The Source of My Astrophysics Career, and Other Things

This post is a little personal, but it also deals with some very important (and unanswered) questions in astrophysics today.

I don't know when any of you decided on Astrophysics as your field of study, but my decision came when I was in high school. My sophomore year in high school, in my chemistry class, I first learned about nucleon binding energy, and also that iron-56 was the most stable element. At about this same time I learned about the nuclear process in stars which, if the star is massive enough, results in an iron core, with successive layers of lighter elements around it. As I thought about this I came up with two potential problems with this theory. The first was that the heavy elements created in the star would have no mechanism to get out of the star. Even if the star went supernova, only the outer layer, which consists mostly of hydrogen, would be blown off, leaving the metal core. This would mean that there would be no way to get elements such as carbon, oxygen and iron into the interstellar medium, and eventually collected together to make the earth.

The second problem I thought of is that if the star fused elements it would eventually have to stop at iron because anything beyond that would be endothermic and would stop nuclear fusion in the star. So again, even if the star went supernova, how could it create elements such as uranium, gold and lead in sufficient abundance that it would result in the abundances that we find in the earth. I thought about these problems for a few years and ultimately after exhausting the resources I then had, in my senior year of high school (ten years ago!) I found online a NASA site where anyone could submit a question to a astrophysicist. I submitted my question and after a few weeks I got an answer:

ANSWER:

Hello Quantum Leap,


Thank you for writing to "Ask a High-energy Astronomer".

That's a very good question!

The short answer is that the energy released in a supernova explosion comes from the gravitational energy released as the star collapses. Think about an object falling from a very high building. You may know that this object as it is falling is transforming its potential energy into kinetic energy-- The same happens for a star iron core which collapses: it transforms its gravitational energy to heat and motion -- When the collapse is stopped by the formation of a neutron star able to sustain its own gravitational pressure because of the fermion pressure, there is a ``bounce'' and the star explodes.

Now the detailed answer is much, much more complicated than that and in fact, is a field of research by itself.

The large amount of gravitational energy available from the collapse of the core of the star is indeed transformed to heat and kinetic energy but the problem was that this is not the part which explode. The part which goes flying off the surface is a relatively loosely bound envelope of matter (hydrogen and heavier elements up to silicon)--How the heat from the gravitational collapse of the core is transfered to the rest of the star was a heated discussion (no pun intended :-)) -- The solution of the ``transfer'' of energy may lie in neutrinos, these elusive particles which are supposedly massless, neutral and react very weakly with matter. These neutrinos are ejected carrying most of the energy of the gravitational collapse and in the process they give a little (around 1%) of their energy to the envelope which then ``explodes''.

The detection of a burst of neutrinos (7 total compared to the billions ejected in the explosion) right before the detection of SN1987A is considered to be the most impressive confirmation of the theory.

For more information you may want to check scientific newspapers dated from around the discovery of the neutrino burst (check "Scientific American" and "Sky and Telescope" index for neutrinos and SN 1987A)


We hope this answers your question.

Koji and Ilana for the "Ask a High-energy Astronomer" team.

[Note: You may have noticed that the answer from the NASA scientists was addressed to Quantum Leap. That gives you an idea about how long I have been using the pseudonym "quantumleap". The "42" came later when I signed up for a Google account and "quantumleap" was already taken, so I added on "42" at the end.]

Now back to the science. Essentially I understood their response to be, "There are some very good ideas about how this works but we really don't know." It was then that I decided on what I wanted to do with my life, I decided to become an astrophysicist and actually try to answer these questions.

Now years later, and after a class on stellar physics, I learned the answer to the first problem. The mechanism to transport the metals (i.e. anything heavier than helium) that I was not aware of in high school, is aptly called dredge-up. The concept is fairly simple, but very powerful in explaining why the metallicity of the interstellar medium is so high. Essentially what happens is the convection zone, or the zone of thermal instability inside a star, which normally is confined to a thin skin on the surface, extends all the way down into the core and proceeds to "dredge-up" the metals in the core. Then as the star ages further it sheds its outer layers, not through supernovas, but just through normal stellar winds, and thus throws out a highly enriched mixture of hydrogen and other heavy elements.

The discovery of "dredge-ups" only became possible when numerical simulations had advanced, and the computers necessary to run them, to the point where enough resolution and computing power could be used to be informative. This is because the standard equations of stellar structure (i.e. in their simplest, analytical form) do not predict dredge-ups, and it only became apparent when numerical simulations demonstrated a sudden increase in the depth of the convection zone, as is shown by the graph below.



The answer to the second problem may be answered by the theory of neutrino energy transport, but to the best of my understanding it has not been adequately answered.