Edwin Powell Hubble

 
Edwin Powell Hubble in 1931. Credit: Johan Hagemeyer
 
Edwin Powell Hubble, one of the leading astronomers of the twentieth century, was born in Marshfield, Missouri, on November 20, 1889. His discovery in the 1920s that countless galaxies exist beyond our own Milky Way galaxy revolutionized our understanding of the universe and our place within it.

   Hubble was the fifth of his parents' eight children. He studied mathematics and astronomy at the University of Chicago and earned a bachelor of science degree in 1910. Hubble was one of the first Rhodes Scholars at Oxford University, where he studied jurisprudence (philosophy of law) and obtained a bachelor of arts degree. In 1913, he returned to the United States and taught at a high school in Indiana for a year, before going back to the University of Chicago in 1914 for graduate work in astronomy. He completed his doctorate dissertation in 1917. After serving briefly in France in World War I, Hubble returned in 1919 with the rank of major, and accepted a staff position at the Carnegie Institution for Science's Mount Wilson Observatory, near Pasadena, California, which housed the newly commissioned 100-inch Hooker telescope, then the largest telescope in the world.

Edwin Hubble seated at the 100-inch reflecting telescope at the Mount Wilson Observatory in California.
 Credits: Edwin P. Hubble Papers, Huntington Library, San Marino, California

 From 1922 to 1936 Hubble solved four of the central problems in cosmology:

  •   From 1922 to 1926 Hubble proposed a classification system for nebulae, both galactic (diffuse) and extragalactic. The galaxy classification system has become the Hubble morphological sequence of galaxy types. The elliptical, spiral and irregular galaxies are arranged in a scheme known as the Hubble tuning fork diagram.
  •   Hubble demonstrated the nature of extragalactic nebulae as collections of stars comparable to our own Milky Way. With his discovery of Cepheid variable stars in NGC 6822 in 1924, with parallel work in M33 and M31, Hubble settled decisively the question of the nature of the galaxies, whose correct solution had previously been given using what many believed to be inconclusive arguments, by Heber Curtis (1872–1942), Knut Lundmark (1889–1958), and Ernst Öpik (1893–1985). 
Hubble's observations were announced in a paper  titled Cepheids in Spiral Nebulae, presented in absentia at a joint meeting of the American Association for the Advancement of Science and the American Astronomical Society on January 1, 1925. After discovering Cepheid variable stars within the Andromeda Nebula (M31) and Triangulum (M33), Hubble used the period–luminosity relationship — discovered in 1912 by Henrietta Swan Leavitt (1868–1921) — to determine estimated distances to the galaxies. He derived a distance of about 285,000 pc for the two nebulae. The distance meant that the nebulae must lie outside our Galaxy, for Harlow Shapley (1885–1972) had already used the same calibrated period–luminosity relation to derive a maximum extent of 100,000 pc for the Milky Way. As Walter Baade (1893–1960) showed nearly three decades later, Shapley's period–luminosity relation had been erroneously calibrated, so the correct values of distances are: about 765 kpc for the Andromeda Galaxy, 730 to 940 kpc for the Triangulum Galaxy, and about 27 kpc for the diameter of the Milky Way.

This graphic depicts Edwin Hubble’s Classification Scheme, also known as the tuning fork diagram. It divides galaxies into elliptical and spiral galaxies. The letters indicate the level of compactness of their spiral arms, with “a” being the most tightly wound and “c” being the least. Galaxies are also subdivided by normal and barred spirals. While this diagram offers a strong foundation for galaxy classification, it doesn’t fit all types, including irregular, dwarf, and massive elliptical galaxies. Credits: NASA; ESA

  •   Hubble showed that the distribution of galaxies was uniform, and they occupy a space that is cosmological. From 1926 to 1936 the distribution of galaxies, averaged over many solid angles, was determined to be homogeneous in distance. The test was made by showing that the coefficient of the log N(m) count distribution with magnitude has a value of about 0.6 at bright magnitudes. This proved that galaxies truly mark a space which is significant to the universe itself. Galaxy counts to the magnitude limit of the Mount Wilson 100-inch telescope were then used to attempt a measurement of the radius of curvature of space by finding deviations of the coefficient from the Euclidean value of faint magnitudes.
  •   Working with Milton L. Humason (1891–1972), Hubble showed that galaxies are moving away from us with a speed proportional to their distance (Hubble’s law). The linear velocity-distance relation was set out in a discovery paper in 1929, followed by a series of papers with Humason between 1931 and 1936 that verified and extended the relation to large (i.e. 60,000 km s-1) redshifts. This discovery lead to the notion of the expanding Universe which is the centre-piece (being the necessary condition) for the cosmological models of the present day. The slope of the Hubble diagram of the velocities of galaxies plotted against their distances is known as the Hubble constant, and its reciprocal, the Hubble time, is an estimate of the age of the Universe. 
By the mid-1930s the redshift-distance relationship was generally interpreted as a velocity-distance relationship such that the spectral shifts of the galaxies were a consequence of their motions. Hubble found a slope to the linear velocity–distance correlation of about 536 km/s/Mpc. The implied Hubble time was about 2 billion years, close to ages being found for the Earth and for some stars at about the same time. But Hubble throughout his career resisted the definite identification of the redshifts as velocity shifts and in consequence the expanding Universe.
Velocity-Distance Relation among Extra-Galactic Nebulae.
  Credit: Edwin Hubble. A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae. From the Proceedings of the National Academy of Sciences, Volume 15 : March 15, 1929 : Number 3

   Hubble also solved the problem of the source of radiation and the nature of the spectra of diffuse nebulae, recognizing the difference between emission and reflection nebulae, and proving that the source of radiation of reflection nebulae is an associated star. He also made the unprecedented identification of globular clusters in M31.

   In 1936, Hubble summarized his discoveries in a successful popular book The Realm of the Nebulae. After a long career entirely at Mount Wilson Observatory, he died as the result of a cerebral thrombosis on September 28, 1953, in San Marino, California. He is commemorated by the name of the Hubble Space Telescope. 

 

Equipped with his five senses, man explores the universe around him and calls the adventure Science.
— Edwin Hubble in "The Exploration of Space". Harper's Magazine. May 1929 
 
Edwin Hubble examining a photograph of the Andromeda Galaxy (M31).
  Credits: Edwin P. Hubble Papers, Huntington Library, San Marino, California

 

Edwin Hubble stands by the 48" Schmidt Telescope at Palomar Observatory in 1949.
  Credits: Carnegie Institution of Washington

References:

Hubble, E. P., Cepheids in Spiral Nebulae, The Observatory, Vol. 48, p. 139-142 (1925) [full text]
Hubble, E. P., A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebulae. From the Proceedings of the National Academy of Sciences, Volume 15 : March 15, 1929 : Number 3 
Kragh, Helge. Hubble, Edwin Powell. In: The Biographical Encyclopedia of Astronomers. Editor-in-Chief Thomas Hockey. 2007. Springer. ISBN 13:978-0-387-31022-0
Sandage, Alan. Edwin Hubble 1889-1953. The Journal of the Royal Astronomical Society of Canada. Vol. 83, No.6 December 1989 Whole No. 621
Biography of Edwin P. Hubble. NASA Goddard Space Center

 
© 2025, Andrew Mirecki

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