This image made by the Hubble Space Telescope shows the tattered remains of a supernova explosion known as Cassiopeia A. (NASA, ESA, Hubble Heritage STScI/AURA-ESA/Hubble Collaboration via AP)
Life in this universe begins and ends with supernovae. In a spectacular eruption powerful enough to outshine a galaxy, a star is killed — and new elements are forged. The shock wave from the star’s death throes can cause nearby clouds of gas to collapse, triggering the birth of new suns. The ashes of the exploded star spread out into the dark void of space, filling it with ingredients for future stars and planets. Supernovae are creative catastrophes.
For years, scientists have been trying to capture the moment one happens. And they’re getting really close.
In a new study in the journal Nature Physics, an international team of astronomers describes the first three to 10 hours after a supernova in the galaxy NGC 7610, a faint smudge in the constellation Pegasus. Their observations, which came from an array of telescopes scattered across the planet and encompass a wide range of the light spectrum, represent the most complete image of a supernova’s immediate aftermath. Scientists were able to take spectra — analyses that separate light out into its component wavelengths — earlier in the event than had ever been done.
This is a “remarkable achievement,” Norbert Langer, an astronomer at the University of Bonn in Germany who was not involved in the study, wrote in an analysis for Nature Physics. “Young supernovae are hard to come by: without the knowledge of when and where they are going to explode, we tend to chance upon them when they are already several days old. By this time, the supernova ejecta have already swept through a large volume, destroying any information about its immediate environment.”
This study, Langer continued, gives scientists a first good glimpse not just at the explosion, but at the events immediately preceding it — opening a new window on stellar evolution.
The supernova was spotted because of the efforts of scientists working on the intermediate Palomar Transient Factory (iPTF), a project that uses the Palomar Observatory near San Diego to scan the sky in search of new spots of light. Each night, the telescope sweeps across a wide swath of the sky, automatically snapping images of what it sees. A computer system compares images of the same patches of space, looking for signs of change. When it spots something interesting, it sends the location to the Weizmann Institute of Science in Israel, where researchers are just waking up.
On the morning of Oct. 6, 2013, two images of the patch of sky around NGC 7610 streamed onto their computers. A bright white spot had appeared to the left of the galaxy.
“Less than 24 hours before there was nothing there, and suddenly you see there is something,” said Ofer Yaron, an astrophysicist at the Weizmann Institute and the lead author of the study.
It looked like a star had gone boom.
That set in motion a stream of efforts to capture the supernova up close. The scientists at the Weizmann Institute sent a message to Dan Perely, a California Institute of Technology researcher who had secured some time on the massive Keck Telescope in Hawaii. With the supernova in his sights, he took four spectra of the supernova between six and 10 hours after the explosion — the earliest ever taken. Over the next days and months, colleagues running telescopes in Spain, Australia and elsewhere conducted follow-up observations to collect more data. Their analyses revealed the nature of what was given the ungainly name Supernova 2013fs in fine detail.
This graphic shows galaxy NGC 7610 (top right), where the supernova SN2013fs exploded. The event was detected by the Palomar Transient Factory sky survey (bottom right) a mere 3 hours after explosion. The detection was so rapid that researchers could image the circumstellar material (middle left) that surrounded the exploded star. (Ofer Yaron)
It was a type II supernova, the kind that occurs when a massive star evolves into a red supergiant, like the star Betelgeuse. In their waning days, these huge, (relatively) cool bodies start to unravel, expelling huge amounts of their mass until they are surrounded by shells of dense gas called circumstellar material, or CSM.
In the past, it’s been difficult to study the CSM, because it gets swept away by ejecta (stellar shrapnel) once the star blows up. But the spectra taken by Yaron, Perley and their colleagues captured this gas shell before it vanished.
By analyzing the wavelengths of light that came from the CSM, scientists can begin to figure out its chemical composition, how dense it is, how quickly the matter accumulates — questions they could never answer before. They can also start to understand the instability of stars in the months and weeks before they go supernova — possibly allowing scientists to predict when stars are going to blow.
“Several years ago if you found a supernova after about a week or so from explosion, that would be regarded as a very early detection,” Yaron said. “Now we are in the area of Day One supernova detection and science. But we want to go down to the hours and maybe even the minute scale.”
This year, the iPTF project, of which Yaron is a part, will give way to the Zwicky Transient Factory. This initiative will operate at the same telescope as its predecessor, but it will use a camera with a much wider field of view, allowing it to photograph more of the sky more often. This survey will be more than an order of magnitude faster than iPTF, according to Yaron.
Even better is the proposed UltraSat mission, an as-yet-unfunded space telescope that would study the sky in the near ultraviolet end of the spectrum. Its field of view would be nearly 30 times bigger than that of the iPTF camera, enabling it to return to the same patch of sky several times a night and allowing for even more rapid detection of “transients” — celestial phenomena like supernovae that flash for a moment, then fade.
“If we can really catch things during the first minutes of the event, and then our follow-up observations across the whole electromagnetic spectrum … that opens up a completely new window to exploration of the transient sky,” Yaron said.