Astronomers using the NASA Hubble Space Telescope's Advanced Camera
for Surveys (ACS) have found two supernovae that exploded so long
ago they provide new clues about the accelerating universe and its
mysterious "dark energy." The supernovae are approximately 5 and
8 billion light-years from Earth. The farther one exploded so long ago the universe may still have been decelerating under its own gravity.

Type Ia supernovae are believed to be white dwarf stars that pull in
gas from an orbiting companion star. The white dwarf siphons off
mass until it hits a critical point where a thermonuclear "burning"
wave of oxygen, carbon, and heavier elements immolates the star in
a few seconds. The physics of the explosions is so similar from star
to star that all Type Ia supernovae glow at a predictable peak
brightness. This makes them reliable objects for calibrating vast
intergalactic distances

Information from studies of Type Ia supernovae confronted
astronomers about five years ago with the stunning, unexpected
revelation that galaxies appeared to be moving away from each other
at an ever-increasing speed. They have attributed this accelerating
expansion to a mysterious factor known as dark energy that is
believed to permeate the universe

Continued studies of supernovae will allow us to uncover the full
history of the universal expansion



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