Discovery of the First X-Ray Source Outside the Solar System Summary

  • Last updated on November 10, 2022

Riccardo Giacconi and his colleagues launched a rocket-borne X-ray telescope that detected X rays from the constellation Scorpius, thereby establishing the importance of X rays to interstellar astronomy.

Summary of Event

All stars in the universe emit electromagnetic radiation as a result of the enormous thermonuclear reactions and complex chemical reactions of which they are composed. Electromagnetic radiation comes in many forms having particular frequencies and wavelengths. All electromagnetic radiation travels at the speed of light (300 million meters per second). The electromagnetic spectrum ranges from low frequency, long-wavelength radiations such as radio, television, microwaves, and visible light to higher-frequency, shorter-wavelength radiations such as ultraviolet, X rays, gamma rays, and cosmic rays. Scorpius X-1[Scorpius X 1] Astronomy;X rays X rays;astronomy [kw]Discovery of the First X-Ray Source Outside the Solar System (June 18-19, 1962) [kw]X-Ray Source Outside the Solar System, Discovery of the First (June 18-19, 1962)[X Ray Source Outside the Solar System, Discovery of the First] [kw]Solar System, Discovery of the First X-Ray Source Outside the (June 18-19, 1962) Scorpius X-1[Scorpius X 1] Astronomy;X rays X rays;astronomy [g]North America;June 18-19, 1962: Discovery of the First X-Ray Source Outside the Solar System[07270] [g]United States;June 18-19, 1962: Discovery of the First X-Ray Source Outside the Solar System[07270] [c]Astronomy;June 18-19, 1962: Discovery of the First X-Ray Source Outside the Solar System[07270] [c]Science and technology;June 18-19, 1962: Discovery of the First X-Ray Source Outside the Solar System[07270] Giacconi, Riccardo Friedman, Herbert Bridge, Herbert Annis, Martin Rossi, Bruno Lindsay, John Gursky, Herbert Harmon, Norman McDonald, Frank Bethune

High-frequency ionizing radiation (ultraviolet, X rays, gamma rays, and cosmic rays) are mostly blocked by Earth’s atmosphere. The thin ozone layer in the upper stratosphere acts as a chemical shield that reacts with these radiations as they bombard Earth’s atmosphere. Consequently, physicists and astronomers who wish to study extraterrestrial high-energy (high-frequency) radiations must place measuring instruments into orbit above Earth’s atmosphere.

With the explosion of astronomical research in the early twentieth century and the harnessing of atomic energy during World War II, scientists gained considerable understanding of electromagnetism and stellar astrophysics. Scientists who had worked on weapons, materials research, radar, and the atomic bomb during the war refocused their efforts on the peaceful applications of science. These efforts included the field of high-energy physics.

During the late 1950’s, a group of physicists at the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts, established a company whose primary focus was high-energy physics and space research. The company, American Science and Engineering, Incorporated American Science and Engineering, Incorporated , was headed by an American cosmic-ray physicist, Martin Annis, and an Italian American physicist, Bruno Rossi (chairman of the board of trustees). In 1959, with the Cold War space race accelerating between the United States and the Soviet Union, Annis and MIT physicist Herbert Bridge persuaded Ricardo Giacconi to join their company to establish a space science research division.

Giacconi had received a doctorate in physics from Milan University in 1954, and in 1956, he moved to the United States, where he served on the physics faculty at Indiana University (1956-1958) and Princeton University (1958-1959). Giacconi and his colleagues focused their research on extraterrestrial radiation sources such as the Van Allen radiation belts surrounding the earth-moon system. In collaboration with the National Aeronautics and Space Administration National Aeronautics and Space Administration;X-ray astronomy[X ray astronomy] (NASA), American Science and Engineering decided to test the emission of X rays from stars. This work had been started in the late 1940’s by the American physicist Herbert Friedman of the Naval Research Observatory. Friedman launched X-ray detectors above the earth’s atmosphere aboard captured German V-2 rockets and demonstrated that the sun emits X rays.

During 1960 and 1961, Giacconi, Frank Paolini Paolini, Frank , and Norman Harmon devised a small, highly sensitive X-ray telescope that could detect faint X-ray emissions from specific regions in space and that could fly aboard conventional Aerobee rockets capable of achieving an altitude of about 160 kilometers. They attempted several launches of X-ray telescopes from White Sands Missile Range in New Mexico beginning in the fall of 1961. Several of these early launches failed because of rocket or equipment malfunctions, but they finally succeeded at midnight on June 18-19, 1962, with a six-minute suborbital flight. The X-ray telescope, a Geiger-counter-like device, received and recorded stellar X-ray emissions on film.

Careful analysis of the X-ray film showed a higher emission of X rays emanating from the southern constellation Scorpius. The scientists named this X-ray source Scorpius X-1, the first extrasolar X-ray source. Friedman’s research group quickly confirmed Giacconi’s discovery. Other high-energy astrophysicists entered the field and discovered additional X-ray sources, including Cygnus X-1 and the Crab nebula in Taurus.

Following the discovery of Scorpius X-1, Giacconi and his colleagues proceeded to develop more ambitious projects for stellar X-ray detection. These experiments involved the development of more sensitive rocket-borne X-ray telescopes, the mapping of more stellar X-ray sources across the sky, and the mapping of X-ray emissions from the sun’s surface. Their ultimate goal was to place a series of orbiting X-ray telescopes around Earth for precise measurements of hundreds of stellar X-ray sources. They planned the development of these satellites in coordination with astronomers John Lindsay and Frank Bethune McDonald of the NASA Goddard Space Flight Center Goddard Space Flight Center in Greenbelt, Maryland.

During the late 1960’s, Giacconi and his colleagues continued their work on a proposed orbiting X-ray telescope. Friedman’s group at the Naval Research Laboratory in Washington, D.C., and McDonald’s group at the Goddard Space Flight Center independently were approaching the same goal. Funding for this project was limited, as NASA’s budget was devoted to the Apollo piloted lunar landing program, but the X-ray satellite project was completely funded by 1970. On December 12, 1970, the Cosmic X-Ray Explorer satellite Satellites, artificial;astronomy was launched from an oil rig located off the coast of Kenya. Kenya was chosen because from there the satellite could easily enter an equatorial orbit, an orbit that would carry the satellite around the earth’s equator, enabling the X-ray telescope to detect X-ray sources from practically every direction around Earth. X-ray data were relayed to a ground-based control station. The X-Ray Explorer, termed Small Astronomy Satellite I Small Astronomy Satellite I (SAS-l), was nicknamed Uhuru, the Kenyan word for freedom, because it was launched on Kenya’s Independence Day.

Uhuru identified and mapped X-ray sources from deep space for prolonged periods of time. The immense data accumulated from the mission not only helped astrophysicists to locate cosmic X-ray sources but also enabled scientists to study periodic changes in each X-ray source over time, giving researchers clues as to the physical nature of these X-ray sources and the stellar chemical processes that stimulate X-ray emission. Many X-ray sources were identified as sunlike stars and galactic nuclei. Still other sources were determined to be superdense collapsed stars called neutron stars. Other scientists speculate that some X-ray sources (Cygnus X-1) may be black holes, gravitational singularities that are collapsed stars so dense that matter and light cannot escape.

Additional Small Astronomy Observatory Satellites were launched during the early 1970’s, each satellite carrying a variety of high-energy detection equipment designed by Giacconi, Friedman, McDonald, and other physicists. Giacconi, McDonald, and Herbert Gursky of the Harvard/Smithsonian Astrophysical Observatory pushed successfully for the funding and development of a more advanced high-energy detection satellite series. The High Energy Astronomy Observatory (HEAO) program High Energy Astronomy Observatory program consisted of three satellites (HEAO-1, HEAO-2, HEAO-3) containing instrument packages for detecting high-energy radiation from space.

HEAO-2, also called the Einstein X-Ray Observatory Einstein X-Ray Observatory[Einstein X Ray Observatory] in honor of the centenary of the great physicist’s birth, was launched in 1978 and was operational through 1981. It contained a powerful X-ray telescope that detected X-ray emissions from quasars (quasi-stellar radio sources), the most distant and oldest objects yet discovered in the universe. The Einstein X-Ray Observatory also was useful for mapping the locations of these quasars and other cosmic X-ray sources. The Einstein X-Ray Observatory was a major culmination of Giacconi’s career that was intricately dependent upon his 1962 discovery of Scorpius X-l, an X-ray emitting variable blue star twice the size of Earth’s sun.


Giacconi’s discovery of the first extrasolar X-ray source was a tremendous astronomical achievement that changed scientists’ view of the universe and led to a greater understanding of stellar astrophysics. The knowledge that many objects, including stars, planets, galaxies, and quasars, emit X rays has enabled scientists to comprehend the nature of these objects and the processes that occur within them. Giacconi’s discovery launched the field of X-ray astronomy, an explosive branch of astronomy that continually yields new information about the universe.

The first X-ray telescopes, launched aboard sounding rockets by Giacconi and Friedman, pioneered later missions (Uhuru, Einstein X-Ray Observatory) that revealed many cosmic X-ray emitters. With succeeding X-ray telescope missions, X-ray sources were discovered in every section of the universe, enabling a comprehensive map of stellar X-ray emission to be generated.

X rays usually are produced by two specific processes, high-temperature plasmas and molecular collisions and interactions. Within stars, temperatures exceed 1 million Kelvins. At such extreme temperatures, matter exists in a vaporized ionized state called plasma. The collision of plasma ions generates X rays. Stars, such as the sun, emit X rays in all directions (isotropically) similar to light emission. At the same time, high-energy particles are ejected from stars, a phenomenon known as solar wind. When these particles strike molecules such as the Moon’s surface or the thick atmosphere of Jupiter, X rays are produced. Intense magnetic fields, such as those found around the gas giants (Jupiter, Neptune), can generate X rays.

The X-ray telescopes designed by Giacconi and his associates were very sensitive receivers whose collecting areas ranged from 1 square centimeter to 150 square centimeters. The greater sensitivity of succeeding X-ray telescopes enabled astronomers to detect very distant galaxies and quasars and helped astrophysicists to analyze exotic stars such as neutron stars and black holes.

Neutron stars were first hypothesized by the Indian astrophysicist Subrahmanyan Chandrasekhar and the Bulgarian American astrophysicist Fritz Zwicky in the 1930’s. Black holes Black holes Astronomy;black holes , whose existence has not yet been verified, have been advocated for several decades by a host of prominent astrophysicists, including the German physicist Karl Schwarzschild, the American physicists John A. Wheeler and Kip S. Thorne, the Soviet physicists Yakov Borisovich Zel’dovich and Igor Novikov, and the English physicists Stephen W. Hawking and Roger Penrose. Both types of stars are superdense supernova remnants whose gravity can bend light.

The X-ray data gleaned from Giacconi and Friedman’s experiments identified pulsars (Crab nebula) and potential black holes (Cygnus X-l), providing experimental support for theoretical astrophysics. Further X-ray studies have indicated the presence of black holes in galactic nuclei (including the centers of our Milky Way galaxy and the neighboring Andromeda galaxy) and enabled scientists to understand better quasars, the oldest, most distant objects yet discovered in Earth’s universe. Scorpius X-1[Scorpius X 1] Astronomy;X rays X rays;astronomy

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Bartusiak, Marcia. Thursday’s Universe. New York: Times Books, 1986. This excellent book is a history of the major astronomical achievements of the twentieth century. It is very enjoyable reading for a general audience. Chapter 2, “A Twilight’s Last Gleaming,” describes the discovery of Scorpius X-l by Giacconi, Gursky, and their colleagues.
  • citation-type="booksimple"

    xlink:type="simple">Grindlay, Jonathan E. “X Ray Astronomy.” In Encyclopedia of Astronomy and Astrophysics, edited by Robert A. Meyers. San Diego, Calif.: Academic Press, 1989. This survey article is a thorough discussion of X-ray astronomy. It provides detailed information and outstanding computer-generated images of X-ray sources obtained from Uhuru and the Einstein X-Ray Observatory.
  • citation-type="booksimple"

    xlink:type="simple">Israel, Werner. “Dark Stars: The Evolution of an Idea.” In Three Hundred Years of Gravitation, edited by Stephen W. Hawking and Werner Israel. New York: Cambridge University Press, 1987. This extensive article is a history of the scientists who conceptualized the superdense neutron stars and black holes. The article discusses complex topics in simple terms and provides a lengthy reference list. The discovery of X-ray sources is discussed briefly.
  • citation-type="booksimple"

    xlink:type="simple">Novikov, Igor. Black Holes and the Universe. Translated by Vitaly Kisin. New York: Cambridge University Press, 1990. This outstanding little book is a clear presentation of black holes and their properties. Written by a leading Soviet theoretical physicist and a pioneer in black hole research, the book is both comprehensive and entertaining.
  • citation-type="booksimple"

    xlink:type="simple">Rolfs, Claus E., and William S. Rodney. Cauldrons in the Cosmos: Nuclear Astrophysics. Chicago: University of Chicago Press, 1988. This textbook for advanced astronomy and physics students is a summary of current knowledge about stars and stellar processes. The chemistry and mathematics are very sophisticated. Chapter 1, “Astronomy—Observing the Universe,” discusses quasars, radio stars, and results from the Einstein X-Ray Observatory.
  • citation-type="booksimple"

    xlink:type="simple">Serway, Raymond A., et al. College Physics. 7th ed. Belmont, Calif.: Thomson-Brooks/Cole, 2006. This algebra-based textbook is an excellent introduction to physics for undergraduate science majors. It is clearly written, contains numerous sample problems, and provides superb illustrations and examples. Chapter 23, “Electromagnetic Waves,” discusses the various types of electromagnetic radiation in detail, including high-energy X rays and cosmic rays.
  • citation-type="booksimple"

    xlink:type="simple">Tucker, Wallace, and Karen Tucker. The Cosmic Inquirers: Modern Telescopes and Their Makers. Cambridge, Mass.: Harvard University Press, 1986. This book is a history of radio, X-ray, and gamma-ray astronomy. It provides the reader with a unique glimpse into the lives of several major American astronomers. Part 2, “A High-Energy Astrophysicist and the Einstein X-Ray Observatory,” is an in-depth summary of Giacconi’s life and scientific career.
  • citation-type="booksimple"

    xlink:type="simple">Zeilek, Michael, and Stephen A. Gregory. Introductory Astronomy and Astrophysics. 4th ed. Fort Worth, Tex.: Saunders College, 1998. This excellent introductory astronomy textbook for undergraduate science majors is a well-written, extensively detailed survey of astronomy. The mathematics and physics are primarily algebra-based. Numerous photographs, illustrations, and references serve to illustrate key concepts. Chapter 18, “Variable and Violent Stars,” discusses X-ray emitting stars and their nature.

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