Expert: Philip Stahl - 7/17/2010

Question
Hi, I have some questions about the formation of our solar system. They say it’s the Nebular Hypothesis. So it started with a molecular cloud (this is a nebula no?), then a gravitational collapse etc…
My main question is what is the role of the supernovae? I’m a bit confused. Is the molecular cloud (nebula) the result or remains of a supernova (a dead star)? Or is it the force of a supernova on this molecular cloud that caused the collapse?

I’ve read a few articles but since they’re arguing the validity of a hypothesis it gets very technical and I get confused. Is there some webpage or article or book that you know of with a clear and concise (I guess I mean simplified) step by step process of how a supernova leads to our solar system?
And if so, how sure are we of its validity? Do you think it could be disproved at some point?
I read this article and it seems pretty sure no?

http://www.sciencedaily.com/releases/2008/10/081002172445.htm
‘Little Bang’ Triggered Solar System Formation

Thank you very much for your time
Dari

Answer
Hello,

Hello,

Alas, this may be a tough one to solve. The problem is that the use of concepts such as "shock waves", nuclear decays and isotopes, and certainly "hydrodynamics code" (as referenced in our linked article) presuppose a familiarity with advanced physics that most lay people lack. Thus, they run up against a "wall of understanding" when confronting something like the nebular hypothesis, or more accurately, new inputs to it.

Anyway, I will try to break the process down into step that don't require an elaborate knowledge of the detailed physics, just some basic intuitive understanding.

First, the role of the supernova is mainly as a *trigger* to precipitate the collapse of a pre-existing nebular cloud. The article you referenced mentioned "shock waves" but while these can certainly do the trick, they are not easy to grasp - and actually are studied usually in the context of plasma physics. (We have specific equations to describe different types and magnitudes of shock waves).
"
In the case of your linked article, the shock "front" (the edge of the shock wave) strikes at a pre-solar cloud of dust, water, carbon monoxide, and molecular hydrogen, raising temperatures "as high as 1,340°F or 1000 K". This is the first step, and then they apply a "cooling law" (probably based on some assumed relationship between temperatures and pressures of the cloud) to conclude that ~ 100k years later one has 10^3 times more density and at least the potential for collapse. They estimate then that the collapse led to a proto-sun about 160k years later.

Re: the molecular cloud and whether the result or remains of supernova, according to the theory, I had to go ferret out the actual article from the archives, as the magazine account was somewhat nebulous (though the graphic did refer to short-lived radio-isotopes being injected into the cloud).  Anyway, the paper was not in The Astrophysical Journal but in the Ap. J. Letters (Oct 20, 2008, not Oct. 10, 2008).

Anyway, from the original article, the authors (Boss et al) claim that short-lived radio-isotopes (SLRIs) such as 26 Al (Aluminum) and 60Fe (iron) at the time of the formation of primitive meteorites required that the isotopes were synthesized in a massive star and then incorporated into chondrites within ~ 1 million yrs. The authors further claim that yes, the supernova shock wave transported the SLRIs *to the presolar cloud core*, and triggered cloud collapse, and injected the isotopes.  

So the shock front was the agent to accomplish setting the stage for the heavier proto-sun. They used a numerical (simulation) code - basically an involved computer program - to run the numbers and found it worked provided that "cooling by molecular species such as H2O, CO2, and H2 is included".

So their cooling "law" had to have defined chemical (molecular) components.

As for validity, well it *sounds* valid, but bear in mind this is by way of a numerical experiment not an actual experiment. (One would also like to see the "hydrodynamic code" they employed to see if any lurking errors might be hanging around). In addition, bear in mind that in science one always wants to see a confirmation.  So we have to wait and see.

It looks very compelling, what with the evidentiary tie in to primitive meteorites, but time will tell.

Unfortunately, I don't know of any basic texts that deal with stellar evolution or how the nebular hypothesis can be simply explained. Michael Zeilik's basic 1st year astronomy text, 'The Evolving Universe' does have a nice, fairly basic section on the nebular hypothesis (excluding the current findings) and is on pages 241-247. He also has a nice inset explanation (p. 242) of what angular momentum is and how it can be demonstrated. (This also is a key concept in the hypothesis).

If interested you can probably find the latest edition at a library or at amazon.com for a cut rate (used book) price.