Yaron’s observations of SN 2013fs appear to provide the clearest evidence that a dying star anticipates its imminent death by shedding more gas than usual during its final months. Yaron compared the presence of the gas cloud to “a volcano that before a big eruption has smaller eruptions and bubbling at the surface.” Five days later, as the shock wave from the supernova spread, this so-called circumstellar material had completely vanished.
“Maybe a large fraction of stars, those between 10 to 20 solar masses that produce Type II supernovas, have these eruptive mass loss episodes,” said Yaron, who suspects that early shock waves emerging from the star’s core could trigger the process.
Yet not all astronomers believe that the gas surrounding SN 2013fs was of a recent vintage. Red supergiant stars — the kind that gave rise to SN 2013fs — “are surrounded by material all their lives, polluted with material at the surface,” said Luc Dessart, an astrophysicist with the French-Chilean Joint International Astronomy Unit at the University of Chile. “There’s lots of stuff there.” For instance, the red supergiant Betelgeuse in the constellation Orion is only 430 light-years away from us, so astronomers can see clearly that it is shrouded by gas that has likely been there for a long time. One day, Betelgeuse will explode as a supernova, said Dessart — but can we predict if it will happen next year? “Probably not,” he said.
One way to be sure is to measure how fast the gas is moving away from the star. If it’s moving quickly, it must have erupted recently. Yaron said that in SN 2013fs, “it is clear that the velocity is higher compared to typical wind velocities of a red supergiant star, which are around 10 to 20 kilometers per second.” But the measurements made by Keck provide only an upper limit on what the velocity of the gas is, not the exact number. Based on the spectra, one simply cannot tell if the gas moved at a speed of “one, 10 or 100 kilometers a second,” said Jerkstrand. In other words, the gas could have been floating around the progenitor star for many, many years.
It could also be that the star that became SN 2013fs simply had an eruptive process completely unrelated to its core collapse, argued Jerkstrand. After all, scientists have traditionally assumed that it’s very unlikely for a star’s core to “communicate” its imminent collapse to the hydrogen mantle — the star is so large and massive that it should take information thousands of years to travel from the core to the surface.
That’s true if we assume that information is carried by photons of light, which bounce around in the optically thick environment of a star. Shock waves, however, could carry enough energy to create massive surface bubbling, said Fuller. In April of this year, Fuller published a study that explores the idea of information transfer, but with a twist. “There are other ways that energy can be transported, and one of those ways is sound waves,” he said.
Fuller used an idea first put forward in 2012 by Eliot Quataert, an astrophysicist at the University of California, Berkeley, who suggested that acoustic waves from the center of a star could trigger surface outbursts. The core is “sort of like a pot of boiling water — if it’s boiling pretty intensely, you can hear it because boiling excites sound waves in the air,” said Fuller. It’s the same with the star’s core right before a star goes supernova: The boiling excites acoustic waves that carry an enormous amount of energy. The waves create little outbursts on the surface that “haven’t been predicted in previous models of stars,” said Fuller.
His model supports Yaron’s analysis of the gas found around SN 2013fs. “His results,” said Fuller, are “similar to what was happening in my models, so I was encouraged that the data might be verifying my theoretical work.”
Supernova Inside Out
Early spectra, however, tell us only about the outside of the exploding star. To see what’s inside and get clues on why it blows up, you need to study a supernova from about 200 days after the explosion, during its so-called nebular phase, and much later, during the remnant phase, explained Jerkstrand. By this time, the “ejecta” — the shreds of the star — have made it out to where they can be seen. In a sense, the star has been turned inside out. These parts form the heavy elements that are the building blocks of much of our universe — including life.
One remnant that is currently forming right before our eyes is that of SN 1987A — the nearest supernova to Earth in over 400 years. Back in 1987, the event appeared to confirm one of the central insights into how stars explode. Theories of Type II supernovas like SN 1987A predict that the collapse of the star’s core will trigger a massive stream of neutrinos, a type of subatomic particle. And, indeed, two neutrino experiments on Earth independently detected a surge of these particles passing through our planet.
Neutrinos are famously shy, interacting only through the so-called weak force. But in recent years theorists have explored how they could change the supernova itself. Because there are so many neutrinos coming out of a supernova’s core, if even a small fraction of them are reabsorbed by the matter surrounding the core, the energy input will make the gas “begin to boil violently like soup in a pot on the stove,” said Hans-Thomas Janka, astrophysicist at the Max Planck Institute for Astrophysics. “Neutrino-heated gas starts to rise outward in mushroomlike structures similar to those in photos of atomic-bomb explosions.”