Hertzsprung Describes Giant and Dwarf Stellar Divisions

Ejnar Hertzsprung devised a classification system in which stellar objects are categorized by size versus stellar spectral type.

Summary of Event

By the beginning of the twentieth century, astronomers had turned their telescopes on the stars and had begun the exceptionally arduous task of cataloging them. This work was an extension of the work of Hipparchus, a Greek astronomer who cataloged the stars (one thousand entries) in 130 b.c.e. His catalog contained the brightest of the few thousand stars he could make out on any clear night with his unaided eye. When astronomers of the early 1900’s began their task, they used powerful telescopes and recorded their observations on sensitive photographic films. Through the combination of these methods, suddenly millions of stars became visible, making the task of cataloging one of the most challenging and demanding undertakings in science. Stars;classes
Giant stars
Dwarf stars
[kw]Hertzsprung Describes Giant and Dwarf Stellar Divisions (1907)
[kw]Giant and Dwarf Stellar Divisions, Hertzsprung Describes (1907)
[kw]Dwarf Stellar Divisions, Hertzsprung Describes Giant and (1907)
[kw]Stellar Divisions, Hertzsprung Describes Giant and Dwarf (1907)
Giant stars
Dwarf stars
[g]Denmark;1907: Hertzsprung Describes Giant and Dwarf Stellar Divisions[01800]
[c]Science and technology;1907: Hertzsprung Describes Giant and Dwarf Stellar Divisions[01800]
[c]Astronomy;1907: Hertzsprung Describes Giant and Dwarf Stellar Divisions[01800]
Hertzsprung, Ejnar
Schwarzschild, Karl
Russell, Henry Norris

The first detailed stellar catalogs of the nineteenth century included star identifications (names or numbers), positions, and motions. By 1875, new scientific techniques had been developed and added even more information about stars. The technique of spectral photography Spectral photography
Photography;spectral and analysis enabled astronomers to tell much about the composition of stars through the analysis of spectral shifts; they could even determine the motion and velocity of stars in the line of sight.

Astronomers Angelo Secchi Secchi, Angelo and William Huggins Huggins, William determined that there were only a handful of basic stellar spectral types in a broad series that appeared to flow from one type to another, so Secchi proposed a spectral classification Stars;spectra
Stellar spectra scheme that included four main types. In 1901, Antonia Maury Maury, Antonia and Annie Jump Cannon Cannon, Annie Jump of the Harvard College Observatory Harvard College Observatory proposed that the classification system of stars include seven spectral types, indexed and classified by letters. Maury proposed a classification system different from Cannon’s. In Cannon’s scheme, which is still used today, stars are classified by letters: O, B, A, F, G, K, and M. The Harvard College catalog became known as the Henry Draper Catalog, Henry Draper Catalog in honor of Henry Draper, a pioneer in stellar spectrophotometry. The Draper Catalog contained information on more than 225,000 stars.

Unable to make a good living in astronomy, Severin Hertzsprung encouraged his son, Ejnar, to pursue a degree in chemical engineering. Ejnar Hertzsprung graduated from the Polytechnical Institute in Copenhagen in 1898 without any formal education in astronomy. His father died four years later, and, upon his death, all of his astronomy books were sold. Ejnar Hertzsprung would later state with some irony, “Nobody imagined that I should become an astronomer.” He became a chemist in St. Petersburg, then in 1901 seized the opportunity to follow an interest he held in photochemistry. He traveled to Friedrich Wilhelm Ostwald’s laboratories in Leipzig, Germany, and during his stay there, he decided he wanted to pursue a career in astronomy. Because of his engineering and photochemistry background, Hertzsprung brought to his astronomy career an interesting variety of useful talents and knowledge, but he had no idea what the duties of an astronomer were. Hertzsprung began his apprenticeship at the Copenhagen University’s observatory under the tutelage of H. E. Lau, Lau, H. E. who taught him the fundamentals of observational astronomy; he also did some work at the Urania Observatory in Frederiksberg, Denmark.

With his training in photographic techniques and use of specific emulsions, which was likely better than that of most other astronomers, Hertzsprung brought a unique variety of talents to bear on photographing stellar fields at the observatories where he worked. He began to collect and study the distribution of stellar images from these plates. At this time, Hertzsprung was considered an amateur astronomer, an apprentice at best, but it was during this period that he would make the pivotal contribution of his life. Based on a collection of observations, Hertzsprung published two papers in Zeitschrift für wissenschaftliche Photographie, a photophysics and photochemical journal, in 1905 and 1907.

The papers, both of which were titled “Zur Strahlung der Sterne” (on the radiation of stars), used Maury’s star catalog and her spectral classification techniques. Hertzsprung demonstrated that stars in a certain class (which Maury called the c class) were more luminous than the remainder. Later, this idea would be developed to form the concept of “spectroscopic parallax,” Parallax method of measuring stellar distances which has become one of the primary means by which the distances to stars and galaxies are determined.

Most significant, however, these papers also contained Hertzsprung’s profound discovery of giant and dwarf stars among the dense masses of star points on his photographic plates. Hertzsprung made the fundamental discovery that all stars could be categorized into two series of stars, one series that has become known as the main sequence Main-sequence stars[Main sequence stars] and the other containing the high-luminosity stars, or giant stars. Later, a diagram would be made of Hertzsprung’s discovery and would bear his name. This diagram has become one of the fundamental tools of modern astronomy.

Hertzsprung began a correspondence with Germany’s most famous astronomer, Karl Schwarzschild, and sent him copies of his papers. Schwarzschild immediately recognized the importance of Hertzsprung’s findings and invited Hertzsprung to visit him at the University of Göttingen in 1909. Later that year, Hertzsprung was appointed lecturer at Göttingen. He resigned after only a few months, however, to join Schwarzschild, who had become director of the Astrophysical Observatory at Potsdam. Hertzsprung was hired as senior staff astronomer at the observatory. Largely because of Schwarzschild’s mentorship, Hertzsprung’s professional career in astronomy was established.

In 1913, American astronomer Henry Norris Russell presented a paper to the Royal Astronomical Society in London in which he presented his ideas—based on his own independent findings—on giant and dwarf stars. The work done earlier by Hertzsprung was unknown to him. Later, the detailed findings of both astronomers were graphically depicted in what has become known as the Hertzsprung-Russell diagram. Hertzsprung-Russell diagrams[Hertzsprung Russell diagrams] The modern Hertzsprung-Russell diagram (also known as the H-R diagram) plots star types on a graph showing spectral classes (such as those defined by Cannon), absolute magnitude, luminosity, and temperature.

Hertzsprung’s 1907 work on the relationship between the radiation of a star and its color based on stars located within a cluster is often overlooked. This information would later establish the age of the stars of the Pleiades and stars in the neighborhood of the Sun. With his method, Hertzsprung later published the first color-magnitude diagram ever released. Using the information on spectroscopic parallax, Hertzsprung determined the distance to the Small Magellanic Cloud, Small Magellanic Cloud an accomplishment for which he was awarded the Royal Astronomical Society’s Gold Medal in 1929.


Hertzsprung established an important link between observational and theoretical astronomy. Through his careful analysis of the data he was collecting (largely in a cataloging effort), Hertzsprung forged a link between the color of a given star and many other key properties, including its intrinsic (natural) brightness, its temperature, and even its size.

As Hertzsprung discovered through the cataloging efforts of the staff at the Harvard College Observatory, in the relative handful of star spectral classes, there were other key features of stars that could be linked and plotted on a relatively simple graph. At first glance it seemed probable that the kinds of stars in the universe were as infinitely diverse as were their profuse numbers, yet, by establishing links between stars’ characteristics, Hertzsprung organized and constrained the very boundaries of the science of stellar astronomy.

Hertzsprung discovered that a significant population of stars existed outside the “main sequence” of stars (of which the Sun is one). He called these giant and dwarf stars because they were significantly different in characteristics from the main-sequence stars. With the establishment of the main sequence, the unique nature of the giant and dwarf stars became strikingly clear. Such a sequencing method would point later to important evolutionary aspects of stellar development and also to the amount of time each star could exist. Such an incredible diversity of science has emanated from the use of the H-R diagram that there are few areas of modern astronomy that do not rely on the diagram as a fundamental cornerstone.

One of the vital aspects of theoretical astronomy that emerged as a corollary to the H-R diagram is the idea that, given that the kinds of stars depicted on and classified by the diagram are apparently universal, a ubiquitous process of stellar Stellar evolution
Stars;evolution development and evolution must be occurring everywhere. This has, in turn, led to speculations of how the Sun will behave and what the duration of the Sun’s lifetime will be. Looking at other stars on the H-R diagram, one can follow the Sun’s evolutionary position from birth to death as a red giant star.

Another important aspect of the diagram is that it allows astronomers and other scientists to understand much about the environment of any star and near space, so that the conditions on any theoretical planet in orbit around any star can be theorized. From this speculation and knowledge of the population distribution of each type of star on the diagram, complex theories have been developed, such as how many of each class of star could support conditions for the development of life in the universe. Stars;classes
Giant stars
Dwarf stars

Further Reading

  • Asimov, Isaac. Extraterrestrial Civilizations. New York: Crown, 1979. Makes use of stellar types and classes to detail how life could evolve and survive around different types of stars (including giant and dwarf). A good example of how deeply the H-R diagram had woven its way into all aspects of astronomy.
  • Bok, Bart I., and Priscilla F. Bok. The Milky Way. 5th ed. Cambridge, Mass.: Harvard University Press, 1981. The definitive book on the Milky Way galaxy, written by two of the world’s experts on the subject. Well written and easy to understand. Includes a detailed deliberation of the H-R diagram and its relevance to the discussion of the nearer stars in the vicinity of the Sun. Illustrated.
  • Chaisson, Eric, and Steve McMillan. Astronomy Today. 4th ed. Upper Saddle River, N.J.: Prentice Hall, 2001. Chapter 17, titled “Measuring the Stars,” includes discussion of Hertzsprung’s work and explains how H-R diagrams are constructed and used to identify stellar properties.
  • Cornell, James, and Alan P. Lightman, eds. Revealing the Universe. Cambridge, Mass.: MIT Press, 1982. Discusses in some detail specific areas of observational and theoretical astronomy. The H-R diagram is included in some of the discussions, specifically on how the shape of stellar interiors predicts the shape of the H-R diagram. Somewhat technical; aimed primarily at readers with a scientific background. Illustrated.
  • Hartman, William K. Astronomy: The Cosmic Journey. Belmont, Calif: Wadsworth, 1978. Addresses all the basic features of astronomy. Offers detailed discussion of not only the H-R diagram but also giant and dwarf stars. For general readers, especially those with a minimal grounding in science. Heavily illustrated.
  • Harwit, Martin. Cosmic Discovery. New York: Basic Books, 1981. Presents a fascinating expedition through the history and foundation of astronomy. Discusses some of the history of Hertzsprung’s development of the H-R diagram and his use of the classification data from the Harvard College Observatory. For a wide audience. Illustrated.
  • Kaler, James B. “Journeys on the H-R Diagram.” Sky and Telescope (May, 1988): 483-485. Offers a concise view of the H-R diagram and its many uses. Presents a complete discussion of the giant and supergiant classes and why they lie outside the main sequence. Illustrated.
  • Zeilik, Michael, and Stephen A. Gregory. Introductory Astronomy and Astrophysics. 4th ed. Monterey, Calif.: Brooks/Cole, 1997. Introductory text provides a useful overview of general astronomy, including basic spectral issues and the use of H-R diagrams.

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