Edwin Hubble, Mariner of the Nebulae – by Albert Lim

N.K. Mayal made an appropriate remark about Edwin Powell Hubble (1889-1953) in a Jan 1954 issue of Sky and Telescope in an article titled “Edwin Hubble: Observational Cosmologist.” He stated in it that “it is tempting to think that Hubble may have been to the observable region of the universe what the Herschels were to the Milky Way system, and what Galileo was to the solar system.” This paper will discuss Hubble’s work and how it has influenced astronomical research.
Hubble was born in Marshfield, Missouri. He studied at a high school in Wheaton, Illinois, and later studied astronomy at the University of Chicago. He went on to Oxford University as a Rhodes scholar where he completed his doctorate in 1917. His thesis was titled “Photographic Investigations of Faint Nebulae” and in it he concluded that spirals are extragalactic.
In 1916, Dr. Adriaan van Maanen, a colleague of Hubble at Mount Wilson Observatories, went on another path and reported that his observations with the 60-inch showed that M101 had a rotation rate of 85,000 years per revolution. He also reported high rotational rates for 7 other spirals and claimed that he had supporting observations by other observers as well as James Jean, the theoretician from England. Hubble was uncomfortable - he realised that van Maanen’s findings were in direct contradiction to his conclusions that spirals are “island universes”. In 1923, Hubble resolve the outer regions of M31 and M33 with Mount Wilson’s 100-inch telescope. Within months, he was able to isolate substantial numbers of cepheid variables in them. Hubble found their distances by using Leavitt's period-luminosity relationship and Harlow Shapley’s zero point calculation. He calibrated the periods and photographic magnitudes for some 22 cepheids for M33 and 12 for M31 to conclude that they are 930,000 light years (285,000 parsecs) away. These values land strong support for Herber D. Curtis who had among other things, argued on the existence of "island universes" during his great debate with Shapley in 1920. Due to an error in Shapley’s determination of zero point however, Hubble’s calculated distances were only half the actual values. It was not until the 1950s, when Walter Baade postulated the existence of Type I and Type II cepheids that it was realised that Shapley had based his zero point on normal Type II cepheids. The cepheids in the spiral arms of M31 were of younger Type I - meaning they are 4 times brighter. The distance to M31 was thus revised and doubled to nearly 2 million light years ! Hubble’s result at 930,000 light years were however clearly “island universe” distances in 1925. Hubble was reluctant to publish his results because they contradicted van Maanen’s work on the rotation of spirals. It was not until the 1st of Jan 1925 that Hubble’s results were finally presented by the famous Henry Norris Russell at a meeting of the American Astronomical Society in Washington - Hubble himself was absent.

At this point, other astronomers began doubting van Maanen’s work on spiral rotations. He responded by measuring and announcing even more spiral rotations. Eventually in early 1930s, Hubble decided himself to investigated 4 of van Maanen’s rotational observations in M81, M51, M33 and M101 to put the matter to rest. Hubble along with Walter Baade and Seth Nicholson later reported that they found no evidence of rotation in the spirals which van Mannen had reported earlier. It was finally concluded by historians that van Maanen was merely ‘seeing’ what he had wanted to see. Hubble's work had finally put astronomical research on the right path with his concept of galactic distances and his theory that spiral nebulae are in fact "island universes".

Hubble pushed on and in 1926 publish his paper on his proposed classification of extragalactic nebulae. (Hubble deliberately avoided using the term ‘galactic nebulae’ or 'galaxy' apparently because Shapley was the first to propose it as being more appropriate - he prefers instead to call them ‘extragalactic nebulae’). In 1936, Hubble published his famous book titled "Realm of the Nebulae". The often seen tuning fork diagram of his classification of galaxies was presented and is still widely in use today. Hubble had classified galaxies into 4 basic types - elliptical, spirals, barred spirals and irregulars which incidentally comprise only 3% of all galaxies. Hubble had believed incorrectly however that galactic evolution was such that the elliptical galaxies from the base of the tuning fork evolved outwards to become spirals and barred spirals. Nonetheless, Hubble's work on his classification of galaxies was significant and is now globally accepted and in use by most astronomical research efforts in these areas.

Fig 1 : Hubble’s Classification of Galaxies showing the familiar tuning fork.

But perhaps the greatest discovery Hubble made in his distinguished career was his discovery of the velocity-distance relationship - rated by many modern astrophysicist as one of the single most important discovery in the twentieth century. Hubble had combined figures he had on radial velocities of 46 galaxies and their distances deduced from cepheids, novae and blue stars to worked out this relationship which gave the "Hubble Constant". His law was simply defined as V = HD, where V is the recessional velocity of the galaxy, D it's distance and H his constant. This was published in his famous paper of 1929 and had great influence in moving astronomical research towards the direction of an expanding Universe. Cosmological theories, including the Big Bang, which followed are intricately dependent on the value of  Hubble Constant - the exact value of which has been and still is a topic of heated debate today. Thus, Hubble not only define the edge of the observable Universe, he also put the age and expansion rate of the observable universe into perspective for researchers. Indeed, Hubble is truly "the Mariner of the Nebulae."