Riccardo Giacconi, a Nobel Prize-winning scientist who was known as the founder of X-ray astronomy, a discipline credited with opening a new window into the cosmos, died Dec. 9 in San Diego. He was 87.

A daughter, Anna Giacconi, confirmed his death and said she did not yet know the cause. Dr. Giacconi was born in Italy but worked primarily in the United States, where he spent most of his life. At the time of his death, he was listed as a faculty member in the physics and astronomy department at Johns Hopkins University in Baltimore.

In 2002, the Nobel committee awarded Dr. Giacconi a share of the physics prize “for pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources.” The other half of the prize recognized Raymond Davis Jr. and Masatoshi Koshiba for “the detection of cosmic neutrinos.”

Dr. Giacconi was widely known as “the father of X-ray astronomy,” and the leader of the International Astronomical Union called him a determined, strong-minded visionary.

“World astronomy bears a huge debt of gratitude to Riccardo Giacconi,” IAU General Secretary Teresa Lago said in a statement.

For centuries, humans knew no more about the stars than their eyes could tell them. The starry universe seemed calm and immutable. But X-ray astronomy has helped create a new view, of turbulence, cataclysm, birth and death, creation and destruction.

Much of the modern understanding of the nature and dynamics of stars and galaxies, and the charged particles streaming between them, has been provided or supported by X-ray astronomy.

X-rays, although invisible to the eye, own just as firm a place on the electromagnetic spectrum as the light we can see. X-rays possess high energies and tell in turn of the high energy processes by which they are produced.

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While they have streamed across the void of space for eons, only recently have scientists succeeded in capturing them, analyzing them and reading the messages that they carry about cosmic events at unimaginable distances.

The expansion of astronomy beyond visible light created a stir in science and beyond. Beyond the knowledge it provides of the cosmic environment , X-ray astronomy is credited with yielding spinoff applications in industry, medicine, security and environmental monitoring.

Blocked by Earth’s atmosphere, the X-rays emitted by the stars and galaxies have been detectable only by reaching high elevations or by launching observing equipment into space.

Dr. Giacconi and his colleagues developed telescopes suited to gathering the faintest X-ray signals from the far reaches of the universe. In addition to recognizing and analyzing their meaning, the scientists showed patient diligence in obtaining the support needed to place their equipment on the appropriate spacecraft.

Among the challenges in creating an X-ray telescope was the high energies of the rays. Optical telescopes employ mirrors to reflect visible light. X-rays would tear through the mirrors, making them unusable.

Ultimately, geometries were devised so that X-rays would arrive at angles small enough to allow them to bounce off reflectors, rather than penetrate them.

Since the middle of the last century, what scientists have come to call the X-ray universe has been explored by the use of rockets and satellites, with the progress coming so rapidly that detector sensitivity has increased one billion times.

Dr. Giacconi was principal investigator for NASA satellites that orbited the Earth and trained their eyes on X-ray sources. He was a key mind behind the Uhuru X-ray satellite, which found what the Harvard-Smithsonian Center for Astrophysics described as “the first evidence for a black hole,” as well was the Einstein Observatory, the first imaging X-ray telescope.

He was credited with playing a major role in the launch and operation of NASA’s Chandra X-ray Observatory. Launched in 1999, it received credit for a profusion of scientific discoveries. In one notable achievement, Chandra found a cloud of hot gas that extended throughout a galactic cluster several million light-years across, with temperatures as high as 40 million degrees.

Riccardo Giacconi was born in Genoa on Oct. 6, 1931. After his parents separated, he lived for most of his youth in Milan with his mother, a high school teacher of math and physics.

Although a sometimes difficult student — by his account, he cut school and reveled in correcting his teachers — he was attracted to physics, particularly the study of elementary particles and cosmic rays.

He received a PhD in physics from the University of Milan in 1954 and began research in cosmic rays early on. A recipient of a Fulbright fellowship, he soon left for brief stays at Indiana University and then at Princeton.

After Princeton, he was brought to American Science and Engineering, a private research company, to start a program in space science. In his 14 years at AS & E, from 1959 to 1973, he was credited with creating what became the new science of X-ray astronomy.

He held leading roles in succeeding years at important scientific institutions and on major research projects. Among these were the Space Telescope Science Institute at Johns Hopkins. He served there from 1981 to 1992. At the institute he had responsibilities for operating the Hubble Space Telescope.

His reasons for leaving the telescope institute were disclosed in the biography he submitted years later for the Nobel committee. It suggested the depths of human emotion that exist beneath what may seem the austere exterior of science.

“In 1991 my son Marc died in an automobile accident,” he wrote, and the telescope institute, Hubble and the city of Baltimore, “were continued and painful reminders of devastating grief. “

He “jumped at” the chance to become head of the European Southern Observatory. It was based in Germany, and he headed it from 1993 to 1999.

In 1999 he began five years as the president of Associated Universities Inc., a Washington-based manager of laboratories.

He was a recipient of NASA’s Distinguished Public Service Award and of the National Medal of Science, which President George W. Bush awarded to him in 2003.

Dr. Giacconi’s survivors include his wife, the former Mirella Manaira, of San Diego; two daughters, Anna Giacconi of San Diego and Guia Trutter of Lake Forest, Ill.; and two grandchildren. The biography that he prepared for the Nobel committee in 2002 was dedicated to his wife and daughters. He also wrote that it was in memory of his son, Marc Antonio.