Henry Norris Russell used the color-luminosity relationship of stars to work out a theory of how stars change over time.

Sir William Herschel (1738-1822) described the starry sky as a garden wherein one sees stars in varying stages of their lives, as one sees plants and trees in varying stages of early growth, maturity, and death in a garden or forest. The assumptions that stars are of different ages and that stars change as they age were important prerequisites of the formation of theories of stellar evolution. The advent of increasingly sophisticated techniques for classifying stars in the late nineteenth and early twentieth centuries brought a wealth of data on spectral types from which a theory of stellar evolution could be built. Stars;evolution
Color-luminosity relationship of stars[Color luminosity relationship]
Astronomy;stellar evolution
Stellar evolution
[kw]Russell Announces His Theory of Stellar Evolution (Dec., 1913)
[kw]Stellar Evolution, Russell Announces His Theory of (Dec., 1913)
Stars;evolution
Color-luminosity relationship of stars[Color luminosity relationship]
Astronomy;stellar evolution
Stellar evolution
[g]United States;Dec., 1913: Russell Announces His Theory of Stellar Evolution[03460]
[c]Science and technology;Dec., 1913: Russell Announces His Theory of Stellar Evolution[03460]
[c]Astronomy;Dec., 1913: Russell Announces His Theory of Stellar Evolution[03460]
Russell, Henry Norris
Hertzsprung, Ejnar
Lockyer, Sir Joseph Norman

Sir Joseph Norman Lockyer, working in the late 1800’s, used the simple classification systems of Pietro Angelo Secchi and Hermann Karl Vogel—which placed stars in one of four categories—to develop a scheme for stellar evolution. This scheme was based on current theory regarding the energy source for stars and the physical forces shaping their life histories. At the time, physicists believed that a star’s radiation consisted of heat and light, which were released as the star contracted under the force of gravity. It was believed that a star was formed when enough interstellar matter accumulated in one place to begin to exert gravitational attraction on itself and to form a sphere. The star would then begin to contract inward under the force of gravity and to heat up and to shine. Eventually, the collapse was halted when a critical density was reached, and the star would begin to cool off and die. Lockyer used the spectral classes of the time to identify a sequence of stages through which it was believed that all stars pass.

Henry Norris Russell had at his disposal a more sophisticated system of classification that involved seven classes of stars. Also, many more stars had been classified while Russell was conducting his research. This was largely the result of a program carried out at Harvard College Observatory Harvard College Observatory at the beginning of the twentieth century, under the direction of Edward Charles Pickering, in which stars were classified based on their spectra. Stellar spectra A star’s spectrum, or the bands of color and darkness produced when its light is spread out by a prism or grating, contains distinctive dark lines that can be used to classify the star. The researchers at Harvard College Observatory looked at thousands of such spectra and classified their associated stars, producing massive catalogs of information on stellar types. Russell was able to use this information in developing his scheme of stellar evolution.

Given that stars cannot be directly examined in the laboratory, astronomers are forced to deduce the characteristics of stars from things that can be observed, such as their spectral types and their brightness. Russell was faced with the question of how to use what is visible about a star to identify the star’s characteristics and thus its stage of evolution. At the time, a star’s spectral type was almost universally believed to be linked to its surface temperature and its color, and it was further believed that the different types were the results of differing temperatures; however, no consensus had been reached on the cause of differences in brightness. Russell showed that differences in brightness were a result of variations in density. Spectral type was thus related to surface temperature and color, and brightness was related to density. Russell plotted data on spectral types versus data on the absolute brightness of stars (that is, a star’s true brightness, after its brightness as seen from Earth is corrected for its distance). In 1913, he produced a plot of spectral type versus brightness. (Ejnar Hertzsprung had made a similar diagram in 1911; this type of plot is known today as a Hertzsprung-Russell, Hertzsprung-Russell diagrams[Hertzsprung Russell diagrams] or H-R, diagram.) Russell then used this plot to view the relationship between brightness (and density) and spectral type (and color and temperature).

Russell presented his diagram to the Royal Astronomical Society in London on June 13, 1913, and to the American Astronomical Society in Atlanta, Georgia, on December 30, 1913. He also offered his interpretation of the diagram, in terms of stellar evolution. Most stars fell either on a diagonal band stretching across the plot (the main sequence) Main-sequence stars[Main sequence stars] or on a horizontal strip across the top of the plot (the giant sequence). Giant stars The names given to these two sequences were based on work by Hertzsprung and others that determined that the stars on the giant sequence were much larger than the stars on the main sequence. On the main sequence, stars vary in brightness and color, with stars ranging from bright blue to dim red. On the giant sequence, stars have a fairly constant brightness but vary in type (color). Russell explained these two groups or sequences of stars in terms of the age of the stars in each sequence.

Russell theorized that a star’s evolution is driven by gravity alone and that a star begins its life as cool, red, dim, and diffuse and then grows increasingly dense, bright, and hot (with an associated color change) as it contracts. Once it has contracted as far as it can, so that no more gravitational energy is available to it, it begins to cool off and become less bright and more red. Russell hypothesized that the large red stars Red stars at one end of the giant sequence are the youngest of stars and that they represent the earliest stages in a star’s life, when it is very diffuse and just beginning its gravitational collapse. As a star collapses, it becomes more dense and begins to change color and spectral type as it moves across the giant sequence; it eventually brightens and leaves the giant sequence for the main sequence. At its hottest point, which Russell believed to be the midpoint of its life, the star is at the top of the main sequence among the brightest and bluest stars. As it then begins to cool, while continuing to become denser, it slides down the main sequence from being a hot blue star Blue stars to being a yellow star Yellow stars (like the Sun) and finally becomes a dim red star, very dense and near the end of its life.

Thus Russell believed that this track, along the giant sequence and down the main sequence, was a path of increasing density and increasing age. According to his theory, there were two sorts of red stars: young, diffuse, large ones of increasing temperature and old, dense, small ones of decreasing temperature. A red star could therefore be at either end of its lifetime. Hertzsprung had demonstrated earlier that the spectra of the two types of red stars were different, thus enabling astronomers to tell whether a red star was old or young.

Russell presented a concise and straightforward scheme of stellar evolution that neatly fit the known data in terms of the accepted explanation for why stars shine and how they form, exist, and die. He was able to use his diagram to illustrate succinctly the life stages of a star as he hypothesized them. His work on the temperature and density of stars, as related to spectral type and brightness, was confirmed by later work. Although his evolutionary scheme later required major revision, it was still an important step in the understanding of the “garden” of varying stars we see.

Many astronomers began to use Hertzsprung-Russell diagrams immediately after Russell first presented one in 1913, and the H-R diagram remains an important tool in astrophysics. Walter Baade Baade, Walter was able to compare H-R diagrams for groups of stars to show that there are, in fact, two populations of stars (one much older than the other) and that each type has its own distinct H-R diagram. This work had important cosmological implications. Robert Julius Trumpler, Bengt Georg Daniel Strömgren, and Gerard Peter Kuiper, among others, studied the H-R diagrams of clusters of stars in the Milky Way to work out theories of stellar formation and evolution.

The discovery that nuclear fusion, Nuclear fusion rather than gravitational collapse, powers stars for most of their lifetimes brought about drastic revisions in Russell’s scheme. Russell’s work was important, however, in that it was an early attempt to deduce, from observable quantities, the life cycles of stars. His use of the H-R diagram was a key step in the development of the science of astrophysics. Astronomers’ knowledge of the causes of a star’s observable properties, as plotted on the diagram, changed as they learned of nuclear fusion and nuclear science, but the method of using the H-R diagram as a clue to a star’s properties and life cycle has remained the same. Russell pioneered a practice that continues into the twenty-first century.

In his explanation of how the H-R diagram reveals the evolution of stars, Russell gave at least a hint of what was to be discovered later about nuclear power fueling the stars. He suggested that perhaps there is a type of energy release related to radioactivity that could counteract the gravitational pull inward for a brief period and give a star a longer lifetime than it would have had otherwise. He thought this would not be an important enough effect to change the overall life cycle of the star as he described it.

Today, however, it is known that although a star starts to form because a cloud of material collapses under the influence of gravity, eventually conditions become hot enough in the center of the forming star that nuclear fusion begins to occur. The star then lives out most of its life cycle in one spot on the main sequence, its gravitational pull inward balanced by the pressure outward resulting from energy being released in nuclear fusion. Gravity becomes important again at the end of the star’s lifetime, when its fate is determined by the amount of mass it contains. (This is also the factor that determines how long the star lives and where on the main sequence it appears—that is, its brightness and color.)

Astronomers have learned a great deal about such exotic objects as white dwarfs, neutron stars, and black holes, which are the end products of evolution for various masses of stars. Without the H-R diagram, and the foundation of knowledge it offers for the understanding of the interrelationships among a star’s density, brightness, temperature, and spectral type, astronomers could not have arrived at their current understanding. Stars;evolution
Color-luminosity relationship of stars[Color luminosity relationship]
Astronomy;stellar evolution
Stellar evolution

• Abell, George O. Realm of the Universe. 5th ed. New York: Saunders College Publishing, 1994. Introductory college textbook includes sections on properties of stars (including a discussion of the H-R diagram) and on stellar evolution (including information on the use of H-R diagrams to test current theories). Glossary and bibliography, numerous diagrams and 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 Russell’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 stellar properties, such as temperature and density, plus an explanation of the spectral classes and the Hertzsprung-Russell diagram. Chapter 9 details current knowledge regarding stellar evolution. Written for the self-motivated learner, includes exercises, drawings, and glossary.
• Moore, Patrick. Patrick Moore’s History of Astronomy. 6th rev. ed. London: Macdonald, 1983. Chapter titled “The Life of a Star” gives the history of the theory of stellar evolution and discusses stellar spectra and different types of stars. Drawings, some color photographs, and a list of landmarks in the history of astronomy. Written for the layperson.
• Pagels, Heinz. Perfect Symmetry. New York: Simon & Schuster, 1985. Section 1, “Herschel’s Garden,” contains information about H-R diagrams and the wealth of current information on stellar evolution that has resulted from their use. Some diagrams, notably a schematic H-R diagram, and bibliography.
• Pannekoek, A. A History of Astronomy. 1961. Reprint. Mineola, N.Y.: Dover, 1989. Chapter titled “Common Stars” discusses stellar spectroscopy at the beginning of the twentieth century, Russell’s work in constructing H-R diagrams, and his theory of stellar evolution. Includes important diagrams, notably Lockyer’s evolutionary scheme and Russell’s original diagram. A classic in the history of astronomy.
• Rigutti, Mario. A Hundred Billion Stars. Translated by Mirella Giacconi. Cambridge, Mass.: MIT Press, 1984. Part 3 is devoted to the story of stellar evolution, including use of the H-R diagram, and part 2 discusses the H-R diagram. Diagrams and some black-and-white photographs. Conversational style; accessible to amateur astronomers. Contains some mathematics.
• Struve, Otto, and Velta Zebergs. Astronomy of the Twentieth Century. New York: Macmillan, 1962. Cowritten by an astronomer who participated in some of the history described, this book contains several chapters on the work of Russell and others in determining the life cycles and natures of stars and the development of the H-R diagram. Diagrams, photographs, glossary, and bibliography.
• Trefil, James S. Space, Time, Infinity: The Smithsonian Views the Universe. New York: Pantheon Books, 1985. Although the emphasis of the book is on the universe, it contains a brief chapter, “We Are Made of Star Stuff,” that discusses the H-R diagram and theory of stellar evolution. Many beautiful full-color illustrations, including color schematic H-R diagram.
• 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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