Re: Fw: Population I Vs. Population II Stars at Tom's request

[Tim Folkerts <tfolkert@FHSU.EDU>]

> The globular clusters have the oldest stars and are, yet, metal-poor.
> The disc contains the youngest stars and are metal-rich.  How can the
> oldest stars, which have had a much longer time to go super-nova and
> mix their interiors with the medium in which they are immersed, be
> less metallic?

You might ask an astronomy list, but I'll take a crack at it, having taught
this stuff occasionally.

Some background:
* globular clusters tend to be outside the disk, where there is very little
extra gas. Thus no new stars are being formed.  As a rough analogy
comets:solar system :: globular clusters:milky way.
* the larger stars live and die the fastest; only heavier stars have enough
pressure & temperature to produce metals (up thru Fe) in their cores; only
the heavier stars undergo supernova explosions to spread these metals (and
to produce the elements beyond Fe).
* "metal poor" means "metal poor in the regions that we can easily
observe", i.e we observe the appropriate lines in spectrum of the
photosphere/chromosphere/corona.  Metals in the cores are basically
unobservable.



Consider a cluster of the earliest stars, which formed at about the same
time.  Since there had been no SN's (super novas) previously, they
initially contain only primordial H & He.  Some of the heavier stars will
SN and seed the surrounding volume with metals.  However, the solar winds
of the existing stars will tend to blow away any external gas, so these
metals don't get gathered into any existing star to be observed
spectroscopically.  The lighter stars that still are around don't produce
metals.  So you don't ever see metals in any of these stars.

In the disk, the gas from SN explosions can combine with interstellar gas.
When this gas condenses into a _new_ star, that star has a premixture of
metal atoms throughout.  The metal atoms in the stellar atmosphere will
then be observable.  The key is that you need _new_ stars forming _after_
the SN.  The only place with significant gas left is in the disk, but away
from the nucleus.


> And, it is observed that these older systems lack any considerable dust
lanes or free gas,
> implying few, if any, supernovae.

I'm not sure, but I suspect there are just no _recent_ SN's.  Since these
stars in clusters formed many billion years ago, the supernovas would also
be many billion years ago.  Any "crab nebulas" would long ago have
dissipated  and/or cooled.


Tim Folkerts