Henrietta Swan Leavitt’s report of her discovery that the pulsating period of a Cepheid variable star is directly proportional to the star’s brightness marked an important step forward in astronomy.

According to the modern cosmological view, the universe is an expanding sphere of approximately one trillion galaxies. Each galaxy consists of one hundred billion to one trillion stars. Each star, including the Sun, is an immense thermonuclear furnace composed mostly of the elements hydrogen and helium. The remaining one hundred-plus elements (for example, carbon, oxygen, iron) are synthesized by fusion reactions in stars and by massive star explosions (supernovas). The Sun is one of approximately four hundred billion stars in the Milky Way galaxy. Cepheid variable stars
Stars;period-luminosity relationship[period luminosity relationship]
Astronomy;stars
Period-luminosity relationship of stars[Period luminosity relationship]
Variable stars
[kw]Leavitt Discovers How to Measure Galactic Distances (Mar. 3, 1912)
[kw]Galactic Distances, Leavitt Discovers How to Measure (Mar. 3, 1912)
Cepheid variable stars
Stars;period-luminosity relationship[period luminosity relationship]
Astronomy;stars
Period-luminosity relationship of stars[Period luminosity relationship]
Variable stars
[g]United States;Mar. 3, 1912: Leavitt Discovers How to Measure Galactic Distances[03040]
[c]Science and technology;Mar. 3, 1912: Leavitt Discovers How to Measure Galactic Distances[03040]
[c]Astronomy;Mar. 3, 1912: Leavitt Discovers How to Measure Galactic Distances[03040]
Leavitt, Henrietta Swan
Shapley, Harlow
Hubble, Edwin Powell
Hertzsprung, Ejnar
Russell, Henry Norris

By 1900, astronomers had firmly established that Earth, other planets, asteroids, and comets revolve around the Sun and that the Sun is only one of billions of stars in the Milky Way. Nevertheless, there were many unresolved problems. Some astronomers believed that the Sun was located at the center of the Milky Way, a throwback to pre-Copernican views of Earth being the center of the universe. Other astronomers correctly theorized that the Sun is an average star not located at the galactic center, but evidence to that effect was lacking. The existence of other galaxies had not been clearly demonstrated, and no completely reliable methods existed for measuring the enormous distances between stars.

In 1902, Henrietta Swan Leavitt became a permanent observatory staff member at Harvard College Observatory. Harvard College Observatory She studied variable stars, stars that change their luminosity (brightness) in fairly predictable patterns over time. During her tenure at Harvard, Leavitt observed and photographed nearly twenty-five hundred variable stars.

Variable stars can be of three principal types: eclipsing binaries, novas, and Cepheid variables. Eclipsing binaries Eclipsing binary stars are double-star systems where two stars orbit a common center of gravity. Periodically, one star will eclipse, or pass before, its companion star relative to the line of sight. The combined light emission from the double-star system will be reduced. Many double-star systems exist, including Sirius (also known as the Dog Star) in the constellation Canis Major and Mizar-Alcor in the handle of the Big Dipper (Ursa Major). Novas Novas are unstable stars that occasionally erupt and release envelopes of matter into space, temporarily increasing in brightness during the process. Cepheid variables and their similar RR Lyrae stars are older main-sequence stars Main-sequence stars[Main sequence stars] that have exhausted their hydrogen fuel and have switched to helium fusion for energy generation. Cepheids are unstable, periodically increasing, then decreasing, their energy and light output. They become brighter, then dim, repeating the cycle every few days or weeks depending on the star. Each Cepheid variable star has a predictable, repeatable cycle of brightening and dimming. These stars are named after Delta Cephei, the first such variable star discovered in the constellation Cepheus in 1768. Polaris, the North Star, is also a Cepheid.

At the time, the principal means of determining stellar distances was a trigonometric method known as parallax. Parallax method of measuring stellar distances A star’s parallax is the line-by-sight angle subtended by the star as Earth orbits the Sun. Using a right triangle formed by the observed star, the Sun, and Earth as the triangle’s vertices, the star’s distance is calculated based on simple trigonometric equations involving the Earth-Sun distance (about 150 million kilometers, or 93.2 million miles) and the subtended angle traced by the star in our sky. If the subtended angle is large, then the star is close. If the angle is small, the star is far. Related trigonometric methods rely on the Sun’s motion relative to background stars and the motion of star clusters in the Milky Way. Trigonometric parallax methods are limited to relatively close stars. Using parallax, the very close stars Alpha Centauri and Barnard’s star are 4.27 and 5.97 light-years distant, respectively. Sirius is 8.64 light-years distant. Because a light-year Light-years[Light years] is the distance light travels in one year, approximately 9.46 trillion kilometers (5.88 trillion miles), parallax is an extremely accurate means of measuring stellar distances. During the twentieth century, scientists obtained more than ten thousand stellar distances using this method. If one considers the immensity of the universe, however, with its radius of perhaps twenty billion light-years, parallax fails at relatively short astronomical distances (for example, fifty thousand light-years).

While studying variable stars, Leavitt measured their luminosities over time. She was equipped with photographs of the Large and Small Magellanic Clouds collected from Harvard’s Peruvian observatory. Magellanic Clouds Magellanic Clouds are very small galaxies visible in the Southern Hemisphere. The Small Magellanic Cloud contained seventeen Cepheid variables having very predictable periods ranging from 1.25 days to 127 days. She carefully measured the brightening and dimming of the seventeen Cepheids during their respective periods. She collected photographs of other Cepheids in the Magellanic Clouds and made additional period-luminosity studies.

While Leavitt was studying Cepheids, Ejnar Hertzsprung of the Leiden University in the Netherlands and Henry Norris Russell of the Mount Wilson Observatory in Pasadena, California, independently discovered a relationship between a star’s luminosity and its spectral class (that is, color and temperature). Together, their experimental results produced the Hertzsprung-Russell diagram Hertzsprung-Russell diagrams[Hertzsprung Russell diagrams] (or H-R diagram) of stellar luminosities, the astronomical equivalent of chemistry’s periodic table. According to their classification scheme, most stars lie along the “main sequence,” which ranges from extremely bright blue stars Blue stars ten thousand times brighter than the Sun to very dim red stars Red stars one hundred times dimmer than the Sun. Additional star classes outside the main sequence include red giants, supergiants, and white dwarfs. Cepheid variables are placed toward the cooler, red-star end of the main sequence.

Thinking along the same lines as Hertzsprung and Russell, Leavitt carefully measured the luminosities and cyclic periods of changing luminosity for each of many Cepheid variables from the Magellanic Clouds. From her careful measurements, she graphically plotted Cepheid luminosity against Cepheid period. She discovered that a Cepheid’s apparent luminosity is directly proportional to the length of its period, or the time it takes to complete one cycle of brightening and dimming. A faint Cepheid variable has a very short cyclic period during which it fluctuates in brightness, usually ranging from one to four days. A more luminous Cepheid has a longer cyclic period, usually twenty to thirty days or more.

Harlow Shapley, an astronomer at the Mount Wilson Observatory, combined Leavitt’s Cepheid period-apparent luminosity relationship to parallax data for Cepheid distances. He measured the distances of moving star clusters containing Cepheids, then related the Cepheid distances to Cepheid period-luminosity data. From these experiments, Shapley constructed a Cepheid period-absolute luminosity curve. With this curve, one can plot a Cepheid variable having a specific measured period and obtain its absolute luminosity. Knowing the Cepheid’s apparent and absolute luminosities, one can instantly calculate its distance and, therefore, the distances of all the stars in the star cluster containing that particular Cepheid variable.

Consequently, the distances to Cepheid variables in the Milky Way and other galaxies were determined quickly. Shapley used Cepheid distances to demonstrate that the center of the Milky Way is directed toward the constellation Sagittarius and that the Sun is located approximately thirty thousand light-years from the galactic center. Edwin Powell Hubble, also of Mount Wilson Observatory, applied the technique to measure the distances to Cepheids located in distant galaxies, thereby obtaining estimates of the distances between our galaxy and others.

Leavitt’s discovery of the Cepheid variable period-luminosity relationship, reported in the March 3, 1912, issue of the Harvard College Observatory Circular, was an important achievement in twentieth century astronomy. The period-luminosity relationship established Cepheid variables as standard reference points for measuring distances between stars and galaxies.

Immediate applications of Leavitt’s work appeared in the studies of Shapley and Hubble. Shapley derived a period-absolute luminosity curve for Cepheids from Leavitt’s results and from distance measurements for star clusters containing Cepheids. Each Cepheid’s period gives away its absolute luminosity, which gives away its distance. With this approach, any star cluster containing Cepheid variables can be measured to obtain its approximate distance from Earth. The rationale is quite simple: If a distant group of stars contains a Cepheid variable, determination of the Cepheid’s distance will give astronomers an estimate of the distance to all the stars in the cluster, relatively speaking. The stars of the cluster may be very far apart from one another, but relative to Earth, they all are approximately the same distance, the distance measured for their Cepheid variable.

Shapley used the approach to measure the distances to RR Lyrae stars, RR Lyrae stars Cepheid-like pulsating stars first discovered in the constellation Lyra. RR Lyrae stars are distributed throughout the Milky Way and within globular clusters, masses of perhaps a million stars each that surround the Milky Way in a halo. Shapley noted that there are far more globular clusters in the direction of the constellation Sagittarius than there are surrounding Earth’s area of space. If the Sun were the center of the Milky Way, then globular clusters would be spaced roughly equally in all directions, but that is not the case. From this observation, Shapley concluded that the Sun is located out in one of the Milky Way’s spiral arms, in fact, the arm seen as the starry haze called the Milky Way. The center of the Milky Way lies hidden and distant, in the direction of the constellation Sagittarius. From RR Lyrae stars located within each globular cluster surrounding our galaxy, Shapley mapped the distance to each globular cluster and thereby produced an approximate distance map for the entire Milky Way. He demonstrated that the Milky Way is a flattened spiral disk with a thickened center. He also measured the approximate diameter of the Milky Way and estimated that the Sun is about fifty thousand light-years (later corrected to thirty thousand light-years) from the galactic center.

Hubble used the work of Leavitt and Shapley to measure the distances to other galaxies, most notably Messier 31, the great nebula in the constellation Andromeda, the largest galaxy near Earth. Hubble measured the distances to RR Lyrae stars located within the Andromeda galactic disk and obtained an approximate intergalactic distance of 750,000 light-years, later recalibrated to one million light-years. This is one of the closest galaxies to Earth out of possibly one trillion. Hubble applied this technique to the measurement of distances to other galaxies. This work contributed to his later studies of galactic redshift velocities, which led to his monumental astronomical discovery that the universe is expanding. Cepheid variable stars
Stars;period-luminosity relationship[period luminosity relationship]
Astronomy;stars
Period-luminosity relationship of stars[Period luminosity relationship]
Variable stars

• Bartusiak, Marcia. Thursday’s Universe. New York: Times Books, 1986. Excellent history of the major astronomical achievements of the twentieth century. Thorough in scope, but intended for a general audience. Chapter 4, “Wrapped in an Enigma,” discusses the work of Leavitt and Shapley.
• Clark, David H., and Matthew D. H. Clark. Measuring the Cosmos: How Scientists Discovered the Dimensions of the Universe. New Brunswick, N.J.: Rutgers University Press, 2004. Relates the stories of the scientists who have contributed to current knowledge about the size, mass, and age of the universe. Chapters 4 and 5 include discussion of the work of Hubble and Leavitt. Features glossary, bibliography, and index.
• Ferris, Timothy. Galaxies. 1982. Reprint. New York: Harrison House, 1987. Excellent introduction to the subject presents a clearly written, beautifully illustrated discussion of astronomy and cosmology. Easily accessible to lay readers. Includes many photographs of galaxies and star clusters.
• Hoyle, Fred. Astronomy. Garden City, N.Y.: Doubleday, 1962. Introduction to the field by one of the giants of twentieth century astronomy. Presents a detailed history of the subject and clearly describes practical methods in astronomy. Includes extensive illustrations and references.
• Kippenhahn, Rudolf. Light From the Depths of Time. Translated by Storm Dunlop. New York: Springer-Verlag, 1987. Presents an outstanding introduction to astronomy and cosmology for the average reader. Simplifies many complex astronomical concepts in a very entertaining fashion. Chapter 4, “Plumbing the Depths of the Milky Way,” discusses the contributions of Leavitt and Shapley.
• Rolfs, Claus E., and William S. Rodney. Cauldrons in the Cosmos: Nuclear Astrophysics. Chicago: University of Chicago Press, 1988. Detailed, graduate-level astrophysics textbook provides a comprehensive summary of stars and the processes that occur inside stars for readers with some background in astronomy. Chapter 1, “Astronomy: Observing the Universe,” is a very informative and simple introduction to the subject.
• Struve, Otto, and Velta Zebergs. Astronomy of the Twentieth Century. New York: Macmillan, 1962. Excellent thorough survey of twentieth century astronomy covers both astronomical history and techniques in a very clear format for the general reader. Uses outstanding photographs, illustrations, and examples to present astronomical concepts.
• Zeilik, Michael, and Stephen A. Gregory. Introductory Astronomy and Astrophysics. 4th ed. Monterey, Calif.: Brooks/Cole, 1997. Information-packed textbook is aimed at serious undergraduate-level astronomy and astrophysics students. Presents excellent discussions of concepts and includes outstanding tables, diagrams, and illustrations.
• Zim, Herbert S., and Robert H. Baker. Stars. Illustrated by James Gordon Irving. 1951. Reprint. New York: St. Martin’s Press, 2001. Classic astronomy handbook is an outstanding introduction and guide for astronomers of all ages, even elementary students. Includes excellent illustrations and invaluable information on constellations for night-sky observing.

Hertzsprung Uses Cepheid Variables to Calculate Distances to the Stars

Shapley Proves the Sun Is Distant from the Center of Our Galaxy

Michelson Measures the Diameter of a Star

Hubble Determines the Distance to the Andromeda Nebula

Hubble Confirms the Expanding Universe