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\title{Summary of research for: "Local Radiation Hydrodynamic Simulations of Massive Star Envelopes at The Iron Opacity Peak"}


\author{Matteo Cantiello}
\affil{Affiliation not available}



\date{\today}

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This is a layman summary of \href{http://arxiv.org/abs/1509.05417}{"Local Radiation Hydrodynamic Simulations of Massive Star Envelopes at The Iron Opacity Peak"} from Yan-Fei Jiang (\begin{CJK}{UTF8}{gbsn}姜燕飞\end{CJK}\selectlanguage{english}), Matteo Cantiello , Lars Bildsten, Eliot Quataert and Omer Blaes. The full article can be downloaded from the \href{http://arxiv.org/abs/1509.05417}{arXiv}. 

This layman summary is part of the \href{https://www.authorea.com/users/2/articles/50890/_show_article}{Public Friendly Open Science} initiative. 

\section{Broader Context}
Stars more massive than about 10 times our Sun (massive stars) experience a short, eventful life. They shine bright and burn their nuclear fuel in just a few million years. When their fuel reservoir is over, they initially collapse due to their own tremendous gravitational pull. This initial collapse is arrested when the stellar core reaches nuclear densities, so that the in-falling outer layers bounce on top of the core and produce a powerful explosion. This is called a "core-collapse Supernova". The energy released in this process is enormous, so large that supernovae become as luminous as their entire galaxy (which is usually made of hundred billion stars) for several weeks. The ashes of this cosmic firework contain most of the elements of which we are made of. Also, the leftovers of this stellar death are exotic corpses: neutron stars and black holes. Astrophysicists love them because using these stellar remnants they can test physics theories in very extreme conditions (extreme density and gravity), something that can not be done in a laboratory on Earth. 


\section{This research}
To understand the details of  how a massive star dies, astrophysicists need to first understand how it lived. One big complication to model the life of very massive stars is their extreme luminosity. These stars can be several million times more luminous than our Sun: before emerging at the surface, this impressive flux of photons has first to cross the star since energy is produced in the deep, hot regions, where most of the nuclear fusion powering the star occurs. 
While in our daily lives we are not used to feel the pressure of a light beam, in these stars the pressure due to the photons streaming out from the core towards the surface can be much larger than the pressure of the gas. This means that the overall structure of the star is predominantly affected by the radiation, with the light being able to literally "lift" the stellar gas, at least in certain regions of the star. A particularly important zone for this process is found in the outer envelopes of massive stars, not too far from the surface. This region is particularly complex, and to correctly capture its physics one has to model both the dynamics of the gas and of the radiation ( "3D radiation hydrodynamics" models are required), which is a particularly hard computational task.

This work presents the first instance of such calculations, performed using the \textbf{NASA supercomputers} \href{http://www.nas.nasa.gov/hecc/resources/pleiades.html}{Pleiades} and \href{http://www.nccs.nasa.gov/discover_front.html}{Discover}. 

The results are exciting: We now understand better how the energy is transported in the outer layers of these stars, which means we will be able to calculate their size with better precision. This is important because depending on their radii, massive stars that are found in binary systems (two stars orbiting each other, and it turns out this is the case for the majority of massive stars) can either interact or not during their evolution, changing substantially their final outcomes.  Also it appears that, due to the presence of such large radiation field, the atmospheres of some of these stars are not static, but have complex motions that could affect their evolution and impact their final death. While more work needs to be done, this paper paves the road to substantial progress in the understanding of massive stars structure and evolution.\selectlanguage{english}
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\includegraphics[width=0.70\columnwidth]{figures/Screen-Shot-2015-09-20-at-9.27.01-PM/Screen-Shot-2015-09-20-at-9.27.01-PM}
\caption{{Calculations showing the evolution of the density in the outer layers of a radiation dominated massive star (Jiang et al. 2015).%
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