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NASA's Hubble Space Telescope has broken the record in the quest to
find the farthest supernova of the type used to measure cosmic
distances. Supernova UDS10Wil, nicknamed SN Wilson, after the 28th U.S.
President, Woodrow Wilson, exploded more than 10 billion years ago
(redshift 1.914). At that time, the universe was in its early formative
years where stars were being born at a rapid rate.
SN Wilson belongs to a special class called Type Ia supernovae. These
bright beacons are prized by astronomers because they provide a
consistent level of brightness that can be used as a cosmic yardstick
for measuring the expansion of space. They also yield clues to the
nature of dark energy, the mysterious force accelerating the rate of
"The new distance record holder opens a window into the early
universe, offering important new insights into how these stars
explode," said astronomer David O. Jones of The Johns Hopkins
University in Baltimore, Md., lead author on the science paper detailing
the discovery. "At that epoch, we can test theories about how reliable
these detonations are for understanding the evolution of the universe
and its expansion."
One of the debates surrounding Type Ia supernovae is the fuse that
ignites them. This latest detection adds credence to one of two
competing theories of how they explode. Although preliminary, the
evidence favors the explosive merger of two burned-out stars, called
The discovery was part of a three-year Hubble program, begun in 2010,
to survey faraway Type Ia supernovae to determine if they have changed
over the 13.8 billion years since the big bang, the explosive birth of
the universe. Called the CANDELS+CLASH Supernova Project, the census
uses the sharpness and versatility of Hubble's Wide Field Camera 3
(WFC3) to assist astronomers in the search for supernovae in
near-infrared light and verify their distance with spectroscopy. The
survey searches for supernovae in two large Hubble programs, the Cosmic
Assembly Near-infrared Deep Extragalactic Legacy Survey and the
Cluster Lensing and Supernova Survey with Hubble, which study thousands
of galaxies. The census is led by Adam Riess of the Space Telescope
Science Institute in Baltimore, Md., and The Johns Hopkins University.
Finding remote supernovae provides a powerful method to measure the
universe's accelerating expansion due to dark energy. So far, Riess's
team has uncovered more than 100 supernovae of all types and distances,
ranging from 2.4 billion years ago to more than 10 billion years ago.
Of those new discoveries, the team has identified eight Type Ia
supernovae that exploded more than 9 billion years ago, including SN
The supernova team's search technique involved taking multiple
near-infrared images spaced roughly 50 days apart over the span of
three years, looking for a supernova's faint glow. The team spotted SN
Wilson in December 2010 in the CANDELS survey. They then used WFC3's
spectrometer and the European Southern Observatory's Very Large
Telescope to verify the supernova's distance and to decode its light,
finding the unique signature of a Type Ia supernova.
Though SN Wilson is only four percent farther than the previous
distance record holder, it pushes roughly 350 million years further
back in time. The last record breaker was announced just three months
ago by a separate team led by David Rubin of the U.S. Department of
Energy's Lawrence Berkeley National Laboratory in California.
"These supernovae are important tools for studying the dark energy
that is speeding up the expansion of space," Riess explained. "This
study gives us a chance to 'stress test' the supernovae themselves to
test how well we understand them."
Astronomers, however, still have much to learn about the nature of dark energy and how Type Ia supernovae explode.
"The Type Ia supernovae give us the most precise yardstick ever
built, but we're not quite sure if it always measures exactly a yard,"
said team member Steve Rodney of The Johns Hopkins University. "The
more we understand these supernovae, the more precise our cosmic
yardstick will become."
By finding Type Ia supernovae so early in the universe, astronomers
can distinguish between two competing explosion models. In one model
the explosion is caused by a merger between two white dwarfs. In
another, a white dwarf gradually feeds off its partner, a normal star,
and explodes when it accretes too much mass.
The team's preliminary evidence shows a sharp decline in the rate of
Type Ia supernova blasts between roughly 7.5 billion years ago and more
than 10 billion years ago. The steep drop-off favors the merger of two
white dwarfs because it predicts that most stars in the early universe
are too young to become Type Ia supernovae.
"If supernovae were popcorn, the question is how long before they
start popping?" Riess said. "You may have different theories about what
is going on in the kernel. If you see when the first kernels popped
and how often they popped, it tells you something important about the
process of popping corn."
In the two white-dwarf scenario, the first supernovae pop off about
400 million years after they are born as stars, and then the rate
gradually declines over time. "There is a cosmic 'high noon' for star
formation at about 10 billion years ago," Rodney explained. "If most of
the supernovae were exploding very shortly after their birth, then we
would see a cosmic 'high noon' for supernova explosions at about the
same time. We are actually finding relatively few supernovae like SN
Wilson at the time of peak star formation, and this favors the double
white-dwarf model, with a modest time delay between formation and
Knowing the type of trigger for Type Ia supernovae will also show how
quickly the universe enriched itself with heavier elements, such as
iron. These exploding stars produce about half of the iron in the
universe, the raw material for building planets and life.
The team's results have been accepted for publication in an upcoming issue of The Astrophysical Journal.
Space Telescope Science Institute, Baltimore, Md.