More than two decades since its launch, the Hubble Space Telescope continues to inspire awe and astonishment, most recently by identifying the farthest supernova to date of the type used to measure cosmic distances. Supernova UDS10Wil—nicknamed “SN Wilson,” in honor of World War I-era President Woodrow Wilson—exploded more than 10 billion years ago and lies 4 percent more distant from us and is 350 million years older than the previous record-holder. According to astronomer David O. Jones of Johns Hopkins University in Baltimore, Md., the discovery “opens a window into the early Universe, offering important new insights into how these stars explode.”
The newly-found object belongs to the “Type Ia” class of supernova, which are prized by astronomers because they offer a consistent level of brightness and can be used to measure the steady expansion of space. The discovery, to be reported in The Astrophysical Journal, is part of a three-year Hubble program, begun in 2010, to survey these faraway exploded stars and determine if they have changed during the Universe’s approximately 13.8-billion-year lifespan. The telescope employed its versatile Wide Field Camera (WFC)-3—installed by the STS-125 crew during the last shuttle servicing visit in May 2009—to conduct the search at near-infrared wavelengths and verify distances with spectroscopy.
Finding remote supernovae is recognized as an important means of measuring the Universe’s constant expansion. To date, a team led by Adam Riess of the Space Telescope Science Institute (STScI) in Baltimore, Md., and Johns Hopkins University, has uncovered more than 100 supernovae of all types and distances, looking back in time from 2.4 billion years ago to over 10 billion years ago. Eight of these are of the Type Ia variety, each in excess of 9 billion years old.
However, the “yardstick” which these discoveries provide is ever-changing; the previous record-holder, about 350 million years younger than SN Wilson, was detected barely three months ago. “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,” admitted team member Steve Rodney of Johns Hopkins. “The more we understand these supernovae, the more precise our cosmic yardstick will become.” SN Wilson now extends that yardstick an additional 4 percent and places the team literally at the knife-edge of the unknown.
Moreover, the discovery of supernovae so early in the Universe’s existence has produced two competing models for how they exploded. The first proposes that a merger between a pair of white dwarfs was the causal factor, whilst the other suggests that a white dwarf gradually “fed” off its partner—a normal star—and exploded when it accreted too much mass. Preliminary evidence from the Johns Hopkins and STScI team favors the first option, because a sharp decline in the rate of Type Ia explosions between 7.5 billion years ago and more than 10 billion years ago. This in turn predicts that most stars in the early Universe were too young to become Type Ia events.
“If supernovae were popcorn, the question is how long before they start popping?” said Riess. “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.” By developing a clearer awareness the type of “trigger” behind Type Ia events, astronomers hope to reveal how quickly the Universe enriched itself with heavier elements, such as iron, which forms a key raw material for planet-building and, ultimately, for life itself.