolved stars are on average 100,000 times more luminous than the Sun, and therefore visible at great distances.
In 1912, Henrietta Leavitt (1868-1921), from the Harvard University Observatory, established a close relationship between the brightness and the period of variability of Cepheids: the more luminous the star, the longer its period, varying from a few days to 50.
This characteristic variability makes Cepheids a reliable standard candle; we can compare them to nearby examples for which we have directly established the distance.
In 1923-1924, Hubble succeeded in photographing several Andromeda Cepheids using the large 2.5 m telescope at the Mount Wilson Observatory in California. Having measured their periods, Hubble derived their luminosities; by simple rule of three, he deduced the Messier distance 31.
On November 23, 1924, the New York Times made a brief statement of the discovery. The official announcement took place in Washington at the annual meeting of the American Astronomical Union in late December. Surprisingly absent, Hubble reported his results in two very brief articles at the beginning of 1925: he placed Messier 31 930,000 light years from the Sun, therefore clearly outside the Milky Way.
Precursors
As early as 1917, American astronomers George Ritchey (1864-1945) and Heber Curtis (1872-1942) had discovered “nova” type stars in a few “nebulae”, including Messier 31. Novae had been observed for centuries.
Their sudden brightness is produced by the nuclear detonation of material ejected from a red giant star falling on a companion white dwarf. A nova appears at its brightest during the explosive phase, then disappears within a few weeks.
The pattern of their variability was, however, too irregular and unpredictable to make them good standard candles. However, Ritchey and Curtis could state that if they appeared thousands of times fainter than the novae in the Milky Way, that meant they were far beyond it.
It was also novae that Hubble was looking for at the beginning of the 1920s. His surprise was to observe in 1923 in Andromeda a recurring variable star, that is to say one which did not disappear after a few weeks. He had first noted it as a nova. Then, he determined that it obeyed the cycle of brightness variation characteristic of Cepheid stars.
The Cepheids of Andromeda had been photographed by a few astronomers as early as 1917. But it is the merit of Hubble to have recognized them in 1924 and to have derived their distances. As often happens in science, something can be seen several times before it is discovered!
Hubble thus put an end to an age-old debate – that between the defenders of the local hypothesis of “nebulae”, claiming that they were located inside a super Milky Way, versus that of the proponents of the extragalactic nature of the majority of “nebulae”.
The reason for the divergence between these two camps? For several centuries, astronomers were unable to elucidate the nature of “nebulae”. Were they diffuse clouds of ethereal substance or independent star systems, like the Milky Way?
An exact determination of their distances was a necessary step to resolve the question. We now know that a minority, such as the Orion Nebula, are true gas clouds located in the Milky Way; the majority are immense stellar systems independent and external to our Milky Way.
Delayed in his studies by the Great War, Hubble received his doctorate from the University of Chicago in 1921. Working at the Mount Wilson Observatory, Hubble had access to its two large telescopes. Surprisingly, Hubble was not considered a good observer; his colleagues noticed, for example, that the focus was not optimal in several of his photos. He was nevertheless methodical, although proud and cautious about the contributions of others.
Enlargement
In 1924, also 100 years ago, the young Russian physicist Alexander Friedmann (1888-1925) reexamined Albert Einstein’s equations of general relativity. Unlike the stationary solution adopted by Einstein, Friedmann demonstrated that space-time is unstable; just like you can’t make a pencil stand on its tip, space is either contracting or expanding.
Unaware of his Russian colleague, the young Belgian cosmologist Georges Lemaître (1894-1966) found the same solutions, but he went further than Friedmann. In 1927, on the basis of preliminary data on the distances and speeds of a few dozen galaxies, Lemaître concluded that the universe was expanding. Then putting the expansion in reverse, he asserted in a short publication in the journal Nature (1931) that everything that exists originated from an extremely small and hot quantum nucleus a few billion years ago.
This was the “primitive atom” hypothesis, which today has become the big bang theory. In less than a decade, we have gone from a fixed and eternal universe to a fantastically large universe, where the major components of the cosmos are carried away in a systematic movement of flight.
Hubble is often credited with discovering the expansion of the universe through work with his meticulous colleague and peerless observer Milton Humason (1891-1972). But until his death in 1953, Hubble remained doubtful of this interpretation of the relationship between the apparent speed of recession and the distance of galaxies. Allan Sandage (1926-2010), Hubble’s colleague for years, was categorical: Hubble never believed in the reality of expansion.
In the 1950s, the German astronomer Walter Baade (1893-1960), working in the United States, identified two types of Cepheids of distinct brightness. Hubble had observed the most luminous ones, but was referring to the least luminous ones.
Baade’s discovery resulted in a nearly three-fold increase in the distances initially derived by Hubble. Today, the distance of the Andromeda Galaxy is established by a set of indicators, including the Cepheids. At 2.5 million light years away, our great companion of several hundred billion stars measures more than 100,000 light years across.
Youthful memory
In 1961, I was at classical college. I had read in an American astronomy magazine that the astronomer Allan Sandage had just published a superb atlas illustrating the galaxies. In a process that I have never clarified, my mother had ordered The Hubble Atlas of Galaxies at the Carnegie Institution for Science in Washington; the atlas had cost $10. I spent hours studying the book with my stammering English. My imagination tried to grasp these colossal stellar assemblages; I was dazzled by their varied shapes and elegant silhouettes.
The photographs taken by the largest telescopes of the time were sublime, and I still find them fascinating and full of enigmas. From time to time I leaf through this atlas, kept as a precious possession. Later, I devoted several decades of my career to research on galaxies.
In a century, the volume of the observable universe has multiplied a million billion times. While on a large scale the expansion of the universe carries away galaxies like floating ice, gravitational attraction continues to dominate at distances of a few million light years or less.
Galaxies can thus assemble in small groups or form clusters of several thousand galaxies. They can also fall on top of each other and merge. This is what will happen to the Milky Way and the Andromeda Galaxy in 4 or 5 billion years; our two spirals will collide to form a large elliptical type galaxy.
By then, we can say that 100 years ago, we discovered the excess and strange dynamics of the universe — a surprisingly recent advance. For example, my grandparents, born just a few years before Hubble, were his contemporaries.
To suggest a text or to make comments and suggestions, write to Dave Noël at [email protected].