Updated "Dust Evolution in the Universe" essay
Amara Graps
Hello sci.astro folks,
I updated by "Dust Evolution in the Universe" essay to include roughly a
page of new material regarding the local interstellar medium and our
interplanetary dust cloud.
This essay may be considered as a good place to start to learn something
about the subject of interstellar/interplanetary dust. If you would like
a more in-depth study, you may go to some of the reference material I
list at the end of the essay.
Now that Cassini is on its circuitous way to Saturn, there are more
opportunities to learn about our interplanetary dust environment. If you
are skilled at the art of persuasion and you enjoy challenges, I invite
you to convince upper-echelon officials at large American and European
space agencies to turn on Cassini's dust detector instrument while the
spacecraft is in cruise mode. Asking, pleading and begging (reason
may work, too) are acceptable forms of persuasion and you would have the
interplanetary dust community indebted to you for life if you are
successful.
I dedicate this updated version of my Dust Essay to the deer that I hit
several nights ago while I was driving home. (*)
Amara
(*) I tried not to!! Unfortunately, I was unsuccessful. By the time I
got my car unstuck from skidding off of the road, and went looking for
the deer, it was gone (!), running away (I think) into the darkness from
where it came. I hope that wherever the deer is, it is healing OK (and
me too).
------------------------------------------
DUST EVOLUTION IN THE UNIVERSE
by Amara Graps (aka am...@amara.com)
Latest update: October 1997
Can be retrieved at: http://www.amara.com/ftpstuff/dustevolve.txt
Copyright Amara Graps 1996-7. All rights reserved.
This essay is about the evolution of dust in the Universe. In
particular, I describe some ways that dust forms in dusty clouds, cycles
through solar systems (even comets), through a star's late evolution and
back into a nebula.
Dust particles are vital components of astrophysical processes and
significant constituents of the Universe. For example, they can drive
the mass loss that occurs when a star is nearing the end of its life,
they are essential parts of the early stages of star formation, and they
form planets around other stars. In our own Solar System, dust plays a
major role in the zodiacal light, Saturn's B ring spokes, the outer
diffuse planetary rings at Jupiter, Saturn, Uranus and Neptune, the
resonant dust ring at the Earth, and the overall behavior of comets.
The study of dust is one of those many-faceted research topics that
brings together different scientific fields. In this case, physics
(solid-state, electromagnetic theory, surface physics, statistical
physics, thermal physics), math (fractal math), chemistry (chemical
reactions on grain surfaces), meteoritics, as well as every branch of
astronomy and astrophysics.
As you can tell, I think dust and its evolutionary processes are pretty
fascinating.
-----------------------------------
Some "Dusty" Clouds in the Universe
-----------------------------------
First, there is no such thing as a "standard cloud of gas and dust".
There are different types of nebulae with different physical
causes and processes. One might see the following classifications:
diffuse nebula
infrared (IR) reflection nebula
supernova remnant
molecular cloud
HII regions
photodissociation regions
diffuse clouds
Some distinctions between those types of nebula are that very different
radiation processes are at work. For example, HII regions, like the
Orion Nebula, where a lot of star-formation is taking place, are
characterized as thermal emission nebulae. Supernova remnants, on the
other hand, like the Crab Nebula, are characterized as nonthermal
emission (synchrotron radiation).
-------------
Introduction
-------------
The processing of dust in the Galaxy is a wonderfully rich study. The
dust grains evolve cyclically- chemically, physically and dynamically.
There are at least four kinds of dust populations (Dorschner, J.,
(1996), pg. 487-8)):
* stardust,
* dust in the clouds of the diffuse interstellar medium (ISM),
* dust in molecular clouds, and
* circumstellar dust in young stellar objects and in planetary systems.
The scenario that I describe here originated with J. Mayo Greenberg and
has been expanded upon by many people since then. It is called the grain
"core-mantle" model. It has some pretty good observational and
theoretical arguments supporting it.
The dust processing cycle, in brief, looks like the following
(Dorschner, J., (1996), pg. 487-8). From the stellar winds of evolved
stars, new dust is formed and is injected into interstellar space. Young
stardust is mixed with old, heavily-processed, diffuse ISM dust, and is
subject to passing supernova shocks and ultraviolet radiation. Dusty
clouds form. The protostar environment is a fertile ground for solids on
all size scales from dust grains to planets (Hanner, M.S. 1995) to form.
Star formation in cool molecular clouds become both a *sink* of old dust
and well as a *source* of new dust.
Now in more detail.
---------------------
Dust Grain Formation
---------------------
The large grains start with the silicate particles forming in the
atmospheres of cool stars, and carbon grains in the atmospheres of cool
carbon stars. Stars, which have evolved off the main sequence, and have
entired the giant phase of their evolution, are a major source of dust
grains in the Galaxy.
How do we know that that dust is formed in the envelopes of late-evolved
stars? We have some good observations. An observed (infrared) 9.7 micron
emission silicate signature for cool evolved (oxygen-rich giant) stars.
And an observed (infrared) 11.5 micron emission silicon carbide
signature for cool evolved (carbon-rich giant) stars. These help provide
evidence that the small silicate particles in space came from the outer
envelopes (ejecta) of these stars. (See Humphreys et al, (1972) ApJ 172,
75, and Evans, A. The Dusty Universe, pg 164-167)
How do we know that dust wasn't formed in interstellar space? We know
because the conditions in interstellar space are not suitable for the
formation of silicate cores. The arguments are that: given an observed
typical grain diameter a, the time for a grain to attain a, and given
the temperature of interstellar gas, it would take considerably longer
than the age of Universe for interstellar grains to form (see pg 147-148
of Evans' book). Evans says in his book that grains are seen to form in
the vicinity of nearby stars in real time. "Real time" meaning a) nova
and supernova ejecta, and b) R Coronae Borealis, which seems to eject
discrete clouds containing both gas and dust.
Then supernovae/novae explosions of the star eject that material out
into space. The dust that you see in supernovae remnants (like from M1,
the Crab nebula) was mostly _not_ produced in the supernovae. It was
present in the star's outer envelope before the supernovae explosion.
The explosion just pushed that material outwards.
Note that new grains _can_ form in the debris of the supernovae explosion
(even easier in the debris of a novae explosion) but that material has
to be quite cool, so it takes some time (say, on the order of a year)
after an explosion for the debris to be cool enough for grains to form.
It's a race between dropping temperature, which favors grain formation,
and dropping density, which opposes it.
When the grains have cooled down to about 15K, those silicates are the
cores for the growth of mantles of ices. The mantles are formed by
accretion of (gas phase) atoms and molecules of oxygen, carbon,
nitrogen, sulfur, along with hydrogen. The grains are nonspherical, and
various sticking processes cause other simple molecules (H2O, CO, H2S,
CH3OH, OCS, OCN- etc.) to stick. The mantle ices are always being
photoprocessed by ultraviolet (UV) radiation from either distant stars
or by UV created by cosmic rays or from local hot stars and/or stellar
winds. The photoprocessing changes the basic composition of the ices and
causes some complex organic refractory residues to form.
So now the heavily photoprocessed material is out in a diffuse
environment (diffuse cloud). That cloud may eventually become a denser
molecular cloud. In the denser clouds, more organic mantle formation
occurs, layering the silicate core- like the rings in a tree trunk. The
innermost layers of the dust grain have been the most irradiated, and
the outermost layers are the result of the most recent processes.
Then, within those molecular clouds, critical densities lead to star
formation, and that dust gets captured and becomes part of the new
star, and the cycle stars all over again. A typical grain anywhere in
space will have undergone at least 20 cycles (And remember that, in
addition, new grains are formed.)
------------------------
Dust as a Driving Force
------------------------
Dust is an important driving force in the development of a protostar's
accretion disk. In particular, it has a large effect on the turbulence
and convection in the disk. Before the dust grains settle into a
midplane region, (and before the T-Tauri phase of the protostar) the
homogenized distribution of the grains are a primary cause of opacity,
leading to steep temperature gradients and convection and turbulence.
(Other driving forces for turbulence in this early stage may be the
angular momentum in the Solar Nebula by infalling matter, and the
thermal energy liberated by gravitational collapse.)
-----------------------
Dust Grain Destruction
-----------------------
How are the grains destroyed? There are some ultraviolet processes which
lead to grain "explosions" (d'Hendecourt et al., 1985 Astr Ap, 152, 130;
Greenberg, J.M., 1976, Ap. Space Sci, 39, 9). Evans' book also describes
evaporation, sputtering (when an atom or ion strikes the surface of a
solid with enough momentum to eject atoms from it), and grain-grain
collisions, which have a major influence on the grain size distribution,
as well.
These destructive processes happen in a variety of places. Some grains
are destroyed in the supernovae/novae explosion (and then some grains
form sometime afterwards). Some of the dust is ejected out of the
protostellar disk in the strong stellar winds that occur during a
protostar's active T Tauri phase. Plus there are some pretty
complicated gas-phase processes in a dense cloud where ultraviolet
photons eject energetic electrons from the grains into the gas.
Dust grains incorporated into stars are also destroyed, but only a
relatively small fraction of the mass of a star-forming cloud
actually ends up in stars. This means a typical grain goes through
many molecular clouds and has mantles added and removed many times
before the grain core is destroyed.
---------------------
Dust Grain Recycling
---------------------
I said previously that a typical grain anywhere in space will have
undergone at least 20 cycles. How does this work, since a typical star
lives for 5+ billion years, and the time since the Big Bang is only 15
billion years (say)?
Mayo Greenberg explains (in Greenberg's chapter of the IAU Symposium
#135: Interstellar Dust book) : "The mean star production rate of 12
solar masses per year implies an interstellar medium turnover time of
~5*10^9 years, so that this is the absolute maximum lifetime of a dust
particle no matter how resistant to destruction. If we use a mean
molecular cloud-diffuse cloud period of 2*10^8 years (10^8 years in
each), then a typical grain anywhere in space will have undergone at
least 20 cycles so that, for example, the typical diffuse cloud dust
particle age is greater than 10^9 years and consists of a mix of
particles which have undergone a wide variety of photoprocessing."
So the grain recycling works through molecular clouds with the formation
and destruction of grain mantles. One time in 20 or so, the grain core
gets incorporated into a star, and is destroyed. The other times, the
grain gets ejected, and only the mantle is destroyed.
--------------------------------------------------
Specifics of Dust Input to the Interstellar Medium
--------------------------------------------------
I found an interesting table in Gehrz's chapter (pg 447) of the IAU #135
Interstellar Dust book: "Types of Dust Grains in Stellar Outflows".
Stellar Type Input to Interstellar Medium, Relative to all Stars
M Stars (Miras) 35%
RLOH/IR stars 32%
Carbon stars 20%
Supernovae 8%
M supergiants 4%
Wolf-Rayet stars 0.5%
Planetary Nebulae 0.2%
Novae 0.1%
RV Tauri stars 0.02%
O,B stars 0
Gehrz concludes in his last section titled: "The Ecology of Stardust in
the Galaxy" that:
1. M stars, RLOH/IR stars and M supergiants are the primary sources of
silicates, while carbon stars, WR stars and novae produce most of the
carbon and SiC. Novae, supernovae, and WR stars may be responsible for
most of the grains with chemical anomalies.
2. The current star formation rate implies that star formation is
depleting the interstellar medium (ISM) gas by some 3 to 10 solar masses
per year.
3. There is a deficit in stardust production/grain destruction.
Supernovae shock waves destroy ISM grains on very short time-scales
(Seab, 1987, _Interstellar Processes_, Hollenbach and Thronson ed.
Reidel) processing 10-30 solar masses per year and destroying 0.1-0.3 solar
masses per year in dust. Gehrz estimates that 0.01-0.08 solar masses per
year of dust is returned to the ISM by stars. He feels that grain growth
in dark clouds is an attractive mechanism to make up the dust deficit.
-------------------------------------
The Local Interstellar Medium (LISM)
-------------------------------------
Studies of the local interstellar medium (LISM) during the last 20 years
have yielded a picture of the Sun located in a cool (~7000 K according
to Axford), low-density region (Ferlet, R., et al. 1991) flowing past
our Solar System, this region being immersed in an irregular, very hot
(~million degree K), and tenuous (~0.05 per cubic center), larger, local
bubble of tens to hundreds of parsecs in size. (Axford and Suess, 1995).
The larger local bubble is thought to have been produced by a
combination of one or more supernovae and stellar winds associated with
a group of O and B stars.
One technique to learn about the dusty cloud environment out of which
our Solar System formed is by examining some of the primitive meteorites
that have reached Earth. Primitive meteorites contain a few parts per
million of pristine interstellar grains that provide information on
nuclear and chemical processes in stars (this information is from Anders
and Zinner, 1993). The meteorites' grains' interstellar origin is shown
by isotope ratios that are highly anomalous to isotope ratios of the
same material on Earth. Microdiamonds, of average size 10 angstroms,
contain anomalous noble gases which shows the signature of the nuclear
r- and p processes. Silicon carbide, of grain size ~1 micron, shows the
signature of the s-process, and most likely comes from red giant carbon
(AGB) stars of 1-3 solar masses. Graphite spheres, of size ~1 micron,
contain highly anomalous carbon and noble gases, as well as large
amounts of fossil Mg-26 from the decay of Al-26, which seem to come from
at least three sources: AGB stars, novae and Wolf-Rayet stars.
-------------------------------------------
Interplanetary Dust and the Zodiacal Cloud
------------------------------------------
Closer to home, the interplanetary dust cloud has been studied for many
years in order to understand its nature, origin, and relationship to
solar systems (our own, as well as extrasolar systems).
The interplanetary dust particles (IDP) not only scatter solar light
(called the "zodiacal light", which is confined to the ecliptic plane),
the IDP also produce thermal emission, which is the most prominent
feature of the night sky light in the 5-50 micron wavelength domain
(Levasseur-Regourd, A.C. 1996) The grains characterizing the infrared
emission near the earth's orbit have typical sizes of 10-100 microns
(Backman, D., 1997). The surface area corresponds to a mass of roughly
10^{18}--10^{19} grams for a density of 3 grams-cm^-3, equivalent to a
single solid body 5-10 kilometers in radius.
The sources of IDP include at least: asteroid collisions, cometary
activity and collisions in the inner solar system, Kuiper Belt
collisions, and ISM grains. (Backman, D., 1997)
The main physical processes "affecting" (destruction or expulsion
mechanims) IDP are: expulsion by radiation pressure, inward
Poynting-Robertson (PR) radiation drag, solar wind pressure (with
significant electromagnetic effects), sublimation, mutual collisions,
and the dynamical effects of planets. (Backman, D., 1997)
The lifetimes of these dust particles are very short compared to the
lifetime of the Sun. If one finds grains around a star that is older
than 10^8 years, then the grains *must* have been from recently released
fragments of larger objects, i.e. they cannot be leftover grains from
the protoplanetary nebula. (Backman, private communication) Therefore,
the grains would be "second-generation" dust. The zodiacal dust in our
Solar Sytem is 99.9% second-generation dust, 0.1% intruding ISM dust,
and 0% primodial grains from the Solar Sytem's formation. (Primordial
grains can now only be found embedded in unaltered meteorites.)
The interplanetary dust cloud has a complex structure (Reach, W., 1997).
It has:
* at least 8 dust trails -- source is thought to be short-period
comets,
* at least 5 dust bands -- source is thought to be the asteroid belt,
in particular the three asteroid families: Koronis, Eos, Themis,
* at least 2 resonant dust rings (the Earth resonant dust ring, for
example, but every planet in our Solar System is thought to have a
resonant ring with a "wake", Dermott, S.F. et al., 1994, 1997)
-----------
Comet Dust
-----------
The isotope ratios of comet and interstellar dust are very similar,
indicating a common origin (see R.F. Knacke chapter: "Comet Dust" in IAU
Symposium No. 135, _Interstellar Dust_ ed. Allamandola and Tielens,
publ. by Kluwer, 1989). Interplanetary dust particle isotope ratios are
quite different.
In this section I expand on the dusty link between the molecular clouds
and comets in a solar system.
Long-period comets are often thought to be pristine, unaltered relics
since their formation. They may be the best probes of ancient Solar
System (and pre-solar) processes available to us. (However, some experts
with good observational evidence are challenging this view. See recent
work by S.A. Stern)
A long-period comet in a highly eccentric orbit with a perihelion
distance of only a few solar radii will experience a range of diverse
conditions as it traverses its orbit (Lewis, 1995). Almost all of the
time it will be so far from the Sun that it will be too cold for
evaporation of ices to occur. When it passes through the terrestrial
planet region, evaporation will be rapid enough to blow away small
grains, but the largest grains may resist entrainment and stay behind on
the comet nucleus, beginning the formation of a dust layer. Near
perihelion, the heating and evaporation rate will be so great, that no
dust can be retained.
Therefore, the thickness of dust layers covering the nuclei of a comet
can indicate how closely and how often a comet's perihelion travels are
to the Sun. If a comet has an accumulation of thick volatile-depleted
dust layers, it may have frequent perihelion passages that don't
approach the Sun too closely.
This thick accumulation of dust layers is actually a good description of
virtually all of the periodic *short-period* comets. The general
conclusion of studies by David Brin (UCSD), Harry Houpis (UCSD), Asoka
Mendis (UCSD), Fraser Fanale (U HI), James Salvail (U HI), Paul Weissman
(JPL), Hugh Keiffer (USGS), and others has been that dust layers with
thicknesses of order meters may accumulate on the surfaces of
short-period comet nuclei.
Note that the accumulation of dust layers over time would change the
physical character of the short-period comet. A dust layer both inhibits
the heating of the cometary ices by the Sun (the dust is very opaque and
a poor conductor of heat), and slows the loss of gases from below. A
comet nucleus in an orbit typical of short period comets would quickly
decrease its evaporation rate to the point that neither a coma or tail
would be detectable. Such a body may be described by terrestrial
astronomers as a low-albedo near-Earth asteroid. It would be able to
retain much of its ices over its entire dynamical lifetime (of ~100 M
yr).
Comet dust can provide clues to comets' orgin, and the formation of our
solar system. There are two main models about where comets came from:
1) the interstellar model and 2) the solar system model. The remaining
portion of this essay came from information in Science News 149, June 1,
1996, pg. 346-347.
The interstellar model is the following. It says that ices formed on
dust grains in the dense cloud that preceded the Sun. The mix of ice and
dust then gathers together into a comet without appreciable chemical
modification. J. Mayo Greenberg first proposed this idea in 1986.
In the solar system model, the ices that formed in the interstellar cloud
didn't initially gather together, but vaporized as the ices to become part
of the accretion disk of gas and dust around the protosun. The vaporized
ices later resolidified and assembled into comets. So the comets in this
model would have a little bit different composition than those comets
that were made directly from interstellar ice.
Comet Hyakutake data has been very good so far for providing some clues
to the above.
Michael Mumma's et al's IR observations from the IRTF detected ethane in
the comet. He thought at first that it showed that the comet originated
near Jupiter's orbit, which had a variety of hydrocarbons. Now he thinks
that the comet originated directly from a chilly part of the
interstellar cloud (which can't exceed 20K). He has some confirming
evidence for this theory from the fact that Hyakutake's nucleus has very
little carbon monoxide, which makes chemical sense if the carbon is tied
up in heavier molecules like ethane.
-----------
-----------
See, I'll bet you didn't know before that those annoying little dust
particles are significant constituents of the Universe and vital
components of astrophysical processes.
-----------------------------------------------------------------------
I condensed parts of this essay from three or four shorter dust essays
and one long thread titled "Supernovae Distribution" running on
sci.astro in January 1996. I want to thank Steve Willner in particular
for his comments on that sci.astro thread.
The interplanetary dust sections were derived, in part, from reports
given at the Exozody Workshop, NASA-Ames Research Center, Mountain View,
California, October 23-25, 1997, and from discussions with the
participants before, during, and after the Workshop. In particular, I
want to thank Dana Backman for many of our discussions.
-----------------------------------------------------------------------
----------
References
----------
Allamandola, Lou and A.G.G.M Tielens eds., _IAU Symposium 135:
Interstellar Dust_, 1989, Kluwer Press. (I used Mayo Greenberg's
chapter: "The Core-Mantle Model of Interstellar Grains and the Cosmic
Dust Connection" as a source for the sections of "Dust Grain Formation"
and "Dust Grain Recycling" above.)
Anders, Edward, and Ernst Zinner, (1993), "Interstellar grains in
primitive meteorites- diamond, silicon carbide, and graphite," in
_Meteoritics_, 28, 490-514.
Axford, W.I. and S.T. Suess, "The Heliosphere,"
http://web.mit.edu/afs/athena.mit.edu/org/s/space/www/helio.review/
axford.suess.html
Backman, Dana, Extrasolar Zodiacal Emission - NASA Study Panel Report,
1997 for Exozody Workshop, NASA-Ames, October 23-25, 1997.
http://astrobiology.arc.nasa.gov/workshops/zodiac/backman/backman.txt
Dermott, S.F. Jayaraman, S., Xu, Y.L., Gustafson, A.A.S., Liou, J.C.,
(1994), "A circumsolar ring of asteroid dust in resonant lock with the
Earth," Nature 369, June 30, 1994, pg. 79.
Dermott, S.F., in talk titled "Signatures of Planets in Zodiacal Light,"
Exozody Workshop, NASA-Ames, October 23, 1997.
Dorschner, Johann, "Properties of Interstellar Dust," _Physics,
Chemistry and Dynamics of Interplanetary Dust_, Gustafson, Bo A.S. and
Martha Hanner, ed., , ASP Conference series, Vol 104, 1996, p 487-506.
Evans, Aneurin, _The Dusty Universe_ , Ellis Horwood, 1994. (excellent
reference)
Ferlet, R., A. Vidal-Madjar, et al. (1991). Structure of the
local interstellar medium. (Astrophysics at FUV and EUV
wavelengths; Proceedings of the Topical Meeting of the
Interdisciplinary Scientific Commission E /Meeting E3/ of the
COSPAR 28th Plenary Meeting, The Hague, Netherlands, June 25-July
6, 1990. A92-19151 06-90) Advances in Space Research (ISSN
0273-1177) 11(11, 1991): 81-86.
Greenberg, J. Mayo and J.I. Hage, (1990) "From Interstellar Dust to Comets:
A Unification of Observational Constraints," ApJ 361, 260-274.
Hanner, M.S. 1995, Highlights Astron. 10, 351.
Levasseur-Regourd, A.C., 1996, "Optical and Thermal Properties of
Zodiacal Dust," _Physics, Chemistry and Dynamics of Interplanetary
Dust_, ASP Conference series, Vol 104, 1996., p. 301.
John S. Lewis, _Physics and Chemistry of the Solar System_, Academic
Press, 1995, pp. 285, 292-293
Reach, W., in talk titled "General Structure of the Zodiacal Dust
Cloud," Exozody Workshop, NASA-Ames, October 23, 1997.
--
********************************************************************
Amara Graps email: am...@amara.com
Computational Physics vita: finger agr...@shell5.ba.best.com
Multiplex Answers URL: http://www.amara.com/
********************************************************************
Joseph Zorzin
Amara Graps wrote:
> As you can tell, I think dust and its evolutionary processes are pretty
> fascinating.
>
From dust to dust and ashes to ashes.
Uh.. no I didn't say it first, not sure who did. It's in the bible,
right?
Yes, it is an interesting subject.
What about the dust in my apartment? Does any of that space dust
contribute to Earth dust? How does it magically find it's way under my
sofa? <G>
Peter Besenbruch
On Tue, 28 Oct 1997 06:59:19 -0500, Joseph Zorzin
<red...@forestmeister.com> wrote:
>Amara Graps wrote:
>
>> As you can tell, I think dust and its evolutionary processes are pretty
>> fascinating.
>>
>
>From dust to dust and ashes to ashes.
>
>Uh.. no I didn't say it first, not sure who did. It's in the bible,
>right?
Derived from Genesis 2:7 (then the LORD God formed man from the dust
of the ground, and breathed into his nostrils the breath of life; and
the man became a living being) and 3:19 (By the sweat of your face you
shall eat bread until you return to the ground, for out of it you were
taken; you are dust, and to dust you shall return.")
The first is a description of how God formed the first human. The
second is addressed to the man as a consequence of eating what was
forbidden. I suppose one could extend this to the dust clouds floating
around out there, or in my living room.<g>
___________________________________________________
Hawaiian Astronomical Society http://www.hawastsoc.org
HAS Deepsky Atlas http://www.hawastsoc.org/deepsky
Delete the "#nobulk#." for the true e-mail address.
Amara Graps
Joseph Zorzin wrote:
>
> What about the dust in my apartment? Does any of that space dust
> contribute to Earth dust? How does it magically find it's way under my
> sofa? <G>
Actually I read somewhere that the source of most household dust is
human skin. I don't remember where I read that though...
Amara
--
************************************************************
Amara Graps am...@quake.stanford.edu
Solar Oscillation Investigations Stanford University
http://quake.stanford.edu/~amara/
************************************************************
"Never fight an inanimate object." - P. J. O'Rourke
Peter Besenbruch
>> >From dust to dust and ashes to ashes.
>> >
>> >Uh.. no I didn't say it first, not sure who did. It's in the bible,
>> >right?
>>
>Besenbruch) wrote:
>> Derived from Genesis 2:7 (then the LORD God formed man from the dust
>> of the ground, and breathed into his nostrils the breath of life; and
>> the man became a living being) and 3:19 (By the sweat of your face you
>> shall eat bread until you return to the ground, for out of it you were
>> taken; you are dust, and to dust you shall return.")
>
On Tue, 28 Oct 1997 20:54:26 -0500, dmpa...@clark.net (David Palmer)
wrote:
>It is in the Book of Common Prayer (1662, Church of England); the prayer
>for Internment:
> ...we threfore commit his body to the ground; earth to earth,
> ashes to ashes, dust to dust; in sure and certain hope of
> the Resurrection...
>http://www.frii.com/~rlaribee/bcptext.html
And also in the 1552 and 1549 Prayerbooks. They, in turn got it from
the Committal of Old Sarum, and so on, back to Genesis.
Steve Willner
In article <345646...@quake.stanford.edu>, Amara Graps
<am...@quake.stanford.edu> writes:
> Actually I read somewhere that the source of most household dust is
> human skin. I don't remember where I read that though...
And I read insect parts, although human skin was also a component.
No better source than "somewhere," I'm afraid. I'd bet that the
correct answer is known, though.
--
Steve Willner Phone 617-495-7123 swil...@cfa.harvard.edu
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email is NOT appreciated.)
Jonathan Silverlight
In article <635917$c...@flare.convex.com>, Richard A. Schumacher (schu...@convex.com) writes:
>
>
>>What about the dust in my apartment? Does any of that space dust
>>contribute to Earth dust?
>
>is meteoric (space dust). Wave a strong magnet over house
>dust. Most of what sticks will be residue from nickel
>iron meteorites. Much of what does not will be from
>stony meteorites.
>
I've raised this before but I can't find the response. Isn't most
of the component that sticks to a magnet some sort of manmade
pollution ?
a friend
>------------------------
>Dust as a Driving Force
>------------------------
>
>Dust is an important driving force in the development of a protostar's
>accretion disk. In particular, it has a large effect on the turbulence
>and convection in the disk. Before the dust grains settle into a
>midplane region, (and before the T-Tauri phase of the protostar) the
>homogenized distribution of the grains are a primary cause of opacity,
>leading to steep temperature gradients and convection and turbulence.
>(Other driving forces for turbulence in this early stage may be the
>angular momentum in the Solar Nebula by infalling matter, and the
>thermal energy liberated by gravitational collapse.)
>
too clearly so bear with me, here.
One is the argument I read once that it is the opacity of the material of
stars which supports them. If stars were completely transparent, they would
quickly radiate away the energy generated by fusion in their cores, cool,
and collapse. This is why the center of the sun is millions of degrees hot
while the surface is only thousands of degrees.
Another is the argument that the reason stars may not be arbitrarily massive
flows from the fact that the unit area luminosity of a new star varies as
the cube of the mass of the star. (Thus a star 10 times as massive as
another would be 1000 times as bright per unit of surface area) Thus in
very massive stars the surface is so blue-violet hot that the outer layers
of the star are blown away by the heat and radiation of lower layers. This
process progressively reduces both the mass and luminosity of the star. It
also puts an upper limit on how massive a star can be - about 100 solar
masses.
The third notion is the one quoted above from Amara's essay on dust. It
seems intuitively that a star condensed from a cloud containing a lot of
dust and thus having more opacity would have a higher upper limit to how
massive a star could form.
But since the dust would be not only vaporized but ionized as well in such a
massive star, would the presence of silicon and carbon change the internal
opacity of the star as compared with hydrogen and helium? If it did, would
a star with high internal opacity have higher internal and surface
temperatures than a more transparent star? Does that mean that the upper
limit on how massive a star could form would be less, because of the higher
surface temperature?
After the big bang the early generations of stars presumably condensed
primarily from gas and not from dust, having fewer or no predecessors to
have produced any dust in their outer atmospheres. Do we then see lesser
amounts of heavier-than-helium elements in the spectra of immensely remote
stars? Or is the observational problem too great?
Further, since dustless, relatively transparent stars would have somewhat
lower surface temperatures because of having significantly lower central
temperatures, they could potentially be more massive than more opaque stars
born of dust. This might mean that a larger proportion of them would exceed
the 8 solar mass floor for the formation of black holes at the end of their
short brilliant lives. Or is the logic the opposite - that their greater
transparency means that they would start repelling further infalling
material with radiation at a lower mass? I am confused. Help from the
participants?
Jack Kessler
Steve Willner wrote:
> > If it did, would a star with high internal opacity have higher
> > internal and surface temperatures than a more transparent star?
>
> Any stellar interiors experts around? I think higher opacity gives
> higher internal temperature and lower surface temperature with a
> correspondingly larger radius, but I'm not sure. Somebody must know.
>
> > Does that mean that the upper limit on how massive a star could
> > form would be less, because of the higher surface temperature?
>
You lost me there, Steve. I can see intuitively that increasing opacity of the
star would increase the interior temperature because initially the same amount of
energy would be generated but it would radiate away more slowly, leaving a larger
residual energy/temperature. I can see intuitively also that this would be
compounded because the higher temperature would increase the rate of the fusion
processes producing the energy. I can even see intuitively that if the surface
area of the star were unchanged that the surface temperature would increase
because once the star reaches equilibrium, the energy radiated must equal the
energy generated.
But the increase of radius I do not see, short of simply applying Charles' Gas
Laws to a whole star. Can it really be that simple - that if two stars have the
same mass but different internal temperatures, that the hotter one will be larger
simply because it is hotter? It makes sense - it just is not comfortably
intuitive.