RE: Article: "Sun is Mostly Iron, Not Hydrogen"

From: Reason (
Date: Thu Jan 10 2002 - 12:54:26 MST

> > It's a fascinating process, the aging of stars. You start off fusing
> > hydrogen to helium in the core. Then you run out of hydrogen,
> and the fusing
> > moves to an expanding shell. The inner helium core grows and collapses
> > without the sustaining radiation pressure of fusion reactions
> until it gets
> > hot and dense enough to ignite. Which it does pretty much all
> at once. Bang!
> > Helium fusion generates much more energy, so the star expands to a new
> > equilibrium. Now you have a helium fusing core surrounded by a hydrogen
> > fusing shell.
> >
> > Later on, you start to run out of helium, and build up a carbon
> core. So the
> > start have a shell of helium fusion and a shell of hydrogen
> fusion outside.
> > As the core gets larger, it collapses until it ignites with
> carbon fusion.
> > Bang!
> >
> > The process of getting from a helium core ignition to a carbon
> core ignition
> > is somewhat faster than getting a helium core ignition.
> So what happens during planetary formation when a dozen or so
> planetesimals get chucked into the sun by orbital resonances? Doesn't
> the addition of preexisting heavy elements tend to spike the cocktail of
> stellar development?

Not noticably. Planetesimals tend are either jupiter-types (in which case,
they're more or less exactly the same composition as the star), or they're
too small to matter. (The sun right now is about 99% of the mass of the
solar system; the rest is pretty much all gas giants). Anything heavier than
carbon is really very rare by mass in the solar system.

(Note that 1 Jupiter = 318 Earths, and 1000 Jupiters = 1 Sun, more or less).

If you want to do the math, consider that the cloud that collapsed to form
the solar system is homogenous (or as near as matters) in elemental makeup &
figure from there.

> Furthermore, I haven't heard you or anyone speak to Jeff's earlier
> calculation that there was something like 535 Earth masses of iron in
> the Sun, based on the 0.15% iron content in the solar corona. Is this
> accurate? If so, how does the presence of this amount of iron (and any
> other metals) spike the Sun's development?

The sun is 330,000 times as massive as the earth, give or take. 0.15% of
that is considerably more that 535 earth masses, but 0.15% is the correct
mass fraction.

Metals make stars smaller and last longer is the short answer. Metals add
opacity to plasma way out of proportion to their absolute mass fraction,
meaning there is a stronger coupling between radiation and matter in the
star, leading to larger stars being much more unstable or not forming in the
first place. If you remove all the metals from the mix, you get monsterously
huge stars (up to 1000 solar masses or more) that go supernova a million
years after forming.

The heavier metals in the sun are evenly distributed, although I think
there's a little more carbon, nitrogen, oxygen in the core. I don't recall.

The good thing about this sort of stuff is it reminds me to take everything
said on this list on topics I don't actually know much about with a healthy
dose of salt.


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