Massive stars end their lives in an explosion called supernova. Understanding the last stages of evolution in a star life, leading to a supernova, and understanding the explosion itself, are some of the leading questions in astrophysics today. Not knowing when and where a supernova will occur makes it even harder to study this phenomena. Nowadays, we have the capability to discover many supernovae a night thanks to many dedicated experiments. Our detection capability has improved recently and we can now discover supernovae only a few hours after explosion!
Supernova SN2013fs was discovered by the Palomar Transient Factory project three hours after explosion. The early discovery allowed us to perform followup observations with more advanced telescopes such as the Keck telescope, which provided several optical spectra over a few hours after detection. These spectra revealed the existence of dense gas in the close vicinity around the supernova (10^15 cm) that was ionized by the flash from the supernova explosion. The spectral signatures of this gas have disappeared a few days after the discovery, leading to the conclusion that there was a dense massive shell of gas around the supernova which was swept away by the stellar material that was ejected in the explosion. Late-time radio observations support this conclusion.
Massive stars loose mass along their lives thus enriching their surroundings with low density gas. Discovering a dense shell of gas so close to the star that exploded suggests that massive stars go through major dynamical processes which make the star loose a lot of mass in a short time, during the last year of their lives. These processes are still not well understood and there are several theories explaining them. Understanding these processes is critical to our understanding of the processes inside dying stars which lead to supernova explosions.
The results of this research have been recently published in Nature (the research was performed by an international team of researchers led by Dr. Ofer Yaron from the Weizmann Institute and in collaboration with Dr. Assaf Horesh from the Hebrew University)