Astronomy

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Astronomy

Quantifying light—luminosity and spectral classes

Astronomy, the oldest of all the sciences, is the scientific study of the universe and all the celestial bodies, gases, dust, and other materials within it. It seeks to describe the structure, movements, and processes of celestial bodies and materials. Included within astronomy are the theories and observations that have been set forth by astronomers over the many thousands of years of its existence. Cosmology, a branch of astronomy, deals with the study of the universe, including theories of how the universe began, such as the big bang theory. Astronomers use physics, mathematics, chemistry, geology, biology, and many other scientific areas when dealing with astronomy. When astronomers deal primarily with physics, for instance, they are often called astrophysicists.

History and impact of astronomy

Ancient ruins provide evidence that the most remote human ancestors observed and attempted to understand the workings of the cosmos, or the entire universe. Although not always fully understood, ancient ruins demonstrate that early humans attempted to mark the progression of the seasons as related to the apparent changing positions of the sun, stars, planets, and Moon on the celestial sphere. Archaeologists speculate that such observations made the determination of times for planting and harvesting more reliable in developing agrarian communities and cultures.

The regularity of the heavens (what is commonly referred to as the sky above the Earth’s surface) also profoundly affected the development of indigenous religious beliefs and cultural practices. For example, according to Aristotle (384–322 BC), Earth occupied the center of the cosmos, and the sun and planets orbited Earth in perfectly circular orbits at an unvarying rate of speed. The word astronomy is a Greek term for star arrangement. Although heliocentric (Sun-centered) theories were also advanced among ancient Greek and Roman scientists, the embodiment of the geocentric theory (Earth-centered theory) conformed to prevailing religious beliefs. In the form of the Ptolemaic model, it was subsequently embraced by the growing Christian church, which dominated Western thought until the rise of empirical science and the use of the telescope during the Scientific Revolution of the sixteenth and seventeenth centuries.

In the East, Chinese astronomers carefully charted the night sky, noting the appearance of celestial phenomena such as comets and novae. As early as 240 BC, the records of Chinese astronomers record the passage of a guest star known now as Halley’s Comet, and in AD 1054, the records indicate that one star became bright enough to be seen in daylight. Archaeoastronomers (astronomers who deal primarily with archaeology) argue that this transient brightness was a supernova explosion, the remnants of

which now constitute the Crab Nebula. The appearance of the supernova was also recorded by the Anasazi Indians of the American Southwest.

Observations were not limited to spectacular celestial events. After decades of patient observation, the Mayan peoples of Central America were able to predict accurately the movements of the sun, moon, and stars. This civilization also devised a calendar that predicted accurately the length of a year, to what would now be measured to be within six seconds.

Early in the sixteenth century, Polish astronomer Nicolaus Copernicus (1473–1543) reasserted the heliocentric theory abandoned by the Greeks and Romans. Although sparking a revolution in astronomy, Copernicus’s system was flawed deeply by an insistence on circular orbits. Danish astronomer Tycho Brahe’s (1546–1601) precise observations of the celestial movements allowed German astronomer and mathematician Johannes Kepler (1571–1630) to formulate his laws of planetary motion, which correctly described the elliptical orbits of the planets.

Italian astronomer and physicist Galileo Galilei (1564–1642) was the first scientist to utilize a newly invented telescope to make recorded observations of celestial objects. In a prolific career, Galileo’s discoveries, including phases of Venus and moons orbiting Jupiter, dealt a death blow to geocentric theory.

In the seventeenth century, English physicist and mathematician Sir Isaac Newton’s (1642–1727) development of the laws of motion and gravitation marked the beginning of Newtonian physics and modern astrophysics. In addition to developing calculus, Newton made tremendous advances in the understanding of light and optics critical to the development of astronomy. Newton’s seminal 1687 work, Philosophiae Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy) dominated the Western intellectual landscape for more than two centuries and provided an impetus for the advancement of celestial dynamics.

During the eighteenth century, important theories of celestial mechanics by French mathematician Joseph-Louis Lagrange (1736–1813), French mathematician Pierre Simon de Laplace, (1749–1827) and Swiss mathematician Leonhard Euler (1707–1783) explained small discrepancies between Newton’s predicted and the observed orbits of the planets. These explanations contributed to the concept of a clockwork-like mechanistic universe that operated according to knowable physical laws.

Just as primitive astronomy influenced early religious concepts during the eighteenth century, advancements in astronomy caused significant changes in Western scientific and theological concepts based upon an unchanging, immutable God who ruled a static universe. During the course of the eighteenth century, there developed a growing scientific disregard for understanding based upon divine revelation and a growing acceptance of an understanding of Nature that was increasingly based upon the development and application of scientific laws. Whether God intervened to operate the mechanisms of the universe through miracles or signs (such as comets) became a topic of lively philosophical and theological debate. Concepts of the divine became identified with the assumed eternity or infinity of the cosmos. Theologians argued that the assumed immutability of a static universe, a concept shaken by the discoveries of Copernicus, Kepler, Galileo, and Newton, offered proof of the existence of God. The clockwork universe was viewed as confirmation of the existence of a God with infinite power who was the prime mover, or creator, of the universe. For many scientists and astronomers, however, the revelations of a mechanistic universe left no place for the influence of the Divine, and they discarded their religious views. These philosophical shifts sent sweeping changes across the political and social landscape.

In contrast to the theological viewpoint, astronomers increasingly sought to explain miracles in terms of natural phenomena. Accordingly, by the nineteenth century, the appearance of comets was viewed no longer as a direct sign from God but rather a natural, explainable, and predictable result of a deterministic universe. Explanations for catastrophic events (e.g., comet impacts, extinctions, etc.) increasingly came to be viewed as the inevitable results of time and statistical probability.

The need for greater accuracy and precision in astronomical measurements, particularly those used in navigation, spurred development of improved telescopes and pendulum driven clocks; this greatly increased the pace of astronomical discovery. In 1781, improved mathematical techniques, combined with technological improvements and the proper application of Newtonian laws, allowed German-born English astronomer William Herschel (1738–1822) to discover the planet Uranus.

Until the twentieth century, astronomy essentially remained concerned with the accurate description of the movements of planets and stars. Developments in electromagnetic theories of light and the formulation of quantum and relativity theories, however, allowed astronomers to probe the inner workings of the celestial objects. German-American physicist Albert Einstein’s (1879–1955) theories of relativity and the emergence of quantum theory influenced Indian-born American astrophysicist Subrahmanyan Chandrasekhar (1910-1995). Chandrasekhar first articulated the evolution of stars into supernovas, white dwarfs, and neutron stars and accurately predicted the conditions required for the formation of black holes, which were subsequently found in the later half of the twentieth century. The articulation of the stellar evolutionary cycle allowed rapid advancements in cosmological theories regarding the creation of the universe. In particular, American astronomer Edwin Hubble’s (1889–1953) discovery of red-shifted spectra from stars provided evidence of an expanding universe that, along with increased understanding of stellar evolution, ultimately led to the abandonment of static models of the universe and the formulation of the big bang theory, which was based on cosmological models.

In 1932, American engineer Karl Janskey (1905–1945) discovered the existence of radio waves emanating from beyond the Earth. Janskey’s discovery led to the birth of radio astronomy, which ultimately became one of the most productive means of astronomical observation and spurred continuing studies of the cosmos across all regions of the electromagnetic spectrum.

Profound questions regarding the birth and death of stars led to the stunning realization that, in a real sense, because the heavier atoms of which humans were comprised were derived from nucleosynthesis in dying stars, humankind, too, was a product of stellar evolution. After millenniums of observing the cosmos, by the dawn of the twenty-first century, advances in astronomy allowed humans to gaze into the night sky and realize that they were looking at the light from stars distant in space and time, and that they, also, were made from the very dust of stars.

The science of astronomy

At its most fundamental, astronomy is based on the electromagnetic radiation emitted by the stars. The ability to gather light is the key to acquiring useful data. The bigger the primary mirror of a telescope, the greater its light-gathering capabilities and the greater the magnification of the instrument. These two attributes allow a large telescope to image fainter, smaller objects than a telescope of lesser size. Thus, astronomers build ever-larger telescopes, such as the 33-ft-diameter (10-m-diameter) Keck telescopes in Hawaii. They also attempt to escape the distorting effects of the atmosphere with orbital observatories like the Hubble Space Telescope and the James Webb Space Telescope, which is expected to be launched in orbit about the Earth on or after 2011 by NASA.

Astronomy is not just about visible light, however. Though the visible spectral region is most familiar to human sight because eyes are optimized for these wavelengths, observation in the visible region shows only a small portion of the activities and processes underway in the universe. When astronomers view the night sky in other regions of the electromagnetic spectrum, it presents an entirely different picture. Hot gases boil when viewed at infrared wavelengths, newly forming galaxies and stars glow with x rays, and mysterious objects generate explosive bursts of gamma rays. Radio wave and ultraviolet observations likewise bring astronomers new insights about stellar objects.

Each spectral region requires different instrumentation and different approaches to data analysis. Radio astronomers, for instance, use 20- and 30-ft-diameter (6- and 9-m) antennas. In addition, they use telescopes like the one in Arecibo, Puerto Rico, in which a 1,000-ft (303-m) diameter natural bowl in the landscape has been lined to act as an enormous radio wave collector. In the Very Large Array in New Mexico, 27 antennas placed as much as one mile apart from one another are linked by computer to make simultaneous observations, effectively synthesizing a telescope with a 22-mi (35-km) aperture—a radio-frequency analog to the Keck telescope. Infrared, x-ray, and gamma-ray telescopes require special materials and designs for both the focusing optics and the detectors and cannot be performed below the Earth’s atmosphere.

Quantifying light—luminosity and spectral classes

Astronomy is based upon the information that scientists can derive by what is observed when gazing at the stars. One of the characteristics of a star that can be determined observationally is its luminosity—the amount of light that the star emits. When combined with other information about a star, such as its size or temperature, luminosity can indicate the intensity of fusion reactions taking place in the stellar core. Luminosity cannot always be determined by direct observation, however, as distance can decrease the apparent luminosity of an object. The Sun, for example, is not excessively luminous as stars go; it only appears brighter than any other stellar object because it is so close to the Earth.

Magnitude is another way of expressing the luminosity of a star. Greek astronomer and mathematician Hipparchus (190–120 BC) developed the magnitude scale for stars, rating their brightness on a scale of 1 to 6. According to the scale, a star of first magnitude is defined as appearing 100 times as bright as a star of sixth magnitude, so the larger the magnitude, the fainter the object. As telescopes have allowed astronomers to peer deeper into the universe, the scale has expanded: the star Sirius, which appears to be the brightest star in the heavens, has an apparent magnitude of –1.27, while the dwarf planet Pluto has a magnitude of 14.

Apparent magnitude, like apparent luminosity, can be deceptive. To avoid invalid comparisons, astronomers have developed the concept of absolute magnitude, which is defined as the apparent magnitude the object would have when viewed at a distance of 32.6 light years (a light year is defined as the distance that light travels in one earth-year). Thus, measuring the distance to various objects is an important task in astronomy and astrophysics.

The color of light emitted by a star indicates its temperature. At the beginning of the twentieth century, astronomers began classifying stars based on color, or spectral classes. The classes are O, B, A, F, G, K, and M. O-type stars are the hottest (63,000°F [34,632°C]) and tend to appear white or blue-white, while M-type stars are the coolest (5,400°F [2,952°C]) and tend to appear red; the Sun, a yellow star, type G, falls in the middle. Another rating—L-type, for dim, cool objects below M-type—has been proposed for addition to the listing.

Astronomers can glean a tremendous amount of information from stellar magnitudes and glasses. Between 1911–13, Danish astronomer Ejnar Hertzsprung (1873–1967) and American astronomer Henry Norris Russell (1877–1957) independently developed what is now known as the Hertzsprung-Russell diagram, which plots the magnitude and color of stars. According to the diagram, most stars fall on a slightly curving diagonal that runs from very bright, hot stars down to very cool, red stars. Most stars follow this so-called main sequence as they gradually burn out. Some stars fall off of the main sequence, for example red giants, which are relatively cool but appear bright because of their enormous size; or white dwarfs, which are bright but so small as to appear faint.

KEY TERMS

Absolute magnitude —The apparent brightness of a star, measured in units of magnitudes, at a fixed distance of 10 parsecs.

Absorption spectrum —The record of wavelengths (or frequencies) of electromagnetic radiation absorbed by a substance; the absorption spectrum of each pure substance is unique.

Apparent magnitude —The brightness of a star, measured in units of magnitudes, in the visual part of the electromagnetic spectrum, the region to which our eyes are most sensitive. Emission spectrum—A spectrum consisting of bright lines generated by specific atoms or atomic processes.

Luminosity —The amount of light emitted from a source per unit area.

Spectroscopy —A technique for studying light by breaking it down into its constituent wavelengths.

Spectroscopy

When we think of astronomy, spectacular, colorful pictures of swirling galaxies, collapsing stars, and giant clouds of interstellar gas come to mind. In reality, however, some of the most useful observational data in astronomy does not involve images at all. Spectroscopic techniques are powerful tools that allow scientists to detect the presence of certain elements or processes in faraway galaxies.

In spectroscopy, incoming light—such as that from a star—is passed through a grating or a prism that splits the light up into its constituent wavelengths, or colors. Normally, a very bright, hot star will emit a continuous spectrum of light that spreads like a rainbow across the electromagnetic spectrum. In the case of lower-density gas masses such as nebulae, however, the light will be emitted only at certain specific wavelengths defined by the elements found in the nebula; hydrogen atoms, for example, generate vivid yellow lines at characteristic wavelengths. The spectra will, thus, consist of a collection of bright lines in an otherwise dark background; this is called an emission spectrum. Similarly, if a cooler atmosphere surrounds a star, the atoms in the atmosphere will absorb certain wavelengths, leaving dark lines in what would otherwise be a continuum. This is known as an absorption spectrum.

Scientists study absorption and emission spectra to discover the elements present in stars, galaxies, gas clouds, or planet-forming nebulae. By monitoring the amount by which spectroscopic lines shift toward red wavelengths or toward blue wavelengths, astronomers can determine whether objects are moving toward or away from the Earth. This technique, based on the Doppler shift, is used not only to help astronomers study the expansion of the universe but also to determine the distance or age of the object under study. By studying the Doppler shift of stellar spectra, astronomers have been able to monitor faint wobbles in the motion of stars that indicate the presence of a companion star or even of extrasolar planets.

Although the sophisticated instruments and analysis techniques of astrophysics assist in the understanding of universe, astronomy is essentially about the observation of light. Using the data produced by a multitude of telescopes around the world and in orbit, astronomers are making new discoveries on a daily basis and, just as often, exposing new puzzles to solve. The basic tools described above help scientists to extract information about stellar objects and thus about the processes at work in the universe.

Resources

BOOKS

Arny, Thomas. Explorations: An Introduction to Astronomy. Boston, MA: McGraw-Hill, 2006.

Aveni, Anthony F. Uncommon Sense: Understanding Nature’s Truths Across Time and Culture. Boulder, CO: University Press of Colorado, 2006.

Chaisson, Eric. Astronomy: A Beginner’s Guide to the Universe. Upper Saddle River, NJ: Pearson/Prentice Hall, 2004. Hawking, Stephen. The Illustrated Brief History of Time.

Updated and Expanded ed. New York: Bantam, 2001. Rees, Martin J. Our Cosmic Habitat. Princeton, NJ: Princeton University Press, 2001.

Sagan, Carl. Cosmos. New York: Random House, 2002.

OTHER

Home page of <http://www.KidsAstronomy.com> (accessed October 1, 2006).

Nemiroff, Robert, and Jerry Bonnell. “Astronomy Picture of the Day.” National Air and Space Administration and Michigan Technological University <http://antwrp.gsfc.nasa.gov/apod/astropix.html> (accessed October 1, 2006).

K. Lee Lerner

Kristen Lewotsky

The Gale Encyclopedia of Science