Akademik

Hubble, who was born in Marshfield, Missouri, was the son of a lawyer. He was educated at the University of Chicago where he was influenced by the astronomer George Hale and, as a good athlete, was once offered the role of ‘Great White Hope’ in a match against the world heavyweight champion, Jack Johnson. Instead he went to England, accepting a Rhodes scholarship to Oxford University where, between 1910 and 1913, he studied jurisprudence, represented Oxford in athletics, and fought the French boxer, Georges Carpentier. On his return to America he practiced law briefly before returning in 1914 to the study of astronomy at the Yerkes Observatory of the University of Chicago. He obtained his PhD in 1917. After being wounded in France in World War I he took up an appointment in 1919 at the Mount Wilson Observatory in California where Hale was director and where he spent the rest of his career.

Hubble's early work involved studies of faint nebulae, which in the telescopes of the day appeared as fuzzy extended images. He considered that while some were members of our Galaxy and were clouds of luminous gas and dust, others, known as spiral nebulae, probably lay beyond the Galaxy. After the powerful 100-inch (2.5-m) telescope went into operation at Mount Wilson he produced some of the most dramatic and significant astronomy of the 20th century. In 1923 he succeeded in resolving the outer region of the Andromeda nebula into “dense swarms of images which in no way differ from those of ordinary stars.” To his delight he found that several of them were Cepheids, which allowed him to use Harlow Shapley's calibration of the period-luminosity curve to determine their distance as the unexpectedly large 900,000 light-years. Although this conflicted sharply with the results of Adriaan van Maanen, Hubble continued with his observations. Between 1925 and 1929 he published three major papers showing that the spiral nebulae were at enormous distances, well outside our own Galaxy, and were in fact isolated systems of stars, now called spiral galaxies. This was in agreement with the work of Heber Curtis. In 1935 van Maanen reexamined his data and, appreciating their unsatisfactory nature, withdrew the final objection to Hubble's results.

In 1929 Hubble went on to make his most significant discovery and announced what came to be known as Hubble's law. Using his own determination of the distance of 18 galaxies and the measurements of radial velocities from galactic red shifts carried out by Vesto Slipher and Milton Humason, he saw that the recessional velocity of the galaxies increased proportionately with their distance from us, i.e., v=Hd, where v is the velocity, dthe distance, and H is known as Hubble's constant. Further measurements made by Hubble in the 1930s seemed to confirm his earlier insight. It was this work that demonstrated to astronomers that the idea of an expanding universe, proposed earlier in the 1920s by Alexander Friedmann and Georges Lemaître, was indeed correct. The expansion of the universe is now fundamental to every cosmological model.

Hubble's law was soon seen as containing the key to the size, age, and future of the universe. Hubble's constant can be found from the mean value of v/d. Hubble himself gave it a value approximately ten times its presently accepted figure. The constant permits a calculation of the observable size of the universe to be made. The limiting value of recession must be the speed of light (c). If we divide this by H we get a ‘knowable’ universe with a radius of about 18 billion light-years. Beyond that no signal transmitted could ever reach us, for to do so it would need to exceed the speed of light.

It is also possible to calculate the time that must have elapsed since the original highly compact state of the universe, i.e., the age of the universe. Hubble's own estimate was 2 billion years but with revisions of his constant, cosmologists now, none too precisely, assign a value of between 12 and 20 billion years.

Hubble also made a major contribution to the study of galactic evolution by producing the first significant classification of galaxies. William Herschel had simply classified them as bright or faint, large or small, while his son John Herschel introduced five categories in terms of size, brightness, roundness, condensation, and resolvability, each with five subdivisions. Hubble published his scheme in 1926. It involved dividing galaxies into two classes, elliptical and spiral. Ellipticals could be subdivided on the basis of their degree of ellipticity, ranging from the circular form (E0) to the elongated (E7). Spirals could be either barred or normal spirals which were subdivided in terms of their degree of openness. Although anomalous objects were later discovered that failed to fit it, Hubble's scheme is still used as the basis for galactic classification.