simulating star formation (was: Re: Hubble telescope finds'never-seen' galaxies)
Jonathan Thornburg
Subject: simulating star formation (was: Re: Hubble telescope finds 'never-seen' galaxies)
Newsgroups: sci.astro.research
References: <mt2.0-16612...@hydra.herts.ac.uk> <mt2.0-25391...@hydra.herts.ac.uk> <mt2.0-10506...@hydra.herts.ac.uk> <mt2.0-17527...@hydra.herts.ac.uk> <mt2.0-17748...@hydra.herts.ac.uk>
Oh No <No...@charlesfrancis.wanadoo.co.uk> wrote:
> Actually I think the equations of motion leading to formation of stars
> and galaxies are well understood. It is, after all, just a classical
> process, and there is no particular reason to think that computer models
> would be wildly inaccurate in modelling such a process.
Alas, life isn't quite that nice. In fact, there are at least two really
hard problems that come up in trying to model star formation
(I suspect the situation is *worse* for galaxy formation):
(1) What are the initial conditions? What is the statistical distribution
of the masses, densities, temperatures, and chemical compositions of,
and the magnetic fields in, a star-forming region? Lacking high-quality
observational data, we're forced to use educated guesses and crude
theoretical models to set the initial conditions for our simulations.
(2) The star-formation process may well be turbulent, so even if we know
the initial conditions, an accurate model would require a *huge* dynamic
range, and would thus be very hard to simulate (i.e., it would require
a huge amount of CPU time & memory). The bombheads may have supercomputers
with 10^5 CPUs, 100s of terabytes of memory and petabytes of disk, but
astronomers don't usually get access to such beasts. :( And writing
code that so perfectly parallelizes as to get good speedup on large
numbers of CPUs isn't easy, either...
Lots of researchers are working hard on these problems -- if you search
on ADS or Annual Reviews or astro-ph or your other favorite astronomy-
-research site for the keywords "star formation" and "model" you'll
find lots and lots of references. The very first reference I found
on Annual Review just now is
Annual Review of Astronomy and Astrophysics
Vol. 45: 481-563 (Volume publication date September 2007)
(doi:10.1146/annurev.astro.44.051905.092549)
"Toward Understanding Massive Star Formation"
Hans Zinnecker and Harold W. Yorke
http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.astro.44.051905.092549
The abstract of
Annual Review of Astronomy and Astrophysics
Vol. 42: 79-118 (Volume publication date September 2004)
(doi:10.1146/annurev.astro.42.053102.134034)
"THE FIRST STARS"
Volker Bromm
http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.astro.42.053102.134034
also looks relevant here:
We review recent theoretical results on the formation of the first
stars in the universe, and emphasize related open questions. In
particular, we discuss the initial conditions for Population III
star formation, as given by variants of the cold dark matter
cosmology. Numerical simulations have investigated the collapse and
the fragmentation of metal-free gas, showing that the first stars
were predominantly very massive. The exact determination of the
stellar masses, and the precise form of the primordial initial mass
function, is still hampered by our limited understanding of the
accretion physics and the protostellar feedback effects. We address
the importance of heavy elements in bringing about the transition
from an early star formation mode dominated by massive stars to the
familiar mode dominated by low-mass stars at later times. We show
how complementary observations, both at high redshifts and in our
local cosmic neighborhood, can be utilized to probe the first epoch
of star formation.
I think assuming that just because it's "classical" we can necessarily
make accurate computer models, is alas seriously over-optimistic. I
am emphatically *not* an expert on star formation or models of same,
so I can't say how likely it is that today's computer models might be
"wildly inaccurate".
--
-- "Jonathan Thornburg [remove -animal to reply]" <jth...@astro.indiana-zebra.edu>
Dept of Astronomy, Indiana University, Bloomington, Indiana, USA
"If the triangles made a god, it would have three sides." -- Voltaire
Oh No
>From: "Jonathan Thornburg [remove -animal to reply]"
><jth...@astro.indiana-zebra.edu>
>Subject: simulating star formation (was: Re: Hubble telescope finds
>'never-seen' galaxies)
>Newsgroups: sci.astro.research
>References: <mt2.0-16612...@hydra.herts.ac.uk> <mt2.0-2539
>s.ac.uk> <mt2.0-17527...@hydra.herts.ac.uk> <mt2.0-17748-1
>2608...@hydra.herts.ac.uk>
>
>Oh No <No...@charlesfrancis.wanadoo.co.uk> wrote:
>> Actually I think the equations of motion leading to formation of stars
>> and galaxies are well understood. It is, after all, just a classical
>> process, and there is no particular reason to think that computer models
>> would be wildly inaccurate in modelling such a process.
>
>Alas, life isn't quite that nice. In fact, there are at least two really
>hard problems that come up in trying to model star formation
>(I suspect the situation is *worse* for galaxy formation):
>(1) What are the initial conditions? What is the statistical distribution
> of the masses, densities, temperatures, and chemical compositions of,
> and the magnetic fields in, a star-forming region? Lacking high-quality
> observational data, we're forced to use educated guesses and crude
> theoretical models to set the initial conditions for our simulations.
It may be educated guess, but actually I think we know the initial
condition. The early universe expanded from a singularity leading to
isotropic conditions. Isotropy is observed, but also I think there are
strong theoretical reasons based in quantum theory that isotropy is
necessarily the initial condition (admittedly it is not standard
cosmology to produce such an argument). Isotropy is unstable under
gravity, so we would expect huge gas clouds to collapse into the first
wave of star formation.
>(2) The star-formation process may well be turbulent, so even if we know
> the initial conditions, an accurate model would require a *huge* dynamic
> range, and would thus be very hard to simulate (i.e., it would require
> a huge amount of CPU time & memory). The bombheads may have supercomputers
> with 10^5 CPUs, 100s of terabytes of memory and petabytes of disk, but
> astronomers don't usually get access to such beasts. :( And writing
> code that so perfectly parallelizes as to get good speedup on large
> numbers of CPUs isn't easy, either...
I don't think we could assume from isotropy that the first wave of star
formation would be turbulent, but you rightly point out that these first
stars would have been very massive, and hence short lived. Pretty
quickly we would have enormous supernova explosions (seen as gamma ray
bursts, I believe). This would have created boundaries of accelerated
star formation where gas clouds collide, and I suspect is what may have
lead to the observed large scale foamy structure.
>
>I think assuming that just because it's "classical" we can necessarily
>make accurate computer models, is alas seriously over-optimistic. I
>am emphatically *not* an expert on star formation or models of same,
>so I can't say how likely it is that today's computer models might be
>"wildly inaccurate".
>
likelihood is not easy for anyone to quantify. You are talking here of
star formation, and what is of greater interest is galaxy formation,
which must have been later.
Regards
--
Charles Francis
moderator sci.physics.foundations.
charles (dot) e (dot) h (dot) francis (at) googlemail.com (remove spaces and
braces)
Gordon Stangler
>
> likelihood is not easy for anyone to quantify. You are talking here of
> star formation, and what is of greater interest is galaxy formation,
> which must have been later.
>
Why must it have been later? Galaxies contain much more then just
stars.
A galaxy has several star collections, either sorted by age (Type I
and Type II stars), location (open or globular clusters), clouds of
dust and gas, hot clouds, cold clouds, dark matter, and many larger
galaxies contain black holes. AFAIK, irregular galaxies may or may
not contain black holes.
It is conceivable that materials gathered together in a proto-galactic
pile, then as stars formed (Type II or Type III, your call), the rest
of the galaxy evolved alongside them. Depending upon initial
conditions, as well as subsequent formation events, it could change
the galaxy dynamically.