Hale Discovers Strong Magnetic Fields in Sunspots

George Ellery Hale discovered that magnetic fields are associated with sunspots, giving astronomers valuable information about the formation of sunspots.

Summary of Event

In the seventeenth century, Galileo first observed dark spots on the surface of the Sun with a small telescope. Since that time, astronomers have invented new ways to observe the Sun and have used these new methods to learn more about what causes the spots and other features of the Sun. In the early twentieth century, George Ellery Hale developed several solar-observing instruments and used them to determine that the dark spots on the Sun exist in an intense magnetic field. This discovery fueled further research and theorizing about the nature and causes of the spots and about the magnetic fields of the Sun in general. Sunspots
Solar astronomy
[kw]Hale Discovers Strong Magnetic Fields in Sunspots (June 26, 1908)
[kw]Magnetic Fields in Sunspots, Hale Discovers Strong (June 26, 1908)
[kw]Sunspots, Hale Discovers Strong Magnetic Fields in (June 26, 1908)
Solar astronomy
[g]United States;June 26, 1908: Hale Discovers Strong Magnetic Fields in Sunspots[02160]
[c]Science and technology;June 26, 1908: Hale Discovers Strong Magnetic Fields in Sunspots[02160]
[c]Astronomy;June 26, 1908: Hale Discovers Strong Magnetic Fields in Sunspots[02160]
Hale, George Ellery
Zeeman, Pieter
Rowland, Henry Augustus

In 1908, Hale observed the solar disk with a special instrument he had developed called the spectroheliograph. Spectroheliograph This instrument worked essentially by filtering the light of the Sun so that the Sun’s various surface features, such as sunspots, could be viewed in the light of one particular wavelength. This was useful for studying the processes going on in the Sun, because each wavelength represents a particular atomic process in a particular type of atom.

When Hale observed the Sun in a wavelength coming from hydrogen atoms, he noted the very interesting fact that there appeared to be huge swirls of hydrogen gas resembling a terrestrial storm or tornado. These “vortices” seemed to be associated with the formation of sunspots. Hale considered what the consequences of this rotating motion might be. Research by the English physicist Joseph John Thomson had shown that hot bodies emit electrons, and, as the Sun is very hot, Hale thought that perhaps the Sun was emitting electrons, which, if caught up in this whirling motion, might create a magnetic field in the sunspots. (Henry Augustus Rowland had proven earlier that a moving electric charge acts magnetically in the same way as an electric current. Hans Christian rsted rsted, Hans Christian had discovered earlier that electric currents affect magnets, and electromagnetic theory explains that electric currents produce a magnetic field.)

Hale had a way to check this hypothesis. A star’s spectrum (the pattern of bright light and dark lines that results when the star’s light is passed through a prism or reflected from a grating) can reveal much about the temperature of the star’s constituent gases, the velocity of its rotation, and other information. In 1896, Pieter Zeeman discovered that if a source of light is placed in a magnetic field, the lines in the spectrum of the light source will be split into two or more components. Zeeman discovered this phenomenon (known as the Zeeman effect) Zeeman effect in the laboratory, but it proved valuable in deducing even more information from a star’s light. When Hale learned of the Zeeman effect, he conducted laboratory work of his own to observe what the effect looked like in certain test cases.

He carried out observations of light from an iron arc (or spark) from other sources and recorded much diversity in the splitting of the spectral lines from the different materials. Some lines split into two parts, some into three, and one even into twenty-one components. Once he had this information, he was ready to compare any observed splitting behavior in solar spectral lines to the laboratory results; if there was a magnetic field associated with sunspots, he could expect a close match between the solar line splitting and the laboratory line splitting. Each dark line in the spectrum represents a particular element undergoing a particular process; therefore, Hale could recognize and identify solar elements by the distinctive wavelengths of light at which their characteristic dark lines appeared in the spectrum.

Because the Sun is much closer and thus much brighter to the earth than other stars, its spectrum can be photographed and examined in greater detail than can be done for other stars. The Sun emits light at many wavelengths, each associated with a different color of light. White light is seen from the Sun because the colors of these many wavelengths are all blended and viewed together; to spread the light back into its separate colored components, one must use a prism or finely spaced grating. With enough care and effort, it is possible to spread the light and observe detail in the dark lines.

Hale expended much care and effort in building a 60-foot (18-meter) tower telescope on Mount Wilson. Solar telescopes This tower had mirrors on the top to catch the Sun’s light and reflect it through a telescope lens down to a spectrograph about 30 feet (9 meters) underground. At the spectrograph, the light was spread out into a spectrum. Because the mirrors that collected the sunlight were high above the ground and were fairly thick, they were not subjected to as much distortion from the Sun’s heat as previous solar telescopes had been, and the height of the mirrors also helped to avoid some of the warm air currents near the ground. The underground room where the final image was obtained had a nearly constant temperature, thus avoiding possible air currents caused by changes and differences in air temperature. Moving air and warped mirrors distort and muddy the images obtained from telescopes, so the net result of all these precautions was to produce an image that was unusually steady and sharp compared with previous images from solar telescopes.

Hale used this tower telescope to observe the Sun’s spectrum and its surface and, in particular, to search for the Zeeman effect of splitting of lines in the Sun’s spectrum. A doubling of certain lines in the spectrum had been observed previously, but this result had been misinterpreted at the time and no one, until Hale, had been able to observe the lines with sufficient precision to arrive at other ideas about why they appeared split.

Hale, an energetic and active man, spent much of the summer of 1908 running up and down the ladder of the telescope tower in the course of his work. He was fortunate in having a period of clear skies during the second half of June, 1908, which enabled him to spend much of his time observing. On June 26, 1908, Hale observed a doubling of spectral lines that he thought was caused by the Zeeman effect. He was greatly excited and immediately compared his observations of the Sun with his laboratory observations of the effect. He found a correlation between the two sets of observations. For the first time, an extraterrestrial magnetic field had been detected and related to laboratory observations. Hale wanted to be absolutely certain of his discovery, so he continued to make observations for two more weeks. By July 6, he was confident that what he had found was indeed the Zeeman effect, indicative of magnetic fields in sunspots. Zeeman considered Hale’s results and agreed that Hale’s hypothesis was the best explanation for the observed phenomena. Astronomers in general were impressed by Hale’s results and excited about the possibilities they raised for further study of the Sun’s magnetic properties.


One important effect that Hale’s discovery had on astronomers was that it enabled them to extend their application of the relatively new field of electromagnetic theory to the cosmos at large. Astronomers work by understanding processes that can be observed on earth and applying them in the heavens. With Hale’s discovery, astronomers realized that they could apply their knowledge of terrestrial magnetic and electric phenomena to the distant stars; this gave them a new tool to use.

Zeeman had found—in addition to the fact that spectral lines split in the presence of a magnetic field—that the distance between the components of the split line is directly proportional to the strength of the magnetic field causing the split. Hale measured the separations in lines split in the laboratory by a magnetic field of known strength, measured the separations in split lines in the spectrum of a sunspot, and derived the strength of the sunspot’s magnetic field. He found impressively large magnetic fields. Also, Hale was able to study how the strength of the field varied at different places in a sunspot; he discovered that the magnetic field is strongest at the center and weakest toward the edges. This finding has implications for what the structure of a sunspot might be like.

Hale then turned his attention to the question of whether the Sun has an overall magnetic field in addition to the fields associated with sunspots. Although Hale worked at this question periodically for the rest of his life, he was never able to answer it. Later astronomers not only determined that the Sun has a magnetic field overall but also learned how this field changes over the Sun’s twenty-two-year sunspot cycle, how the field is affected by the Sun’s rotation, and the role that the field has in sunspot formation and other types of solar activity. Magnetic fields have poles (like the north and south poles of magnets), and astronomers have worked on discovering the polarity of the Sun’s magnetic field. Understanding the Sun’s magnetic properties has been crucial in understanding the Sun’s activity.

Another question Hale considered was whether the magnetic fields associated with sunspots could be strong enough to cause magnetic storms on Earth. Hale was not in a position to answer this question, given that he would have needed records on both solar and terrestrial magnetic events over a period of time. The question, however, indicates why the study of the Sun is important to astronomers. Sunspots are associated often with energetic events on the Sun’s surface, which can send charged particles and energetic radiation out into space to interact with the earth’s atmosphere. This interaction can cause benign phenomena such as the aurora borealis, or northern lights, and it can also interfere with terrestrial communications systems. In the future, the radiation from solar flares could prove hazardous to inhabitants of long-term space colonies. Astronomers are interested in studying the magnetic processes that drive sunspot and flare formation because of the effects of these phenomena on humans. Sunspots
Solar astronomy

Further Reading

  • Bhatnagar, Arvind, and William Livingston. Fundamentals of Solar Astronomy. Hackensack, N.J.: World Scientific Publishing, 2005. Presents a history of solar astronomy and then discusses basic methods and techniques used in the field. Provides information that amateur astronomers can use to build simple solar telescopes. Includes glossary, bibliography, and index.
  • Kaufmann, William J. Discovering the Universe. New York: W. H. Freeman, 1987. Intended as a textbook for an introductory descriptive course on astronomy, this volume discusses Hale’s discovery and considers other solar features as well as sunspots and the role that magnetic fields play in these features. Includes photographs and drawings as well as various study aids, such as chapter summaries, review questions, and a glossary.
  • Mitton, Simon, ed. The Cambridge Encyclopedia of Astronomy. New York: Crown, 1977. Includes a section on sunspots that discusses Hale’s participation in discovering the role that magnetic fields play in sunspots and other solar disturbances. Discusses current ideas on solar magnetism. Includes many photographs and drawings, as well as a brief physics primer at the end of the book that gives a concise overview of the magnetic and hydrodynamic facts used to explain solar activity.
  • Noyes, Robert W. The Sun, Our Star. Cambridge, Mass.: Harvard University Press, 1982. Describes Hale’s discovery of the magnetic field in sunspots as well as his work with the spectroheliograph and discusses the Sun and its magnetic properties. Intended for both scientists and nonscientists. Includes photographs, drawings, and graphs.
  • Struve, Otto, and Velta Zebergs. Astronomy of the Twentieth Century. New York: Macmillan, 1962. Coauthor Struve witnessed some of the astronomical events this book covers. Contains a chapter on the Sun that discusses Hale’s work and its implications for theories of solar magnetic fields and their effects. Discusses the growth of the solar observing facilities at Mount Wilson.
  • Wentzel, Donat G. The Restless Sun. Washington, D.C.: Smithsonian Institution Press, 1989. Discusses the Zeeman effect and gives a thorough presentation of current views on the Sun’s magnetic field. Includes diagrams, drawings, and black-and-white photographs. Presents a good discussion of solar physics, written for the nonscientist.
  • Wright, Helen. Explorer of the Universe: A Biography of George Ellery Hale. 1966. Reprint. Melville, N.Y.: American Institute of Physics Press, 1994. Drawing heavily on letters and diaries written by the participants in the events described, this engaging book is an excellent source for the story of Hale’s discovery of the magnetic field associated with sunspots. Includes drawings and photographs of sunspots and solar spectra as well as bibliographies.
  • Wright, Helen, Joan N. Warnow, and Charles Weiner, eds. The Legacy of George Ellery Hale: Evolution of Astronomy and Scientific Institutions, in Pictures and Documents. Cambridge, Mass.: MIT Press, 1972. Relying heavily on photographs and original documents, this collection of material on Hale’s astronomical work includes the text of an address Hale gave in 1909, “Solar Vortices and Magnetic Fields.” Also valuable for the letters, photographs, and newspaper clippings about Hale and his work.

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