Monday, 27 January 2014

Edwin Hubble

Edwin Hubble (1889–1953), American astronomer, who made important contributions to the study of galaxies, the expansion of the universe, and the size of the universe. Hubble was the first to discover that fuzzy patches of light in the sky called spiral nebula were actually galaxies like Earth’s galaxy, the Milky Way. Hubble also found the first evidence for the expansion of the universe, and his work led to a much better understanding of the universe’s size. See also Astronomy.
Edwin Powell Hubble was born in Marshfield, Missouri. He attended high school in Chicago, Illinois, and received his bachelor’s degree in mathematics and astronomy in 1910. He was awarded a Rhodes Scholarship to study at the University of Oxford in England, where he earned a law degree in 1912. He returned to the United States in 1913 and settled in Kentucky, where his family had moved. From 1913 to 1914 Hubble practiced law and taught high school in Kentucky and Indiana. In 1914 he moved to Wisconsin to take a research post at the University of Chicago’s Yerkes Observatory.
In 1917 Hubble earned his Ph.D. degree in astronomy from the University of Chicago and received an invitation from American astronomer George Hale to work at Mount Wilson Observatory in California. Around the same time that Hubble received the invitation, the United States declared war on Germany, marking the beginning of official U.S. military involvement in World War I (1914-1918). Hubble volunteered to serve in the U.S. Army, rushing to finish his dissertation and reporting for duty just three days after passing his oral Ph.D. exam. He was sent to France at first and remained on active duty in Germany until 1919. He left the Army with the rank of major.
In 1919 Hubble finally accepted the offer from Mount Wilson Observatory, where the 100-in (2.5-m) Hooker telescope was located. The Hooker telescope was the largest telescope in the world until 1948. Hubble worked at Mount Wilson for the rest of his career, and it was there that he carried out his most important work. His research was interrupted by the outbreak of World War II (1939-1945); during the war he served as a ballistics expert for the U.S. Department of War.
While Hubble was working at the Yerkes Observatory, he made a careful study of cloudy patches in the sky called nebulas. Now, astronomers apply the term nebula to clouds of dust and gas within galaxies. At the time that Hubble began studying nebulas, astronomers had not been able to differentiate between nebulas and distant galaxies, which also appear as cloudy patches in the sky.
Hubble was especially interested in two nebulas called the Large Magellanic Cloud and the Small Magellanic Cloud (see Magellanic Clouds). In 1912 American astronomer Henrietta Leavitt had used the brightness of a certain type of star in the Magellanic Clouds to measure their distance from Earth. She used Cepheid stars, yellow stars that vary regularly in brightness. The longer the time a Cepheid star takes to go through a complete cycle, the higher its average brightness, or average absolute magnitude. By comparing the brightness of the star as seen from Earth with the star’s actual brightness (estimated from the length of the star’s cycle), Leavitt could determine the distance from Earth to the nebula. She and other scientists showed that the Magellanic Clouds were beyond the boundaries of the Milky Way Galaxy.
After World War I, with the Hooker telescope at his disposal, Hubble was able to make significant advances in his studies of nebulas. He focused on nebulas thought to be outside of the Milky Way, searching for Cepheid stars within them. In 1923 he discovered a Cepheid star in the Andromeda nebula, now known as the Great Andromeda Spiral Galaxy. Within a year he had detected 12 Cepheid stars within the Andromeda Galaxy. Using these variable stars, he determined that the Andromeda nebula was about 900,000 light-years away from Earth. (A light-year is the distance light can travel in one year, a measurement equal to 9.46 trillion km or 5.88 trillion mi). The diameter of the Milky Way is about 100,000 light-years, so Hubble’s measurements showed that the Andromeda nebula was far outside the boundaries of Earth’s galaxy.
Hubble discovered many other nebulas that contained stars and were located outside of the Milky Way. He found that they contained objects similar to those within the Milky Way Galaxy. These objects included round, compact groups of stars called globular clusters and stars called novas that flare suddenly in brightness. In 1924 he finally proposed that these nebulas were in fact other galaxies like our own, a theory that became known as the island universe. From 1925 he studied the structures of these external galaxies and classified them according to their shape and composition into regular and irregular forms. The regular galaxies, 97 percent of the total, had elliptical or spiral shapes. Hubble further divided the spiral galaxies into normal spiral galaxies and barred spiral galaxies. Normal spiral galaxies have arms that come out from a central, circular core and spiral around the core and each other. The arms of barred spiral galaxies come out from an elongated, bar-shaped nucleus. There are no distinct boundaries between the types of galaxies—some galaxies have the characteristics of both spiral and elliptical galaxies, and some spiral galaxies could be classified as either normal or barred. Irregular galaxies—galaxies that seem to have no regular shape or internal structure—made up only 3 percent of the galaxies that Hubble found.
Hubble began to measure the distance from Earth to the galaxies that he classified. He used information provided by Cepheid stars within the galaxies to measure their distance from Earth. He compared these distance measurements to measurements of the galaxies’ movement with respect to Earth. Several astronomers, in particular American astronomer Vesto Slipher, studied the speed of the galaxies in the 1910s and 1920s, before Hubble classified them as galaxies. The astronomers measured the galaxies’ speed by measuring the redshift of the galaxy. Redshift results from the radiation that an object emits. This radiation will appear to shift in wavelength if the object is moving with respect to the observer. If the object is moving away from the observer, each wave will leave from slightly farther away than the wave before it did, increasing the distance between the waves. The wavelength of an object’s radiation will seem shorter if the object is moving toward the observer. This is called the Doppler effect. When the radiation emitted by the object is visible light, a lengthening in wavelength corresponds to a reddening of light. Therefore, the light of astronomical objects moving away from the observer is said to be red-shifted. Slipher and the other astronomers found that all of the galaxies were moving away from Earth. Hubble also did his own redshift measurements.
In 1929 Hubble compared the distances of the galaxies to the speed at which they were moving away from Earth, and he found a direct and very consistent correlation: The farther a galaxy was from Earth, the faster it was receding. This relationship was so consistent throughout the 46 galaxies that Hubble initially studied, as well as in virtually all of the galaxies studied later by Hubble and other scientists, that it is known as Hubble’s Law. Hubble concluded that the relationship between velocity and distance must mean that the universe is expanding. In 1927 Belgian scientist Georges Lemaître had developed a model of the universe that incorporated the general theory of relativity of German American physicist Albert Einstein. Lemaître’s model showed an expanding universe, but Hubble’s measurements were the first real evidence of this expansion.
The relationship of the velocity of galaxies to their distance is called the Hubble constant. If astronomers knew the precise value of Hubble’s constant, they could determine both the age of the universe and the radius of the observable universe. Many teams of scientists have attempted to measure the value since Hubble proposed his theory. In 1999 a group of scientists measured Hubble’s constant to be 70 kilometers per second-megaparsec, with an uncertainty of 10 percent—the most precise measurement to date. This result means that a galaxy appears to be moving 260,000 km/h (160,000 mph) faster for every 3.3 million light-years that it is away from Earth. The universe may be infinitely large, but if objects really do move faster as they move farther from the center of the universe, at some distance objects will be moving at the speed of light. That distance would be the limit to the observable universe, because light from an object moving at the speed of light could never reach Earth. The radius of the observable universe is called the Hubble radius.
During the 1930s, Hubble studied the distribution of galaxies. His results showed that galaxies should be scattered evenly across the sky. He explained that there seemed to be fewer galaxies in the area of the sky that corresponds to the plane of the Milky Way because large amounts of dust block light from external galaxies.
Hubble was an active researcher until his death. He was involved in building the 200-in (508-cm) Hale telescope at the Mount Palomar Observatory, also in southern California. The Hale telescope was the largest telescope in the world from when it went into operation in 1948 until the Keck telescope at the Mauna Kea Observatory in Hawaii was completed in 1990. The Hubble Space Telescope (HST), a powerful telescope launched in 1990 and carried aboard a satellite in orbit around Earth, was named after Hubble and has helped scientists make many important observations.

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