A Japanese and two American astrophysicists won the Nobel Prize in physics Tuesday for using some of the most obscure particles and waves in nature to increase understanding of the universe.

Riccardo Giacconi, 71, of the Associated Universities Inc. in Washington, D.C., will get half of the $1 million prize for his role in "pioneering contributions to astrophysics, which have led to the discovery of cosmic X-ray sources."

Raymond Davis Jr., 87, of the University of Pennsylvania shares the other half of the prize with Japanese scientist Masatoshi Koshiba, 76, of the University of Tokyo. The two men pioneered the construction of giant underground chambers to detect neutrinos, elusive particles that stream from the sun by the billion.

Neutrinos offer a unique view of the sun's inner workings because they are produced in its heart by the same process that causes it to shine. Davis' early experiments, performed during the 1960s in a South Dakota gold mine, confirmed that the sun is powered by nuclear fusion.

His experiments were described in the citation as "considerably more difficult than finding a particular grain of sand in the whole of the Sahara desert."

Koshiba won his share of the prize for his work at the Kamiokande neutrino detector in Japan. That experiment confirmed and extended Davis' work, and also discovered neutrinos coming from distant supernova explosions, some of the brightest objects in the universe.

The Italian-born Giacconi, a U.S. citizen, was cited for building the first X-ray telescopes that provided "completely new -- and sharp -- images of the universe," the academy said.

His research laid the foundation for X-ray astronomy, which has led to the discovery of black holes and allowed researchers to peer deep into the hearts of the dusty young galaxies where stars are born.

This year's Nobel winners have "opened new windows to space," Mats Jonsson, chairman of the awards committee said.

When academy officials reached Giacconi by phone at his home outside Washington, he said he was "dumbstruck" to learn of the prize. Koshiba also was phoned at home in Tokyo, spokesman Erling Norrby said.

Koshiba said he knew he won the prize when the phone rang Tuesday evening at his home in Tokyo.

"All I can say is I'm so happy," Koshiba said. "This wonderful outcome was only possible because of my young assistants' hard work."

Davis suffers from Alzheimer's disease and was not available to comment Tuesday morning.

This year's Nobel awards started Monday with the naming of Britons Sydney Brenner, 75, and Sir John E. Sulston, 60, and American H. Robert Horvitz, 55, as winners of the medicine prize, selected by a committee at the Karolinska Institute.

The researchers shared it for discoveries about how genes regulate organ growth and a process of programmed cell deaths that shed light on how viruses and bacteria invade human cells, including in conditions such as AIDS, strokes, cancer and heart attacks.

The winner of the Nobel Prize in chemistry will be named on Wednesday morning and the Bank of Sweden Prize in Economic Sciences in Memory of Alfred Nobel later the same day.

The literature prize winner will be announced on Thursday and the winner of the coveted peace prize -- the only one not awarded in Sweden -- on Friday in Oslo, Norway.

The award committees make their decisions in deep secrecy and candidates are not publicly revealed for 50 years.

Alfred Nobel, the wealthy Swedish industrialist and inventor of dynamite who endowed the prizes left only vague guidelines for the selection committees.

In his will he said the prize being revealed on Tuesday should be given to those who "shall have conferred the greatest benefit on mankind" and "shall have made the most important discovery or invention within the field of physics."

The Royal Swedish Academy of Sciences, which also chooses the chemistry and economics winners, invited nominations from previous recipients and experts in the fields before cutting down its choices.

The prizes, which include a gold medal and a diploma, are presented on Dec. 10, the anniversary of Nobel's death in 1896, in Stockholm and in Oslo.