Ejnar Hertzsprung discovered that the color of a star is related to its luminosity, and this led to his presentation of the first Hertzsprung-Russell diagram.

In the late nineteenth and early twentieth centuries, the science of astronomy was changing. Up until that time, astronomy had been devoted chiefly to the study of the motions of heavenly bodies and the mechanical laws and forces that govern those motions. With the advent of spectroscopy and photography in the late nineteenth century, astronomers began to study the nature of the stars and other bodies rather than merely their motions. Hertzsprung-Russell diagrams[Hertzsprung Russell diagrams]
Stars;color-luminosity relationship[color luminosity]
Color-luminosity relationship of stars[Color luminosity relationship]
[kw]Hertzsprung Notes Relationship Between Star Color and Luminosity (1905)
[kw]Star Color and Luminosity, Hertzsprung Notes Relationship Between (1905)
[kw]Color and Luminosity, Hertzsprung Notes Relationship Between Star (1905)
[kw]Luminosity, Hertzsprung Notes Relationship Between Star Color and (1905)
Hertzsprung-Russell diagrams[Hertzsprung Russell diagrams]
Stars;color-luminosity relationship[color luminosity]
Color-luminosity relationship of stars[Color luminosity relationship]
[g]Denmark;1905: Hertzsprung Notes Relationship Between Star Color and Luminosity[01150]
[c]Science and technology;1905: Hertzsprung Notes Relationship Between Star Color and Luminosity[01150]
[c]Astronomy;1905: Hertzsprung Notes Relationship Between Star Color and Luminosity[01150]
Hertzsprung, Ejnar
Russell, Henry Norris

Spectroscopy Spectroscopy
Astronomy;spectroscopy is the study of the spectra of stars. A star’s spectrum is the band of colored light and dark lines that results when the star’s light is spread out by a prism or grating. The patterns of lines can be used to classify a star and provide clues to the star’s properties, such as what it is made of, how hot it is, and whether it is rotating and at what speed. Scientists used photography to record these spectra in order to study and compare them and to work out the relationships between a star’s spectrum and its properties. Spectral photography
Photography;spectral As data were gathered for large numbers of stars, scientists could begin to search for order and patterns in the data and for relationships among various properties of stars. Ejnar Hertzsprung’s work was part of this quest to understand the nature of stars as revealed in their light.

Hertzsprung was well suited to work in the emerging study of stars as revealed through their spectra, as he had studied chemistry and specialized in photochemistry. He began the study of astronomy in 1902, after several years of working as a chemist. He was drawn to astronomy by his interest in observing how black body radiation relates to the radiation of stars. (Black body radiation is the radiation emitted by a body that absorbs all wavelengths of light and radiates all wavelengths when heated. It shows a relationship between the temperature of the body and the wavelength at which the most radiation is emitted; the wavelength is related to the color of the body.) He studied stellar spectra photographed in Denmark and star classification work done at Harvard College Observatory, Harvard College Observatory and he wrote two papers, in 1905 and 1907, both titled “Zur Strahlung der Sterne” (on the radiation of stars).

Hertzsprung presented several results that would prove important to the growing science of astrophysics. Astrophysics His findings included the discovery that there are two different types of stars: giants and dwarfs. He had studied the colors, brightnesses, motions, and distances of stars to arrive at this discovery, which led to his development of a diagram plotting the intrinsic brightness against the temperature for a group of stars. This type of plot, which was developed independently in the United States by Henry Norris Russell and presented in 1913, is known as a Hertzsprung-Russell, or H-R, diagram and is a basic tool of astrophysics today. Hertzsprung first made such a plot in 1906 and did further work to investigate the relationship between the color of a star and its brightness, or luminosity.

Hertzsprung used open star clusters Open star clusters in his determination of the relationship between a star’s brightness and its color. An open star cluster is a group of stars clustered together in one place in the sky; it seems reasonable to assume that the stars in a cluster are in roughly the same spatial location. When observing a star’s brightness, one usually cannot know what the intrinsic brightness of the star is; one only knows how bright the star appears from Earth. To know the intrinsic brightness, one must apply a correction for the distance of the star; stellar distances are not easy to discover. Yet stars in a cluster can be assumed to be at about the same distance, so one does not need to worry about making the distance correction to arrive at the absolute brightness, as the distance correction will be roughly the same for all members of the cluster.

Hertzsprung studied the colors of stars in several clusters whose spectra he had photographed at the Urania Observatory in Copenhagen. He was able to measure the wavelength of peak light emission from individual stars, and then he used this wavelength as an index to the stars’ colors. (The wavelength is the distance from crest to crest of each electromagnetic wave making up the light; these wavelengths are very small. Red light has a longer wavelength than green light, which, in turn, has a longer wavelength than blue light. The wavelength is related to the color.) Once he had indexed the stars’ colors, he constructed a diagram in which he plotted the color versus the magnitude (brightness) for the stars in two star clusters, the Pleiades and the Hyades.

In 1911, these diagrams were the first of their type to be published, when Hertzsprung was a senior staff astronomer at the Astrophysical Observatory at Potsdam, East Germany. On these two plots, Hertzsprung made the important observation that stars do not appear in all possible combinations of color and brightness (that is, the points are not scattered all over the plot), but there is a relationship between how bright a star is and what color it is. Thus the brightnesses and colors of stars in the two clusters form a narrow diagonal band across the plot. The band stretches from bright blue stars Blue stars in the upper left corner to dim red stars Red stars in the lower right corner. This band was called the main sequence, Main-sequence stars[Main sequence stars] or sometimes the dwarf Dwarf stars sequence, to differentiate it from the giant Giant stars sequence of large stars, which was revealed on other H-R diagrams. The discovery of this relationship gave astronomers major data about the population of the heavens. The task of determining the relationship between color and brightness was to keep astronomers occupied for years. The H-R diagram is still an essential tool of astrophysicists today.

Another area in which Hertzsprung contributed was that of the determination of stellar distances, which has traditionally been a difficult problem in astronomy. Hertzsprung made a discovery that led to the use of a star’s spectrum to determine its absolute brightness and, from that, its distance.

At Harvard College Observatory in the late nineteenth century, researchers studied hundreds of photographic plates of spectra and classified hundreds of thousands of stars. Stars;spectra In particular, Antonia Maury Maury, Antonia developed a sophisticated system involving twenty-two categories of stars, with three subdivisions for each category designating the width of spectral lines. Hertzsprung used her classifications in his spectral work. He found that stars that had Maury’s designation c for their narrow spectral lines had high absolute brightnesses. A star’s apparent brightness as seen from the earth depends on its distance; its absolute brightness is a measure of how bright it actually is. Once the absolute brightness of a star is known, it can be compared to the apparent brightness and the star’s distance can be calculated. (The process is similar to the way in which the distance of a lightbulb would be estimated based on how bright it appears and the knowledge that it is a 100-watt bulb.) Later studies elaborated on this method, and it became the method of spectroscopic parallax, which enables one to determine a star’s distance from its spectrum. Parallax method of measuring stellar distances

Hertzsprung continued his work on the color-luminosity relationship by comparing the H-R diagrams for the Pleiades, Hyades, and Praesepe star clusters, and he noticed differences in the types of stars in the three clusters. Astronomers today interpret these differences as indications that the stars in the Pleiades are younger than those in the other two clusters and are able to use H-R diagrams as a tool for determining the ages of clusters. It is now known that the brighter and bluer a star is, the shorter its lifetime; therefore, a cluster with bright blue stars still left in it must be younger than the maximum age such stars would reach. In general, by seeing the brightness and color of the stars highest on the main sequence (closest to the bright blue end) for a given cluster, one can estimate an upper limit to the age of the cluster. In addition, a method was later developed in which the brightness-color diagram for a cluster is compared with that of another cluster at a known distance; this comparison can yield an estimate of the distance to the first cluster. In working on the Pleiades cluster, Hertzsprung did related work on the stars’ spectra that involved making many measurements of wavelength and led to estimates of the total mass of the cluster. H-R diagrams thus provide a rich source of information about star clusters.

Hertzsprung’s plot of stars on the main sequence, together with his demonstration that a giant sequence Giant stars exists, provided support to Russell as well as information that Russell used in formulating his theory of stellar evolution Stellar evolution
Stars;evolution based on the H-R diagram. Russell’s evolutionary picture turned out to be mistaken and to require drastic overhauls in the light of later knowledge of nuclear processes. Yet it was an important first step in the use of H-R diagrams to plot the distribution of stars and to determine the significance of this distribution for stellar evolution. Hertzsprung also showed that there is a gap, today called the Hertzsprung gap, Hertzsprung gap separating main-sequence stars from giant-sequence stars on an H-R diagram.

The discovery of a connection between spectral type and luminosity was followed up by Walter Sydney Adams Adams, Walter Sydney and Arnold Kohlschütter Kohlschütter, Arnold at Mount Wilson Observatory in California. They found differences in the ratios of intensities of certain lines in the spectra of giants and dwarfs and used these differences for stars of known difference to calibrate a plot of ratio versus magnitude (brightness). Therefore, if one measured the ratio for a star, one could consult the plot and determine the star’s absolute brightness and then use the apparent brightness to arrive at an estimate of the star’s distance. This distance technique proved to be a powerful one.

Hertzsprung’s work with stars’ spectra and their colors yielded results that were vital for the further studies conducted in astrophysics in the twentieth century. The recognition of a color-luminosity relationship was a first step toward understanding the reasons behind the relationship, reasons that have had profound implications for ideas on stellar formation and evolution. Hertzsprung-Russell diagrams[Hertzsprung Russell diagrams]
Stars;color-luminosity relationship[color luminosity]
Color-luminosity relationship of stars[Color luminosity relationship]

• Abell, George O. Realm of the Universe. 5th ed. New York: Saunders College Publishing, 1994. Introductory college textbook. Contains sections on properties of stars and development of the H-R diagram and on the uses of the diagram in the study of stellar evolution, in particular the application of the diagram to the determination of the ages of star clusters. Includes glossary and bibliography as well as numerous diagrams, drawings, and color plates.
• 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.
• Degani, Meir H. Astronomy Made Simple. Rev. ed. Garden City, N.Y.: Doubleday, 1976. Chapter 7 contains useful information on H-R diagrams and on the stellar properties, such as brightness, that are used in constructing the diagrams. Contains drawings and graphs, glossary, and exercises for self-motivated learners.
• Moore, Patrick. Patrick Moore’s History of Astronomy. 6th rev. ed. London: Macdonald, 1983. The chapter titled “Exploring the Spectrum” gives background on some of Hertzsprung’s spectral work and introduces the reader to the complexities of analyzing a star’s radiation. The chapter titled “The Life of a Star” introduces the H-R diagram and its applications to stellar evolution studies. Written for the average reader. Includes a list of landmarks in the history of astronomy.
• Pannekoek, A. A History of Astronomy. 1961. Reprint. Mineola, N.Y.: Dover, 1989. The chapter titled “Common Stars” discusses the development and uses of spectroscopy and the construction of H-R diagrams by Hertzsprung and Russell. Also discusses the uses of H-R diagrams in studies of stellar evolution.
• Rigutti, Mario. A Hundred Billion Stars. Translated by Mirella Giacconi. Cambridge, Mass.: MIT Press, 1984. Part 2 contains useful information on stellar classification and on the H-R diagram. Part 3, on stellar evolution, discusses the application of the H-R diagram to the problem of stellar evolution in general and to the ages of star clusters in particular. Includes diagrams and some black-and-white photographs.
• Struve, Otto, and Velta Zebergs. Astronomy of the Twentieth Century. New York: Macmillan, 1962. Cowritten by an astronomer who has direct knowledge of some of the events described, this book offers a good overview of the development of the H-R diagram and the understanding of the relationships between a star’s spectrum and its other properties. Includes drawings and other illustrations, a time line of twentieth century astronomy, a glossary, and a bibliography.
• Vaucouleurs, Gerard de. Discovery of the Universe. London: Faber & Faber, 1957. Sections 6 through 9 contain an overview of astronomy from the late nineteenth century to the mid-twentieth century. Gives more information on applications of the H-R diagram and follow-ups to Hertzsprung’s work, including spectroscopic parallax. Contains diagrams, drawings, and an annotated bibliography.
• Zeilik, Michael, and Stephen A. Gregory. Introductory Astronomy and Astrophysics. 4th ed. Monterey, Calif.: Brooks/Cole, 1997. This introductory text provides a useful overview of general astronomy, including basic spectral issues and the use of H-R diagrams.

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Russell Announces His Theory of Stellar Evolution