Archival Hubble Space Telescope images taken more than a decade ago have revealed an ancient star in the act of exploding.
Take deep pictures of swaths of night sky using one of mankind's most powerful telescopes, and you never know what you’ll turn up. Such serendipity has led to the discovery a distant star gone supernova just hours before the image was snapped.
In 2010 the Hubble Space Telescope took long images of a galaxy cluster known as Abell 370. The light from this galactic hive comes from 4 billion light-years away. But within the image of this swarm is something else — the light of far more distant background galaxies and the stars within them, their light bent and magnified by the intervening cluster’s mass.
And within that background light are photons coming from a star that “just” exploded — 11.5 billion years ago, when the universe was only 2.3 billion years old.
Thanks to relativistic effects of gravity, the chance alignment of this distant star behind the galaxy cluster results in its light being split into four images — an “Einstein cross” alignment. One of these images is too faint to detect; the other three images took different, bent paths to Earth. That means that the three images that are visible capture the supernova at different times.
The photons following the most direct path show the supernova days after collapse. The photons that make up the other two images took slightly longer paths and thus show the exact same event, but 2 days and 8 days earlier, respectively. The earliest image shows the supernova just 6 hours after the stellar core collapsed.
For a star to explode, several things have to happen first. First, the stellar core that has long fought against gravity’s inward pull runs out of fusion fuel and collapses. Regardless of whether it forms a neutron star or squishes further into a black hole, the compact object at the core will meet the outer layers of gas that are still falling in. A reverse shock reverberates outward, eventually breaking through the star’s surface and scattering the outer layer’s gaseous remains.
“What we saw was the emission from after the breakout, when the supernova ejecta expands and cools,” explains study lead Wenlei Chen (University of Minnesota). “Three supernova images show that the expanding surface changed from faint blue (small and hot) to bright red (large and cool).”
These observations reveal the original star’s size. From the temperatures they measured, Chen and colleagues report in Nature that the supernova’s progenitor was a typical red supergiant. Such a size estimate has previously only been possible for a handful of nearby supernovae.
“For me, the most important thing is still the “gee whiz” factor that you can actually find these things and use them to infer properties of supernova progenitors at redshift 3!” says supernova and cosmology expert Robert Kirshner (Harvard University).
While the star that exploded isn’t itself unusual, it’s the second gravitationally lensed supernova to be found in the Frontier Field set of Hubble images, of which this one is a part. (The other supernova, spotted previously, is Supernova Refsdahl.) Using these two supernovae, Chen’s team is able to estimate how many stars are dying in the early universe. The result: when the universe was only a couple billion years old (what astronomers term a redshift of 3), about eight stars would go supernova every 10,000 years within a space 3 million light-years on a side. That number is consistent with our ideas — based on observations of nearer stars — about how stars are born and, ultimately, die.
“As for rates, well, the statistics are very limited: two objects!” Kirshner notes. “Still, this demonstrates the principle, it shows the one previous object was not a fluke, and it opens the path to serious exploration of massive star birth and death at redshift 3. Pretty amazing!”