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CONGRESS REPORT
ASTRONOMY AND NUCLEAR PHYSICS
Dust in Space
Harassed housewives and househusbands are always asking themselves why the
shelves in the living room are full of dust yet again, even though they were thoroughly
cleaned just last week. Astrophysicists ask themselves how dust got into the universe
in the first place and what purpose it serves there. More than 300 scientists from all over
the world discussed these issues in the fall at a meeting in Heidelberg entitled ”Cosmic
Dust – Near and Far.” This interdisciplinary meeting was organized by the Heidelbergbased MAX PLANCK INSTITUTES
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P HOTO : NASA/JPL-C ALTECH /K. S U (U NIVERSITY
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The Helix Nebula
looks like a flower of
the cosmos. This cloud
created a star that inflated into a giant at
the end of its life. In
infrared light, the dust
(colored green) shows
up particularly clearly.
very four years or so, dust researchers convene at a conference like this one to discuss every
aspect of this eminently important
ingredient of the universe. Where
does cosmic dust come from? How is
it distributed in galaxies? What does
comet dust reveal about the emergence of our solar system? These are
just some of the questions the scientists are trying to answer.
This year, the conference took place
in Germany for the first time. The organizers could not have found a more
suitable place than Heidelberg. The
Max Planck Institute for Astronomy
has always been dedicated to astronomical observations in the infrared
range of the spectrum, with a special
focus on dust in star-forming regions.
On the other hand, the Max Planck
Institute for Nuclear Physics enjoys
an outstanding reputation worldwide
for the analysis of meteorites and
moon rock, and has been building
dust detectors for space probes for a
long time. These include Giotto (Halley’s comet), Galileo (Jupiter) and
Cassini (Saturn).
When dust is heated by stars in its
vicinity, it emits infrared radiation. The
total mass of the dust can be calculated from the intensity of the radiation.
The further away a galaxy is, however, the more difficult verification becomes. It was thus hailed as a great
breakthrough when, a few years ago,
scientists succeeded in measuring the
thermal emission of dust in the most
distant known quasars. The latter are
the ultrabright central regions of galaxies in which a black hole heats up
the gas surrounding it and makes it
shine. An area approximately the size
of our solar system emits radiation
that can be several thousand times
brighter than all the stars in our Milky
Way together. This is why these celestial bodies can be observed from distances of many billions of light-years.
The most distant quasar discovered
to date, called SDSS J1148+5251,
emitted the light we receive from it
today when the universe was 870 million years old. What astrophysicists
are looking at here is essentially the
nursery of the universe, currently assumed to be 13.7 billion years old. As
Fabian Walter from the Max Planck
Institute for Astronomy reported, it is
possible to show that this quasar contains huge quantities of dust equivalent to several hundred million solar
masses. That is approximately as
much as in the entire Milky Way.
It is not only the hot quasar that
makes the dust shine, but also the
many young stars that emerge from
the dust. Estimates give an annual
birth rate of around 3,000 stars with
the mass of our Sun. The stars being
created in the Milky Way have a total of just about five solar masses.
THE BIG BANG PRODUCED
ONLY LIGHT ELEMENTS
To the amazement of the astrophysicists, dust in these quantities has
been found around several other
quasars in the young universe. We
know today that hydrogen and helium, the lightest elements, were almost the only elements created in
the Big Bang. These volatile materi-
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CONGRESS REPORT
als cannot form dust particles. This
requires heavier elements, such as
carbon, oxygen or silicon. Therefore,
these elements must have been produced in huge quantities before the
quasars flared up. In principle, only
supernovas – exploding stars – could
be considered candidates here.
According to model-based calculations, each supernova would have to
deliver around one solar mass of
dust in order to explain the quantities that have been observed in the
quasars. At the conference, several
astrophysicists saw problems here,
one of which was shown by Isabelle
Cherchneff from the ETH (technical
university) in Zurich.
The first stars can only have consisted of hydrogen and helium. Models predict that this is why they were
considerably more massive than today’s stars with up to 300 solar masses. Through nuclear fusion, they incubated heavy elements in their cores
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The huge billowing clouds
of the Eagle Nebula (left)
provide the raw material
for new stars. In the four
center images of the supernova remnant Cassiopeia A, the colors mark
the different parts (blue:
silicon gas, green: argon
gas, red: dust). On the
right, the Hourglass Nebula surrounds the giant
star Eta Carinae, which
will explode as a supernova in the distant future.
and released them into
the environment when
they exploded. As star
lifetime falls markedly
with increasing mass, the
first-generation
giants
became only a few million years old.
“After two years, the
temperature in the explosion
cloud
had
dropped far enough to allow the first
molecules to form,” says Cherchneff.
Up to one-third of a star’s total mass
is in simple compounds, such as carbon monoxide, molecular oxygen
(O2) or silicon oxide (SiO). These
molecules collide and remain attached to each other and then gradually grow into dust particles.
The efficiency of this process was
the subject of lively discussion. Model-based calculations performed by
several theoreticians say that 1 percent to 20 percent of a star’s mass
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becomes dust – enough to
explain the large quantities in the first quasars.
However, most of the
particles are later destroyed. When a star explodes, a shock wave races
out into space, heating
and compressing the surrounding gas. As a result,
the gas itself sends out a
collision front that returns to the exploded star. In simple terms, the collision front of the supernova is reflected by the surrounding gas and crosses
the dust clouds several tens of thousands of years after the explosion.
The particles it contains are heated to
high temperatures and bombarded
with fast atomic nuclei that are
caught up in the collision front.
How many of the dust particles survive this inferno is the subject of research. According to model-based calculations by Takashi Kozasa from the
University of Hokkaido, all the particles measuring up to 0.05 micrometers
(thousandths of a millimeter) in diameter are completely destroyed. The extent of the destruction grows with the
bombardment of the atomic nuclei,
and thus with the density of the gases
containing the dust.
As neither the particle size distribution nor the gas densities are
known, the model simulations deliver
an unsurprisingly wide variety of solutions. Simone Bianchi from the Astrophysical Institute in Florence concludes that “not even 10 percent of
the dust created survives the return
shock wave.” Supernovas are therefore very effective dust factories, but
most of their product is very shortlived by cosmic standards. The first
stars would have been able to contribute the necessary amount of dust
to the young universe only if most of
them had indeed been more massive
than their successors are today.
For the foreseeable future, it will not
be possible to observe the dust produced by the first generation of stars
directly, as it is far too dim. It is already extremely difficult to find dust
in supernova clouds close to Earth.
This was done successfully for the first
time only in 1987, in the Large Magellanic Cloud, which is approximately
163,000 light-years away.
A year after supernova 1987A, astronomers watched how the first dust
arose in the gas cloud. However, this
phase was already complete after
one further year. “There is only a
very small window of time in which
the dust can be created,” says Cherchneff. The reason seems plausible
enough: in the expanding explosion
cloud, the temperature must fall to
around 1,200 degrees Celsius before
permanent particles can form. As expansion continues, however, the material is rarefied more and more, so
that particles collide only rarely and
cannot continue to grow. Since supernova 1987A, it has only been
possible to show dust in a few other
cases of explosion clouds. A team of
astronomers working with Jeonghee
Rho from the California Institute of
Technology in Pasadena scored the
most recent success. Using the
Spitzer space telescope at the end of
2007, the team found dust clouds in
the supernova remnant Cassiopeia A,
which is around 11,000 light-years
away, and in two other objects. As
Rho reported, the total dust mass in
the three objects amounts to a few
hundredths of a solar mass. That falls
short by more than a power of ten
from what the theoreticians expect
of the first generation of stars.
Either the first supernovas were the
more productive dust factories, which
is possible due to their presumably
higher masses, or there were other
sources of dust in the young universe
after all. “Maybe the black holes in
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CONGRESS REPORT
ASTRONOMY AND NUCLEAR PHYSICS
P HOTOS : P ETER H OPPE
Left: Isotope diagram of presolar SiC particles.
Black and green mark giant stars with 1.5 to 3 solar
masses and varying chemical composition;
supernovas and novas are shown in light blue;
carbon-rich stars are red.
Silicate
Al2O3
D IAGRAM : P ETER H OPPE
SiC
1 m
the centers of the quasars played a
role here that we are as yet unaware
of,” speculates Isabelle Cherchneff.
In the universe of today, there is a
second significant source of dust:
stellar wind. At the end of their lives,
massive stars inflate to become giants and eject some of their outer
envelope into space. “Dust particles
form just a short distance away from
the star, and pressure from the starlight drives them out into space,” explains Susanne Hoefner from the
University of Uppsala. A famous example of this is Eta Carinae, which is
7,700 light-years away and is possibly the most massive and brightest
star in the Milky Way.
When a star like this eventually
explodes as a supernova, which is
what researchers expect Eta Carinae
to do, the shock wave destroys most
of the dust produced previously.
However, stars with less mass, like
our Sun, inflate in their final stage
to become red giants, produce dust
and then simply burn up, ending as
white dwarves. Their dust clouds
can spread out unhindered and thus
enter the interstellar medium. Eli
Dwek from the Goddard Space
Flight Center at NASA sums up our
current knowledge thus: “In the
universe of today, stars with less
mass, from 2 to 5 solar masses,
dominate dust production.”
However, the first stars were unable to do this because their outer
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envelope consisted only of hydrogen
and helium. They had to manufacture the heavy elements first and
discharge them into space, where
they acted as raw material for subsequent generations.
Once the dust has escaped the
gravitational field of its star, it moves
freely through interstellar space.
Should it find its way into one of the
spiral arms of the Milky Way, it collects there. This is because these arms
disrupt the general gravitational field
of the galaxy and slow the material
down. Spiral arms can be pictured as
interstellar jams where the dust collects and thickens into large clouds.
COVETED MATERIAL FROM
THE NEW BORN STAGE
If one of these clouds exceeds a certain size, it contracts under its own
gravity. As it decreases in size, it rotates faster and faster until, finally,
the centrifugal force pulls it into the
shape of a flat disk. A star is created
at the center, and large planets and
smaller asteroids and comet bodies
condense in the surrounding disk.
One fascinating aspect is that material dating from the emergence of
the solar system remains unchanged
in some comets and meteorites. Planetary rock, on the other hand, particularly on Earth, undergoes significant changes with the effects of
wind, weather and plate tectonics.
But how do the researchers access
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Bottom: Scanning electron microscope images
of presolar dust particles found in meteorites.
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the highly desirable original material? There are several options: First,
there are large numbers of meteorites on Earth. These are fragments of
asteroids that have left their original
orbit, which is mainly between Mars
and Jupiter, and collided with the
Earth. There has been comet dust in
terrestrial laboratories for years.
Comet particles enter the upper atmosphere, which slows them down,
and they float slowly down to the
ground. Researchers estimate the total annual mass at 30,000 tons. These
particles can be collected by highflying aircraft with specially developed traps. In these cases, however,
it is not known from which comet
the particles originate.
This situation changed fundamentally when, in 2006, the Stardust
space probe returned to Earth after
collecting dust from the tail of the
Wild 2 comet. A total of 10,000 particles ranging in size from 1 to 300
micrometers and with a total mass of
one thousandth of a gram were
brought to Earth in this way.
In Heidelberg, the leader of the
Stardust project, Don Brownlee from
the University of Seattle, presented
photos and measurements of these
precious specks of dust. Although
examination of them is still in the
early stages, there are already some
very surprising results. Most of the
material consists of silicate minerals,
such as olivine and pyroxene. They
A false color image
of a 9x9 micrometer
section of the Acfer 094
meteorite. The presolar
particle shows up clearly
with its unusual proportions of the oxygen
isotopes 170 and 160.
contain elements and isotopes in the same proportions as the Sun.
However, some particles
contained clearly distinguishable quantities of such minerals
as forsterite (Mg2SiO4) and enstatite
(MgSiO3), as well as pockets rich in
calcium and aluminum.
These materials are created at temperatures of 1,100 degrees Celsius or
higher. Their presence was thus extremely surprising, because it had
been assumed that the comets had
emerged a long way from the Sun, at
temperatures far below freezing. So
how did the comets have these hightemperature phases?
The researchers do not yet have a
clear answer to this question. Brownlee quoted the work of his colleague
Frank Shu from the University of
California at Berkeley: according to
his theory, powerful X-rays from the
young Sun generated in the surrounding disk a particle wind that
blew material into the outer areas.
The high-temperature minerals were
thus created very close to the Sun,
were then transported behind the
Mars orbit and taken up into the existing comet bodies. The researchers
hope that further analysis of the
Stardust particles will yield more
revelations about the astonishing
composition of the comet material.
Finally, in an impressive lecture,
Peter Hoppe from the Max Planck
Institute for Chemistry
in Mainz described how
the developmental narrative of meteorites and
the interplanetary dust
particles trapped in the upper atmosphere can be reconstructed. These
contain small quantities of dust that
was created before our solar system
was formed in the surroundings of
long-dead stars. The different sources of dust, principally supernovas
and giant stars, leave very characteristic fingerprints called isotope
ratios.
Isotopes are different variants of
an element. The element oxygen, for
example, is characterized by eight
protons in its nucleus. However,
there are three oxygen isotopes with
eight, nine or ten neutrons in the
nucleus. Chemically, isotopes behave
in the same way, but physically they
do not.
GRAPHITE PARTICLES
REVEAL THEIR ORIGINS
In the presolar nebula, these isotopes
occurred in certain frequency ratios,
which are reflected in the solar material and in most meteorites. However, meteorite rock also contains
small crystals in which these isotope
ratios deviate markedly from the solar value. This is caused by the different original sources that produced
isotopes in different proportions. The
isotope ratios calculated by theoreti-
cians in complex computer simulations are also found to occur in the
dust particles.
Silicon carbide (SiC) particles are
the most extensively studied, and
contain, in addition to the main
component, many more elements
and their isotopes. Researchers now
distinguish between several types of
SiC particles whose origins can be
determined with isotope ratios, as
demonstrated by Hoppe. For example, it is possible to use nitrogen
(14N/15N) and carbon (12C/13C) isotope ratios and the isotope ratios of
other elements to unambiguously
identify giant stars with 1 to 3 solar
masses and supernovas and novas
as sources. Graphite particles reveal
their origins with special carbon
isotope ratios.
Entering the isotope ratios measured on a diagram clearly reveals
the fingerprints of the sources. Longgone stars leave traces in the dust,
from which our solar system, and
therefore also our Earth arose.
The conference in Heidelberg
demonstrated how exciting research
into cosmic dust is – and it is also
interesting because astrophysicists,
chemists, geologists and mineralogists all contribute their results. Interdisciplinary cooperation between
these areas does not happen as a
matter of course, and encouraging
it was one of the functions of this
THOMAS BÜHRKE
conference.
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