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FOR IMMEDIATE RELEASE: April 10, 2003

Astronomers Stretch Celestial 'Yardstick' to New Lengths

Flares of distant supernovae may reveal major changes in
early evolution of universe

Astronomers' "yardstick" for measuring vast distances
across the cosmos grew longer today as scientists at
The Johns Hopkins University announced they had
identified and closely analyzed two distant new
instances of a kind of exploding star known as a Type
Ia supernova.

The new supernovae belong to a group of star types
known as "standard candles" that astronomers prize for
their usefulness in gauging cosmic distances. They are
approximately 4.7 and 7.6 billion light years from
Earth, and are found in the constellation Ursa Major,
which contains the Big Dipper.

According to Johns Hopkins astronomers, the supernovae
they discovered will be just the first of many to be
identified with a new camera in the orbiting Hubble
Space Telescope, the Advanced Camera for Surveys
(ACS). That prospect has them excited about filling
a prominent hole in their knowledge of the history
of the universe.

"We're trying to fill in a blank region where the
universe's rate of expansion switched from
decelerating due to gravity to accelerating growth
driven by dark energy," explains John Blakeslee, an
associate ACS research scientist at Johns Hopkins
and lead author of a new paper due out in the June
Astrophysical Journal. "That's a real challenge, but
the ACS is making it very straightforward to find
distant supernovae and get detailed information about
them."

Blakeslee and coauthor Holland Ford, professor of
physics and astronomy at Johns Hopkins, noted that
astronomers have previously identified more distant
supernovae, but do not have the same level of
detailed information on those supernovae.

"We have enough data on the new supernovae to
constrain both their distance and the amount of dust
obscuration," Blakeslee said.

Dust obscuration is important to distance measurements
because astronomer's system of "standard candles"
contrasts the inherent brightness of a star or nova
with its apparent brightness from Earth. 

"If you measure the brightness of a candle, and then
move the candle away from you and measure its
brightness again, the candle will appear four times
fainter every time the distance is doubled," explains
Ford. 

Astronomers use their detailed models of star formation,
development, and death to predict how bright certain
stars should be at various points in their life cycle,
and then compare that with their apparent brightness to
get a better feel for how far away the stars are. The
first such "standard candle," identified early in the
20th century, was a type of star known as a Cepheid
variable. Astronomers linked the amount of time it
took for a Cepheid to go through its cycle of varying
brightness to its actual brightness.

Type Ia supernovae are white dwarf stars that have been
drawing mass from a companion star. The white dwarf
siphons off mass until it reaches a critical mass
known as the Chandrasekahr limit.

"Hitting this limit causes a deflagration that consumes
the star in about five seconds," Ford says. "A
thermonuclear burning wave, burning oxygen and carbon
and higher elements, goes through the star."

As a result, the star explodes and shines as brightly
as several billion stars for several days, enabling
astronomers to see it across huge gulfs. Blakeslee
estimated that light from the most distant supernovae
he and Ford detected with ACS had been traveling
towards Earth since the universe was less than half
its current age of 13 billion years old.

Astronomers Zlatan Tsvetanov of Johns Hopkins and
Dan Magee of the University of California-Santa Cruz
compared earlier Hubble images of the same patch of
sky with new ACS images to initially identify the
supernovae. Blakeslee led the follow-up observations
with ACS and other Hubble instruments and the
analysis that enabled them to get a detailed fix on
the new supernovae's intensity and their distance
from Earth.

Information from studies of Type Ia supernovae
confronted astronomers about five years ago with
the stunning, unexpected revelation that stars and
galaxies appeared to be moving away from each other
at an ever-increasing rate in Earth's cosmological
neighborhood. They've attributed this accelerating
expansion to a mysterious factor known as "dark
energy" believed to permeate the universe. 

Looking farther away into the universe (and, because
of the distances involved, further into the past),
they've seen evidence that gravity was at one point
holding back the acceleration of the expansion of the
universe. They have very little data, though, on the
period of transition between these two phases, when
the repulsion produced by dark energy surpassed the
drag created by the pull of gravity. 

"Continued studies of supernovae will allow us to
uncover the full history of the universal expansion,"
Blakeslee says. "The sharper images, wider viewing
area, and keener sensitivity of ACS should allow
astronomers to discover roughly ten times as many
of these cosmic beacons as was possible with Hubble
previous main imaging camera."

Note to editors: Higher resolution photos of the two
supernovae are available online. See
     http://hubblesite.org/news/2003/12/

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