When massive stars (on the order of ten or more times the mass of our Sun) end their lives, they go out with a bang. In an instant, these stars send out a massive shock wave as type II supernovae, spreading the contents of their interiors — hydrogen, helium, and heavier elements that include silicon, oxygen, and iron — into the interstellar medium, sprinkling the materials of future stars and solar systems throughout the galaxy.
Supernovae have been observed, both within our galaxy and in other galaxies, for thousands of years, and their results can be seen as nebulae, neutron stars, and black holes. But what is it that actually makes these stars go bang? The answer is: We don’t know.
But Ofer Yaron of the Weizmann Institute of Science in Israel and his colleagues have just brought us a little closer to finding that answer. In a paper recently published in the journal Nature Physics, Yaron and his colleagues report their measurements of supernova SN2013fs, which exploded in the nearly galaxy NGC 7610 in 2013. Their results represent some of the earliest post-explosion follow-up observations of a supernova event, including the earliest spectra of a supernova ever, shedding light on the dying star’s final days.
Reading the gas
What we do know is that the evolution of a star prior to its explosion likely holds key clues about the processes that precede type II supernova. The star’s behavior, such as its growth into a red supergiant and the mass loss it experiences during this phase, affect the results we see when the star does explode. But the red supergiant phase is actually quite short (cosmically speaking; this phase can last between a few hundred thousand to maybe a million years), so we rarely see stars in this part of their life cycle.
Because supernovae are instantaneous and unpredictable, we also rarely catch them right as they’re happening. The chance to see a supernova just as it’s occurring, rather than days or weeks later, could translate into the data needed to trace back the star’s evolution and even understand the instant of the explosion itself.
One of the processes astronomers are looking to trace is the red giant’s history of mass loss. Mass can be lost through expansion as the star ages, as well as via eruptions of the star’s upper atmosphere. This mass loss can cause a “shell” of circumstellar material that blankets the star. And when the supernova occurs, the way it lights up this material can thus tell astronomers about how the material was lost, highlighting the star’s most recent history like the last few rings in a tree’s trunk.
When the supernova occurs, the shock wave it produces causes a process called photoionization, which strips electrons away from the gas surrounding the star. Shortly thereafter, all these free electrons recombine with the gas atoms of the shell (in a process aptly called recombination), which causes the gas to shine. Studying the resulting spectrum of the gas reveals information about the elements in this gas shell, as well as its density, motions, and the distance of the gas from the star.
As the shock wave moves through the shell surrounding the star, it lights up different features, all of which provide 3-dimensional information about the structure of the cloud. All of this information can be used to reconstruct a picture of the environment around the star just before the supernova occurred.
The key, though, is catching the supernova in its earliest stages, because as the shock wave progresses through the material around the dying star, it quickly distorts it and blows it away, erasing the information there like shaking a cosmic Etch A Sketch.
SN2013fs was first detected in October of 2013 by the intermediate Palomar Transient Factory (iPTF) survey. The event was quickly followed up in multiple wavelengths, including X-ray, ultraviolet, optical, and infrared. These follow-up observations include the earliest spectroscopy of a type II supernova ever obtained. The explosion was first identified just three hours after it occurred, and the first spectrum was taken within six hours of the initial event.
The observations are consistent with a shell of material surrounding the star out to a distance of about 1015cm — that’s a little more than 66 times the Earth-Sun distance. Models indicate that the bulk of this material was ejected within the last few hundred days of the star’s life. But because the velocity of the gas cloud around the star could not be directly measured, it’s still difficult to decouple the effects of a short burst of mass loss right before the supernova event from longer-term, slower mass loss due to a stellar wind over hundreds of years.
Fortunately, surveys such as the one that identified SN2013fs are on the rise, and the better and more comprehensive these surveys become, the more likely it is that additional “young” supernova events will receive the follow-up necessary to begin piecing together the physics that lead to these cataclysmic events in the first place.