Three U.S.-born scientists won the Nobel Prize in physics on Tuesday for their studies of exploding stars that revealed that the expansion of the universe is accelerating.

The Royal Swedish Academy of Sciences said American Saul Perlmutter would share the 10 million kronor ($1.5 million) award with U.S.-Australian Brian Schmidt and U.S. scientist Adam Riess. Working in two separate research teams during the 1990s _ Perlmutter in one and Schmidt and Riess in the other _ the scientists raced to map the universe's expansion by analyzing a particular type of supernovas, or exploding stars.

They found that the light emitted by more than 50 distant supernovas was weaker than expected, a sign that universe was expanding at an accelerating rate, the academy said.

"For almost a century the universe has been known to be expanding as a consequence of the Big Bang about 14 billion years ago," the citation said. "However the discovery that this expansion is accelerating is astounding."

Perlmutter, 52, heads the Supernova Cosmology Project at the University of California, Berkeley.

Schmidt, 44, is the head of the High-z Supernova Search Team at the Australian National University in Weston Creek, Australia.

Riess, 42, is an astronomy professor at Johns Hopkins University and Space Telescope Science Institute in Baltimore, Maryland.

The physics prize was the second Nobel to be announced this year. On Monday the medicine prize went to American Bruce Beutler and French scientist Jules Hoffmann who shared it with Canadian-born Ralph Steinman for their discoveries about the immune system. Steinman died three days before the announcement.

The prestigious Nobel Prizes were established in the will of Swedish industrialist Alfred Nobel, and have been handed out since 1901.

Last year's physics award went to Russian-born scientists Andre Geim and Konstantin Novoselov for groundbreaking experiments with graphene, the strongest and thinnest material known to mankind.

The prizes are handed out every year on Dec. 10, on the anniversary of Nobel's death in 1896.