Edwin Powell Hubble used the Cepheid variables to determine the distance to the Andromeda galaxy, a finding that eventually led to the development of the big bang theory.

The question of how the cosmos began has always fascinated humanity. The Babylonians measured the positions of stars and the planets and concluded that those bodies affected human lives. Their answer was based on myths and therefore was not scientific. The ancient Greeks were the first to explain the cosmos in a scientific manner: They saw the cosmos as a series of spheres, with the stars attached to the outer sphere. They considered the entire universe to be a few million kilometers in diameter. This work culminated with Ptolemy Ptolemy and his theory of the geocentric cosmos, in which everything—the Moon, the planets, the Sun—moves around Earth. His theory, which accounted for all the known motions of the objects in the sky, was so successful that scientists, astronomers, philosophers, and religious leaders alike accepted it as valid for almost fifteen hundred years. Astronomy;galaxies
Galaxies;Andromeda nebula
Andromeda nebula
Cepheid variable stars
Stellar distances
[kw]Hubble Determines the Distance to the Andromeda Nebula (1924)
[kw]Andromeda Nebula, Hubble Determines the Distance to the (1924)
[kw]Nebula, Hubble Determines the Distance to the Andromeda (1924)
Astronomy;galaxies
Galaxies;Andromeda nebula
Andromeda nebula
Cepheid variable stars
Stellar distances
[g]United States;1924: Hubble Determines the Distance to the Andromeda Nebula[05920]
[c]Science and technology;1924: Hubble Determines the Distance to the Andromeda Nebula[05920]
[c]Astronomy;1924: Hubble Determines the Distance to the Andromeda Nebula[05920]
Hubble, Edwin Powell
Shapley, Harlow
Leavitt, Henrietta Swan
Sitter, Willem de

Edwin Powell Hubble at the Mount Wilson Observatory near Pasadena, California.

(NASA/GSFC)

This consensus changed gradually starting in the early sixteenth century with the work of Nicolaus Copernicus, Copernicus, Nicolaus who quietly asserted the case for heliocentricity—that is, that Earth is not the center of the universe but merely a planet that moves around the Sun. The edge of the universe was as far away as the planet Saturn. In the seventeenth century, Galileo Galileo championed this cause, leading to the famous controversy over cosmological understanding and its theological implications. In time, although Galileo’s specific arguments were found to be inaccurate, the principle of heliocentricity was proved as a fact.

As technology improved, the importance of the universe continued to diminish. In 1838, Friedrich Wilhelm Bessel Bessel, Friedrich Wilhelm measured the angular movement (stellar parallax) of star 61 Cygnus against its background stars and determined that it was six light-years away from Earth. The inability to measure this stellar parallax was one of the reasons the ancients concluded from their observations that Earth was not moving, and, therefore, the geocentric theory was correct. The stars were simply too far away for them to measure the parallax. Parallax method of measuring stellar distances

From 1915 to 1920, Harlow Shapley used the 100-inch (254-centimeter) telescope at the Mount Wilson Observatory Mount Wilson Observatory to study globular clusters. Globular clusters
Hooker telescope These densely packed assemblies of gravitationally bound stars are, in turn, bound to Earth’s galaxy, the Milky Way. Milky Way galaxy Shapley noted that the clusters are not evenly distributed in the sky. He knew also that certain stars, the Cepheid variables, have an interesting property: Their average luminosity correlates with the period of their variability. Luminosity is the amount of energy a star gives off per second and determines the star’s absolute brightness. If two stars have the same luminosity, the one closer to Earth will appear brighter. The brightness of a star as viewed from Earth is termed its apparent brightness. A star’s apparent brightness is dependent on its absolute brightness and its distance from Earth, and astronomers developed an equation to show that relationship.

For the Cepheids, the luminosity varies in a predictable manner and is in concert with the period of oscillation. Henrietta Swan Leavitt established this relationship in 1904, when she cataloged a large number of variable stars. Variable stars She saw that for the Cepheids, the longer the period of light variation, the greater the average brightness of the star. If a variable’s distance could be determined to calibrate the relationship, this would establish the star’s absolute brightness. Astronomers could then reverse the process and use the period-luminosity relationship Period-luminosity relationship of stars[Period luminosity relationship] to measure the distances of other Cepheid variables and objects associated with them.

The method to determine the distance works in the following manner. An astronomer measures a Cepheid’s apparent brightness and period. The period-luminosity curve yields the star’s absolute brightness, and the two brightnesses are entered into the distance-brightness equation. The calculation yields the Cepheid’s distance from Earth. Because globular clusters contain Cepheids, Shapley was able to measure the distance to the clusters. Knowing the distances and the direction of the clusters, Shapley plotted their location relative to the location of Earth in the Milky Way. Because the number of clusters increased toward the portion of the Milky Way in the constellation Sagittarius, Shapley concluded that Earth’s location is about two-thirds the distance from the center of the galaxy in Sagittarius and that the Milky Way is shaped like a disk about 100,000 light-years in diameter.

With the realization that the Milky Way is a grouping of billions of stars came the question of whether there are more of these groupings. Up to this point, astronomers did not have telescopes powerful enough to find out, but in 1919, Edwin Powell Hubble gained access to what was then the world’s most powerful telescope: the Mount Wilson 100-inch telescope. By 1924, he was able to resolve the fuzzy patches of light that Charles Messier had cataloged into individual stars a century and a half before. These “nebulas” were not clouds of glowing gas like those found within our galaxy; rather, they were galaxies similar to the Milky Way.

Hubble was able to detect Cepheid variables in the Andromeda nebula, and, after determining the apparent brightness and period of oscillation for several Cepheids, he was able to calculate the distance to the nebula at roughly 800,000 light-years. This is more than eight times the distance to the most distant stars in our galaxy. Hubble’s work proved that the light patches are separate, gravitationally bound groups of stars and independent of our own galaxy. Hubble went on to measure the distances to other nearby galaxies.

Two decades later, Walter Baade Baade, Walter determined that the Andromeda nebula is actually 2 million light-years from Earth. He discovered that there are two types of Cepheid variables: the classic variety and another set with a different brightness-period relationship. These other Cepheids are brighter for a given period of oscillation than the classic Cepheids. Hubble assumed that his Cepheids were fainter than they were, and therefore closer than they were.

Once Hubble determined that the nebulas were other galaxies, he gathered information about them. This led to their classification and the plotting of their possible evolution. With improvement in photography and other techniques, astronomers were able to analyze the light spectra of galaxies. The spectrum of a galaxy looks like a rainbow with dark lines through it and is the total of all frequencies of light coming from the galaxy. The lines are caused by the absorption of certain light frequencies by cooler gases in the galaxy. If there is relative motion between the light’s source and the detector, the spectrum is shifted; this is known as the Doppler effect. Doppler effect If a galaxy is moving toward Earth, the entire spectrum shifts toward the blue end of the spectrum. If the galaxy is moving away, the shift is toward the red end of the spectrum. The amount of shift is proportional to the velocity of the galaxy. With this method, Hubble determined that the farther the galaxy is from Earth, the faster it is receding. This suggests that the galaxies are moving away from one another and leads to the conclusion that the universe Universe;expansion is expanding, as maintained by Dutch astronomer Willem de Sitter.

In order to answer the question of the origin of the cosmos, one must determine its characteristics. The universe could be small or large. It may have a boundary or it may continue forever. It may be static or dynamic. Therefore, astronomers devote much of their time to determining the distances of various objects—such as planets, stars, nebulas, and galaxies—from Earth and how those distances change over time. Such distances obviously cannot be measured directly, so astronomers have developed ingenious methods of indirect measurement, such as the use of trigonometry. As larger telescopes have been built, astronomers have devised new methods that utilize the data these instruments allow them to gather, as Hubble did in using the Mount Wilson telescope to locate the Cepheid variables in the Andromeda nebula.

Astronomers continued the work that Hubble began, establishing the distances and the velocities of nearby galaxies. This permitted them to measure the more distant galaxies through such methods as the comparison of image sizes. Linking this information with the galaxies’ velocities—found using the Doppler effect—scientists have determined that the universe is from 13 to 18 billion years old and is expanding.

Two theories have been put forward to explain the age and expansion of the universe: the big bang theory Big bang theory of Georges Lemaître Lemaître, Georges and George Gamow Gamow, George and the steady state theory Steady state theory of Thomas Gold Gold, Thomas and Fred Hoyle. Hoyle, Fred The steady state theory proposes that the universe is expanding but remains the same because new matter is created to take the place of the galaxies that are moving away from Earth. Astronomers are uncomfortable with the assertion that the matter appears from nowhere—something that is very difficult to verify.

Most astronomers now accept the big bang theory as the best explanation of how the universe began. According to this theory, at some time between 13 and 18 billion years ago, a very hot, very dense “object” exploded. As the temperature dropped, matter as it is now known came into existence: electrons, protons, neutrons, and other particles. As the universe cooled, hydrogen formed, along with a small amount of helium. The matter formed into the first stars and galaxies. There were no planets, because the elements—such as silicon, oxygen, and iron—necessary for planets’ formation did not exist. Before planets could form, stars had to undergo supernova explosions that would cause the formation of these elements and others.

Although astronomers continue to gather information and form theories, the eventual likely fate of the universe remains unknown. It may continue to expand forever. It could “die” as its stars use up their hydrogen fuel and become white dwarfs, neutron stars, or black holes. Or, if the universe has enough mass, its expansion will stop because of the gravitational attraction of that mass, and it will then collapse into its original condition. It may then start to expand again to produce another universe, one that may be completely different from our present universe, with physical laws we cannot even imagine. Astronomy;galaxies
Galaxies;Andromeda nebula
Andromeda nebula
Cepheid variable stars
Stellar distances

• Baade, Walter. Evolution of Stars and Galaxies. Cambridge, Mass.: Harvard University Press, 1963. Baade’s work on stellar populations led to the discovery that there are two varieties of Cepheid variables and resulted in the doubling of the estimated distance to the Andromeda nebula to 2 million light-years. Compiled from a series of lectures Baade gave in the fall of 1958.
• Clark, David H., and Matthew D. H. Clark. Measuring the Cosmos: How Scientists Discovered the Dimensions of the Universe. New Brunswick, N.J.: Rutgers University Press, 2004. Relates the stories of the scientists who have contributed to current knowledge about the size, mass, and age of the universe. Chapters 4 and 5 include discussion of the work of Hubble and Leavitt. Features glossary, bibliography, and index.
• Ferris, Timothy. The Red Limit: The Search for the Edge of the Universe. Rev. ed. New York: Harper Perennial, 2002. Well-presented volume discusses the history of the major discoveries in astronomy, paying particular attention to the individuals who made these discoveries. A comprehensible, accurate discussion of astronomy written in an engaging style for readers who have no familiarity with modern cosmological ideas. Includes extensive glossary, selected bibliography, and index.
• Kaufmann, William J., III. Galaxies and Quasars. San Francisco: W. H. Freeman, 1979. Explains in nontechnical language the implications of Hubble’s discoveries. Discusses the Cepheid variable luminosity-period relationship, the Doppler effect, and the geometry of the universe. Includes many figures and photographs.
• Silk, Joseph. The Big Bang. 3d ed. New York: W. H. Freeman, 2000. Presents a sweeping account of the formation and evolution of the universe. Recounts the history of astronomical speculation about the universe and examines evidence for the big bang theory. Includes glossary and index.
• _______. On the Shores of the Unknown: A Short History of the Universe. New York: Cambridge University Press, 2005. A history of the universe and the development of humankind’s knowledge about it that is accessible to lay readers. Includes illustrations and index.

Slipher Obtains the Spectrum of a Distant Galaxy

Leavitt Discovers How to Measure Galactic Distances

Slipher Presents Evidence of Redshifts in Galactic Spectra

Michelson Measures the Diameter of a Star

Hubble Shows That Other Galaxies Are Independent Systems

Hubble Confirms the Expanding Universe