A supernova imaged in X-ray (left) and visible light (right).

A supernova imaged in X-ray (left) and visible light (right). (Source: NASA, Swift Science Team, Stefan Immler)

On September 29th, Dan Milisavljevic gave a talk titled Reverse Engineering Supernovae. Milisavljevic got his PhD in astronomy in 2011. He then went on to work at the Harvard-Smithsonian Center for Astrophysics where he focuses on supernovae and their remnants.

Throughout history, people have observed supernovae as very bright spots that appeared suddenly in the night sky. The oldest confirmed record of a supernova comes from second century China (1).

Understanding supernovae is essential because they play a key role in shaping galaxies, producing heavy elements, and triggering star formation among other things.  Although supernovae are relatively easy to see, they are surprisingly difficult to study in detail. Once a star explodes it is too late to gather information on its previous state. So astronomers have to hope that someone had collected data on that specific patch of sky recently. Even if they find something, real astronomical data looks little like the crisp, colorful artist’s renditions that one often sees. Thus the best way to discover what a star was like before it exploded is often to study the remnants and reverse engineer the events that created them: this is what Milisavljevic does (2).

He specifically studies core collapse supernovae, which occur in stars greater than eight solar masses. A very simplistic and perhaps outdated explanation of this type of collapse is that stars are kept from collapsing by the pressure exerted by the radiation they produce. Stars burn successively heavier elements in shells until they begin to burn iron in their core. At this point, continued fusion consumes energy rather than releasing it, causing the outward radiation pressure to diminish. Gravity takes over and the star collapses until it reaches its degeneracy pressure, a fundamental limit beyond which it cannot be compressed at which point the star violently bounces back tearing itself apart (2).

This model is now being challenged by data from supernova remnants and computer simulations. Years of simulations show that the bounce-back is too weak to produce an explosion on the scale of a supernova. Additionally, the distribution of nickel in the remnants is not consistent with the model above.  Milisavljevic believes this is because stars are far less ordered and far more turbulent internally than was previously believed. Milisavljevic is currently working to create three-dimensional digital models of the debris left over from certain supernovae to better describe this phenomenon (2) .

References:

1. Cohen, J. (2011). Astronomers Solve Puzzle of First Supernova on Record. Retrieved October 1, 2015.

2. Dan Milisavljevic, personal communication, September, 29, 2015.