Payne-Gaposchkin, Cecilia Helena

Payne-Gaposchkin, Cecilia Helena (1900-79), English-born American astrophysicist. Known for her study of variable stars, she was also the first astronomer to measure by means of spectral analysis the relative abundances of chemical elements in a star. She was the first to show that more than 60 percent of the known galactic novas in the Milky Way are concentrated in the quadrant containing the galactic center. She joined the Harvard University Observatory in 1923 and was granted a doctorate in 1925 from Radcliffe College. In 1938 she was named Phillips Astronomer at Harvard University, and in 1956 she became professor of astronomy. At Harvard she developed a new method for determining stellar magnitudes from photographic plates.

Spectroscopy

Absorption Spectrometer Spectrometers are instruments that generate, examine, or record spectrums. In this instance, an absorption spectrometer is being used to determine the spectrum created by an unknown substance. The instrument’s lenses focus and prevent diffraction of the light, while a central prism splits white light into a spectrum of its constituent colors. The colors appearing on the screen represent the radiation wavelengths which the sample did not absorb.© Microsoft Corporation. All Rights Reserved.

The next major breakthrough in the study of the Sun was the development of ways to study sunlight. In the mid-17th century English scientist Isaac Newton used a prism—a specially cut chunk of glass—to break sunlight down into its different colors. This range of colors is called the Sun’s spectrum, and the study of spectra is called spectroscopy. In 1802 British scientist William Wollaston found that the solar spectrum was cut by several dark gaps. By 1815 German physicist Joseph von Fraunhofer had cataloged the wavelengths of more than 300 of the gaps, called absorption lines. Fraunhofer assigned letters to the most prominent absorption lines. In the mid-19th century German scientists Gustav Kirchhoff and Robert Bunsen related the absorption lines in the Sun’s spectrum to chemical elements. In 1925 English-born American astronomer Cecilia Payne (later Cecilia Payne-Gaposchkin) compared the spectrum of the Sun to that of other stars to show that virtually all bright, middle-aged stars have the same composition.

Cecilia Payne-Gaposchkin American astrophysicist Cecilia Payne-Gaposchkin became well known for her studies of variable stars and the Milky Way. She also compared the Sun to many other stars, finding that our Sun is an average middle-aged star. In 1923 she joined Harvard University and was associated with that institution for much of her career.UPI/Corbis

The spectrum of the Sun’s corona was studied for the first time in the mid-19th century. During the solar eclipse of August 7, 1869, American astronomers Charles A. Young and William Harkness independently discovered that the corona’s spectrum featured an especially bright line of green light. Bright lines in a spectrum are called emission lines. They are the fingerprints of elements in the substance producing the light. The corona’s bright green emission line comes from highly ionized iron, indicating that the corona has very high temperatures.

C Studying the Sun’s Photosphere and Sunspots

Detailed studies of the Sun’s photosphere and the sunspots began with Galileo’s telescopic camera obscura of the 17th century. The next revolution in this area occurred in the 1840s, when German scientist Heinrich Schwabe discovered that the number and positions of sunspots vary over an 11-year period. In 1859 British astronomer Richard Carrington discovered solar flares. Carrington’s discovery helped explain that geomagnetic storms (increased intensity of Earth’s magnetic field) are related to events on the Sun. In 1908 American astronomer George Ellery Hale showed that sunspots contain magnetic fields that are thousands of times stronger than Earth’s magnetic field.

Solar atmosphere

Photosphere

Although there are no fires on the surface of the Sun, the photosphere seethes and roils, displaying the effects of the underlying convection. Photons flowing from below, trapped by the underlying layers, finally escape. This produces a dramatic drop in temperature and density. The temperature at the visible surface is about 5,800 K but drops to a minimum about 4,000 K at approximately 500 kilometres above the photosphere. The density, about 10−7 gram per cubic centimetre (g/cm3), drops a factorof 2.7 every 150 kilometres. The solar atmosphere is actually a vacuum by most standards; the total density above any square centimetre is about 1 gram, about 1,000 times less than the comparable mass in the atmosphere of the Earth. One can see through the atmosphere of the Earth but not through that of the Sun because the formeris shallow, and the molecules absorb only radiation that lies outside of the visible spectrum. The hot photosphere of the Sun, by contrast, contains an ion called negativehydrogen, H−, a hydrogen nucleus with two electrons attached. The H− ion absorbs radiation voraciously through most of the spectrum.

The photosphere is the portion of the Sun seen in ordinary light. Its image reveals two dominant features, a darkening toward the outermost regions, called limb darkening, and a fine rice-grain-like structure called granulation. The darkening occurs simply because the temperature is falling; when one looks at the edge of the Sun, light from higher, cooler, and darker layers is seen. The granules are convective cells that bring energy up from below. Each cell measures about 1,500 kilometres across. Granules have a lifetime of about 25 minutes, during which hot gas rises within them at speeds of about 300 metres per second. They then break up, either by fading out or by exploding into an expanding ring of granules. The granules occur all across theSun. It is believed that the explosion pattern shapes the surrounding granules in a pattern called mesogranulation, although such a pattern is still in dispute. A larger, undisputed patterned called supergranulation is a network of outward velocity flows, each about 30,000 kilometres across, which is probably tied to the big convective zone rather than to the relatively small granules. The flow concentrates the surface magneticfields to the supergranulation-cell boundaries, creating a network of magnetic-field elements.

The photospheric magnetic fields extend up into the atmosphere, where the supergranular pattern dominates the conducting gas. While the temperature above the average surface areas continues to drop, it does not fall as rapidly at the network edges, and a picture of the Sun at a wavelength absorbed somewhat above the surfaceshows the network edges to be bright. This occurs in almost all wavelengths outside the visible.

Fraunhofer was the first to observe the solar spectrum, finding emission in all colours with many dark lines at certain wavelengths. He assigned letters to these lines, by which some are still known, such as the D-lines of sodium, the G-band, and the K-lines of ionized calcium. Further studies by the German physicist Gustav R. Kirchhoffled to the understanding that the lines reveal which atoms are in the photosphere and, by comparison with laboratory data, their state of ionization and excitation.

The spectral lines seen are those expected to be common at 6,000 K, where the thermal energy of each particle is about 0.5 volt. The most abundant elements, hydrogen and helium, are difficult to excite, while atoms such as iron, sodium, and calcium have many lines easily excited at this temperature. When Cecilia Payne, a British-born graduate student studying at Harvard College Observatory in Cambridge, Mass., U.S., recognized the great abundance of hydrogen and helium in 1925, she waspersuaded by her elders to mark the result as spurious; only later was the truth recognized. The strongest lines in the visible spectrum are the H- and K- (Fraunhofer's letters) lines of ionized calcium. Such is the case because calcium is easily ionized, and these lines represent transitions in which energy is absorbed by ions in the ground, or lowest energy, state. The sodium D-lines are quite a bit weaker because most of the sodium is ionized and does not absorb radiation.

The intensity of the lines is determined by both the abundance of the particular elementand its state of ionization, as well as by the excitation of the atomic energy level involvedin the line. By working backward one can obtain the abundance of most of the elementsin the Sun. This set of abundances occurs with great regularity throughout the universe;it is found in such diverse objects as quasars, meteorites, and new stars. The Sun is roughly 90 percent hydrogen by number of atoms and 9.9 percent helium. The remaining atoms consist of heavier elements, especially carbon, nitrogen, oxygen, magnesium, silicon, and iron, making up only 0.1 percent by number.

Chromosphere and corona

The ordinary solar spectrum is produced by the photosphere; during an eclipse the brilliant photosphere is blocked out by the Moon and three objects are visible: (1) a thin,pink ring around the edge of the Sun called the chromosphere, (2) a pearly, faint halo extending a great distance, known as the corona, and (3) pink clouds of gas called prominences suspended above the surface. When flash spectra (spectra of the atmosphere during an eclipse) were first obtained, astronomers found several surprising features. First, instead of absorption lines they saw emission lines (bright lines with nothing between them). This effect arises because between the spectrum lines the chromosphere is transparent, and only the dark sky is seen. Second, they discovered that the strongest lines were due to hydrogen, yet they still did not appreciate its high abundance. Finally, the next brightest lines had never been seen before; because they came from the Sun, the unknown source element came to be called helium. Later helium was found on Earth.

Developments and trends of the 20th century

Astronomy

Some of the most spectacular advances in modern astronomy have come from research on the large-scale structure and development of the universe. This research goes back to William Herschel's observations of nebulas at the end of the 18th century.Some astronomers considered them to be “island universes”—huge stellar systems outside of and comparable to the Milky Way Galaxy, to which the solar system belongs. Others, following Herschel's own speculations, thought of them simply as gaseous clouds—relatively small patches of diffuse matter within the Milky Way Galaxy, which might be in the process of developing into stars and planetary systems, as described in Laplace's nebular hypothesis.

In 1912 Vesto Melvin Slipher began at the Lowell Observatory in Arizona an extensive program to measure the velocities of nebulas, using the Doppler shift of their spectral lines. (Doppler shift is the observed change in wavelength of the radiation from a source that results from the relative motion of the latter along the line of sight.) By 1925 he had studied about 40 nebulas, most of which were found to be moving away from the Earth according to the red shift (displacement toward longer wavelengths) of their spectra.

Although the nebulas were apparently so far away that their distances could not be measured directly by the stellar parallax method, an indirect approach was developed on the basis of a discovery made in 1908 by Henrietta Swan Leavitt at the Harvard College Observatory. Leavitt studied the magnitudes (apparent brightnesses) of a large number of variable stars, including the type known as Cepheid variables. Some of them were close enough to have measurable parallaxes so that their distances and thus their intrinsic brightnesses could be determined. She found a correlation between brightness and period of variation. Assuming that the same correlation holds for all stars of this kind, their observed magnitudes and periods could be used to estimate their distances.

In 1923 the American astronomer Edwin P. Hubble identified a Cepheid variable in the so-called Andromeda Nebula. Using Leavitt's period–brightness correlation, Hubble estimated its distance to be approximately 900,000 light-years. Since this was much greater than the size of the Milky Way system, it appeared that the Andromeda Nebula must be another galaxy (island universe) outside of our own.

In 1929 Hubble combined Slipher's measurements of the velocities of nebulas with further estimates of their distances and found that on the average such objects are moving away from the Earth with a velocity proportional to their distance. Hubble's velocity–distance relation suggested that the universe of galactic nebulas is expanding, starting from an initial state about 2,000,000,000 years ago in which all matter was contained in a fairly small volume. Revisions of the distance scale in the 1950s and later increased the “Hubble age” of the universe to more than 10,000,000,000 years.

Calculations by Aleksandr A. Friedmann in the Soviet Union, Willem de Sitter in The Netherlands, and Georges Lemaître in Belgium, based on Einstein's general theory of relativity, showed that the expanding universe could be explained in terms of the evolution of space itself. According to Einstein's theory, space is described by the non-Euclidean geometry proposed in 1854 by the German mathematician G.F. Bernhard Riemann. Its departure from Euclidean space is measured by a “curvature” that depends on the density of matter. The universe may be finite, though unbounded, like the surface of a sphere. Thus the expansion of the universe refers not merely to themotion of extragalactic stellar systems within space but also to the expansion of the space itself.

The beginning of the expanding universe was linked to the formation of the chemical elements in a theory developed in the 1940s by the physicist George Gamow, a former student of Friedmann who had emigrated to the United States. Gamow proposed that the universe began in a state of extremely high temperature and density and exploded outward—the so-called big bang. Matter was originally in the form of neutrons, which quickly decayed into protons and electrons; these then combined to form hydrogen andheavier elements.

Gamow's students Ralph Alpher and Robert Herman estimated in 1948 that the radiation left over from the big bang should by now have cooled down to a temperature just a few degrees above absolute zero (0 K, or -459° F). In 1965 the predicted cosmic background radiation was discovered by Arno A. Penzias and Robert W. Wilson of the Bell Telephone Laboratories as part of an effort to build sensitive microwave-receiving stations for satellite communication. Their finding provided unexpected evidence for theidea that the universe was in a state of very high temperature and density sometime between 10,000,000,000 and 20,000,000,000 years ago.

Evolution of stars and formation of chemical elements

Just as the development of cosmology relied heavily on ideas from physics, especially Einstein's general theory of relativity, so did theories of stellar structure and evolution depend on discoveries in atomic physics. These theories also offered a fundamental basis for chemistry by showing how the elements could have been synthesized in stars.

The idea that stars are formed by the condensation of gaseous clouds was part of the 19th-century nebular hypothesis (see above). The gravitational energy released by this condensation could be transformed into heat, but calculations by Hermann von Helmholtz and Lord Kelvin indicated that this process would provide energy to keep the Sun shining for only about 20,000,000 years. Evidence from radiometric dating, startingwith the work of the British physicist Ernest Rutherford in 1905, showed that the Earth isprobably several billion years old. Astrophysicists were perplexed: what source of energy has kept the Sun shining for such a long time?

In 1925 Cecilia Payne, a graduate student from Britain at Harvard College Observatory, analyzed the spectra of stars using statistical atomic theories that related them to temperature, density, and composition. She found that hydrogen and helium are the most abundant elements in stars, though this conclusion was not generally accepted until it was confirmed four years later by the noted American astronomer Henry Norris Russell. By this time Prout's hypothesis that all the elements are compounds of hydrogen had been revived by physicists in a somewhat more elaborate form. The deviation of atomic weights from exact integer values (expressed as multiples of hydrogen) could be explained partly by the fact that some elements are mixtures of isotopes with different atomic weights and partly by Einstein's relation between mass and energy (taking account of the binding energy of the forces that hold together the atomic nucleus). The German physicist Werner Heisenberg proposed in 1932 that, whereas the hydrogen nucleus consists of just one proton, all heavier nuclei contain protons and neutrons. Since a proton can be changed into a neutron by fusing it with anelectron, this meant that all the elements could be built up from protons and electrons—i.e., from hydrogen atoms.

In 1938 the German-born physicist Hans Bethe proposed the first satisfactory theory of stellar energy generation based on the fusion of protons to form helium and heavier elements. He showed that once elements as heavy as carbon had been formed, a cycle of nuclear reactions could produce even heavier elements. Fusion of hydrogen into heavier elements would also provide enough energy to account for the Sun's energy generation over a period of billions of years. Although Bethe's theory, as extended by Fred Hoyle, Edwin E. Salpeter, and William A. Fowler, is the best one available, there is still some doubt about its accuracy because the neutrinos supposedly produced by the fusion reactions have not been observed in the amounts predicted.

According to the theory of stellar evolution developed by the Indian-born American astrophysicist Subrahmanyan Chandrasekhar and others, a star will become unstable after it has converted most of its hydrogen to helium and may go through stages of rapid expansion and contraction. If the star is much more massive than the Sun, it will explode violently, giving rise to a supernova. The explosion will synthesize heavier elements and spread them throughout the surrounding interstellar medium, where they provide the raw material for the formation of new stars and eventually of planets and living organisms.

After a supernova explosion, the remaining core of the star may collapse further under its own gravitational attraction to form a dense star composed mainly of neutrons. This so-called neutron star, predicted theoretically in the 1930s by the astronomers Walter Baade and Fritz Zwicky, is apparently the same as the pulsar (a source of rapid, very regular pulses of radio waves), discovered in 1967 by Jocelyn Bell of the British radio astronomy group under Antony Hewish at Cambridge University.

More massive stars may undergo a further stage of evolution beyond the neutron star: they may collapse to a black hole, in which the gravitational force is so strong that even light cannot escape. The black hole as a singularity in an idealized space-time universe was predicted from the general relativity theory by the German astronomer Karl Schwarzschild in 1916. Its role in stellar evolution was later described by the American physicists J. Robert Oppenheimer and John Wheeler. During the 1980s, possible black holes were thought to have been located in X-ray sources and at the centre of certain galaxies.

Developments and trends of the 20th century

Astronomy

Evolution of stars and formation of chemical elements