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 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,” said Blakeslee.

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: Hubble photos showing the flare of one of the two new
supernovae are available online. See
http://www.jhu.edu/news_info/news/home03/apr03/supernovae.html .