Supernova explosions release as much energy in a second as our Sun will in its entire 10-billion year existence. Without supernovae, “there would be no computer chips, trilobites, Mozart or the tears of a little girl,” wrote science writer Clifford A. Pickover.
When a massive star explodes at the end of its life, the explosion ejects essential elements – carbon, oxygen, and iron – that form the basis for life across the universe. The supernovae also release tremendous amounts of radiative, thermal, and kinetic energy into the surrounding environment. According to a new study led by Francisco Rodríguez Montero and colleagues at the University of Oxford, supernovae explosions could pack up to six times more punch on the surrounding interstellar gas with the help of cosmic rays.
Source of the mysterious form of radiation known as cosmic rays
When supernovae explode, they emit light and billions of particles into space. While the light can freely reach us, particles become trapped in spiral loops by magnetic shockwaves generated during the explosions. Crossing back and forth through shock fronts, these particles are accelerated almost to the speed of light and are thought to be the source of the mysterious form of radiation known as cosmic rays.
Influence of cosmic rays in galaxy evolution not well understood.
Due to their immense speed, cosmic rays experience strong relativistic effects, effectively losing less energy than regular matter and allowing them to travel great distances through a galaxy. Along the way, they affect the energy and structure of interstellar gas in their path and may play a crucial role in shutting down the formation of new stars in dense pockets of gas. However, to date, the influence of cosmic rays in galaxy evolution has not been well understood.
In the first high-resolution numerical study of its kind, the team ran simulations of the evolution of the shockwaves emanating from supernovae explosions over several million years. They found that cosmic rays can play a critical role in the final stages of a supernova’s evolution and its ability to inject energy into the galactic gas that surrounds it.
“Initially, the addition of cosmic rays does not appear to change how the explosion evolves. Nevertheless, when the supernova reaches the stage in which it cannot gain more momentum from the conversion of the supernova’s thermal energy to kinetic energy, we found that cosmic rays can give an extra push to the gas, allowing for the final momentum imparted to be up to 4-6 times higher than previously predicted,” explains Rodríguez Montero.
The results suggest that gas outflows driven from the interstellar medium into the surrounding tenuous gas, or circumgalactic medium, will be dramatically more massive than previously estimated.
Creation of super-bubbles
Contrary to state-of-the-art theoretical arguments, the simulations also suggest that the extra push provided by cosmic rays is more significant when massive stars explode in low-density environments. This could facilitate the creation of super-bubbles powered by successive generations of supernovae, sweeping gas from the interstellar medium and venting it out of galactic discs. Superbubbles are colossal cavities stretching hundreds of light years, carved out of interstellar gas and dust by radiation. The gas is often heated to nearly 2 million degrees Fahrenheit, causing it to emit high-powered X-rays. These chemically enriched super-bubbles may eventually seed the intergalactic medium with heavy elements, explaining why gas in the voids between galaxies still have some metal content. It’s a mystery, however, exactly what generates all the powerful radiation that superbubbles emit.
The N 44 superbubble in the Large Magellanic Cloud –a small neighbouring galaxy to the Milky Way–shown in the image above has been produced by the combination of two processes, reports the ESO: “Firstly, stellar winds — streams of charged particles from the very hot and massive stars in the central cluster — cleared out the central region. Then massive cluster stars exploded as supernovae creating shockwaves and pushing the gas out further to form the glowing bubble.
“Although the superbubble is shaped by destructive forces, new stars are forming around the edges where the gas is being compressed. Like recycling on a cosmic scale, this next generation of stars will breathe fresh life into star cluster NGC 1929.”
“Our results are a first look at the extraordinary new insights that cosmic rays will provide to our understanding of the complex nature of galaxy formation,” concludes Rodríguez Montero.
Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona via Royal Astronomical Society
Image at top of page: An artist’s impression of a supernova, NASA/ESA/G. Bacon, STScI