Vesto Slipher


The man who came upon the flight of galaxies

Vesto Slipher In August 1914, at the observatory in Flagstaff, Vesto Slipher discovered that most galaxies moved away from ours. A first step to the future model of the Big Bang. But the role of this observer of genius in this major advance will be eclipsed by the contributions of Edwin Hubble or Georges Lemaitre.

A hundred years ago, they had no kind of certitude of the nature of the "nebulas" catalogued for more than a century by Messier, Herschel and Dreyer. In particular, they had no idea of what could be the spirals that Lord Rosse had succeeded in discerning in 1845 and which abounded out of the plain of the Milky Way on the photographs which astronomers were starting to produce at the end of the 19th century. The discovery of the extra-galactic character of these objects and the expansion of the Universe is generally atributed to Edwin Hubble. But it was only after the Second World War that they commenced to construct the legendary history of modern cosmology, and this late attribution is not truly merited. The contribution of Hubble dating from the end of the 1920s had been preceded by work whose inspirators - notably Vesto Slipher for his observational work - have remained in the shade for a long time.

When in 1897 the young Vesto Slipher, a lad of the country, started his studies in science at the University of Indiana, in the mid-west, astrophysics was still largely an activity of amateurs. Like astro-photography, practised for several decades by pioneers such as Isaac Roberts in England or Jules Janssen in France, spectroscopy made its entry into the arsenal of professional astronomers, up until then concerned with the astronomy of position.

It was the epoque when the team of William Pickering at Harvard were undertaking the constitution of the monumental catalog of the luminous spectra of 300 000 stars - sponsored by the rich widow of Henry Draper - the whole thing will carry the name of this amateur from New York. It is the epoch equally when the Dane Ejnar Hertzsprung and the American Henry Norris Russell worked independently on the famous diagram relating temperature and luminosity of stars.

This is also the epoch where Percival Lowell, a rich patrician from Boston, took up a passion for Mars and its claimed canals after the resoundig publication of observations by Giovanni Schiarepelli, director of the Milan Observatory. On his return from a diplomatic engagement to the far-east, he had constructed a private observatory in Arizona, at Flagstaff, where he recruited some young observers.

Vesto Slipher arrived in Flagstaff as a trainee in 1901. He used therefore the magnificent 24 in. (60 cm) telescope manufactured by the house of Alvan Clark and Sons. And with the aid of a spectrograph with three prisms, ordered by Lowell from John Brashear, celebrated manufacturer of Pittsburgh, he undertook the long series of observations which often gained him congratulations from several of his colleagues. With the death of Lowell, in 1916, Slipher became director of Flagstaff Observatory, a post which he occupied until his retirement in 1956, a period covering the discovery of Pluto by Clive Tombaugh in 1930.

Martian Chlorophyll.

Slipher's first tasks bore on the program determined by Lowell: spectroscopic observation of planets. He brought into focus the difficult technique of measuring Doppler differences, therefore the process permitted him to understand the sensibility of photographic plates in the near infra-red. He arrived therefore to measure the period of rotation of planets, confirming that of Mars, Jupiter and Saturn (well-known visually) and put forward new values for Uranus and Neptune. In the case of Venus, he excluded the value generally agreed (close to 24 hours) which went back to Cassini. His opinion was confirmed only a half a century later. He tient also the detection of oxygen and chlorophyll (but parvient maybe to measure the water vapor). He failed in any case to recognize methane and ammonia in the atmosphere of the giant planets, which the German Rupert Wildt succeeding in doing a number of years later.

When these planetray observations allowed him some free time, Slipher applied his experimental techniques to the stellar Universe, studying especially the radial velocity of stars, and also spectroscopic binaries, by which the character of double stars only manifested by the periodic widening of their spectral radiation. This was how he made his first outstanding discovery : the existence of interstellar gas. The absorbent lines, which this gas created in the spectre of spectroscopic double stars, always appear thin and quite distinct, contrasting with those periodic thick lines of the binary couple. The German Johannes Hartmann and the Dutchman Jacobus Kapetyn, with whom Slipher corresponded, had only foreseen that the explanation resided in the presence of gas between the stars and Earth. The American observer displayed equally the existence of interstellar on noticing the similarity between the spectra of the blue stars of the Pleiades Cluster and that of the nebulosity which surrounds it and which reflects their light.

At the instigation of Lowell, he attacked the "white" nebulas. Lowell thought that these , which one distinguished from the "green" nebulas (planetary nebulas)' were systems of solar type in formation. Vesto Slipher undertook the arduous task of recording the spectra of spiral galaxies. He understood that the difficulty of these extensive and weak objects did not come so much from insufficient opening of the main instrument as one believed than from that of the camera of spectrography itself. With an objective of f/2.5 for this latter, he succeeding in reducing the time of installation by an important factor. Nevertheless this work demanded from him long sessions of guidage which extended over numerous nights - up to 60 hours in certain cases. On 1st January 1913 obtained at last for the large nebula of Andromeda, on which he had tried, a spectra which appeared clearly of rad-comparable to that of reference spectra. Until then, the radial velocities measured for the stars did not exceed several dozen of km/s.

Straightaway Slipher found for this nebula a shift to the blue corresponding to a speed of approach of 300 km/s. Later the same year, he established in return for a nebula in the constellation of Virgo a shift towards the red, corresponding to a speed of recession of 1000 km/s. "This nebula is in train of leaving the Solar System", he wrote bizarrely to Lowell.

Nebulas in Flight.

In August 1914, at the very moment that war broke out in Europe, Slipher presented his measurements for 15 spiral nebulas at the congress of the American Astronomical Society in Evanston, Illinois. With the exception of the Andromeda Nebula, the speeds are all positive indicating, he declared with more insight, "a general flight relative to us or from the Milky Way". Three years later, in neutral Netherlands, Wilhelm de Sitter , having taken notice of recent work by Einstein, produced for the first time the hypothesis of an expansion of the Universe. Slipher extended his measurements to 25 spirals. All were moving away, the most rapid at near to 2000 km/s. He will add later that these results concerning the nature of nebulas "favor the concept of island Universes" .

It is these results of which Georges Lemaitre will take notice of from 1925 during his time at the Massachusetts Institute of Technology, which Hubble will later include with his own measurements of distance in his article of 1929. A publication which does not even cite the essential contribution of Slipher to the fundamental discovery of contemporary cosmology - applauded at the meeting at Evanston at which the young Hubble had participated.

Vesto Slipher died in 1969 in Flagstaff, which had become a pleasant tourist town of which he had been an active citizen, contributing notably to the creation of the first schools before the war as well as a reputed geological and ethnological museum - the North Arizona Museum. He had made judicious investments in property and hotels, and a ranch where he raised horses.