杏吧原创

THE nearest most of us get to stars is watching them twinkle at night. But
now鈥攊f you know the right people鈥攜ou can hold grains of stardust in
your hands. Sparkling in this stellar debris are grains of diamonds, rubies and
sapphires鈥攋ewels that can help explain the origins of the Sun, the planets
and ourselves.

These cosmic gems come from stars that are the ancient ancestors of our own
star, the Sun. More than 99 per cent of the elements that make up the Earth, the
pages of this magazine, and you, were created in the nuclear hearts of stars
that burnt out long before our Solar System was born. When these stars died,
they released their elements into space as dust and gas. Eventually, this
material clumped together to form our own Solar System, in a building frenzy
that erased most of the chemical memory of its origins. But some dust remains
untouched, locked inside meteorites, frozen in time.

Since their discovery barely a decade ago, these precious grains have fuelled
an explosion of research on everything from the evolution of the Galaxy to the
formation of the Solar System. Weighing less than a nanogram (one billionth of a
gram), they are telling us about stars that weigh 10²⁸ tonnes or more鈥攖he
stars from which we were all ultimately born.

The first hints that meteorites might contain pristine stardust came in 1964,
when Grenville Turner and John Reynolds, then at the University of California at
Berkeley, noticed something very unusual about the xenon gas trapped in a
meteorite called Renazzo (named after the village in Italy where it was found).
Renazzo contained unexpectedly large amounts of the heaviest and lightest
isotopes of xenon; a signature now called Xe-HL. The same pattern subsequently
showed up in many other meteorites. At the time, most people thought that high
temperatures during the Solar System鈥檚 birth wiped out all chemical traces of
its past, so the origin of the Xe-HL was a mystery.

In the early 1970s, some researchers speculated that Xe-HL might be a
signature from before the birth of the Solar System. But confirmation did not
come until 1987, when Edward Anders, Roy Lewis and their colleagues at the
University of Chicago discovered that the xenon is trapped inside tiny diamonds
just a few millionths of a millimetre across. The smallest of these diamonds
contain only a few hundred atoms. Because of the strange xenon isotope signature
of the diamonds, Anders and his colleagues suggested that the diamonds came from
a time before the Solar System was born, and that these gems had somehow escaped
the 鈥渆raser鈥� that had wiped out other records of its origins. The hunt was on
for other types of pre-solar grains.

Each of the diamonds could have come from a different part of a single star,
or perhaps from many different stars, but it was hard to tell which. The
diamonds were far too small to examine individually. But following Anders鈥檚
breakthrough, more types of pre-solar grains were discovered in several
different meteorites. Among these are grains of silicon carbide (SiC), which are
the giants of the pre-solar world at up to several thousandths of a millimetre
across.

Even so, analysing them individually is a great challenge. The
ultimate stardust buster is an ion microprobe, which can fire a beam of ions
much finer than a human hair at a tiny pre-solar grain. In 1988, Ernst Zinner at
Washington University in St Louis and his colleagues were the first to turn an
ion microprobe on individual presolar grains. What they discovered was truly
remarkable鈥攖he carbon in the SiC grains from the Australian meteorites
Murray and Murchison had wildly different isotopic signatures from each other
and anything seen before in natural materials.

Carbon has two stable isotopes, carbon-12 and carbon-13. On Earth鈥攁nd
probably in most of the rest of the Solar System鈥攖he ratio of the two only
varies between about 88:1 and 92:1. But in pre-solar grains of SiC, this ratio
ranges from about 2:1 to 7000:1. As far as we know, natural processes on Earth
cannot produce such a range. Only nuclear reactions in stars can do this, and
even then the range found in these grains requires that they formed in and
around several different types of star.

But what kind of stars did they鈥攁nd ultimately we鈥攃ome from? The
best candidates are carbon-rich red giant stars. Astronomers have now measured
the carbon isotopes in about 30 of these stars and their range of isotope ratios
looks just like that for the majority of the SiC grains (20:1 to 80:1). No other
stars have the same distribution. Red giants are typically one to five times as
massive as the Sun, and are approaching the end of their lives. By this stage,
the stars are so bloated that they have enormous, but relatively cool,
atmospheres which allow SiC and other grains to form. They also generate strong
winds that drive off the gas and dust from their surfaces into space.
Eventually, they will blow off their entire atmosphere, leaving a white dwarf
star behind.

Astrophysicist Roberto Gallino of the University of Turin in Italy has long
been interested in red giants. Since 1991, when they first heard of the SiC
grains, Gallino and his colleagues have been comparing the lab data for
pre-solar SiC to their theoretical predictions for the isotopic compositions of
red giants鈥� atmospheres. They have found that the isotopic composition of many
elements incorporated in the SiC agrees very closely with predictions.

Galactic puzzle

But the isotopes of silicon didn鈥檛 fit quite so easily. Rather than recording
the direct formation of silicon in their stars, the silicon isotopes seem to be
tracing the isotopic evolution of the whole Galaxy. As time goes on, the
chemical composition of the Galaxy changes. The very earliest stars were formed
from the simple elements produced in the big bang鈥攎ainly hydrogen and
helium. All the rest of the elements have been produced from these primordial
building blocks by succeeding generations of stars. So stars that formed at
different times started off with different proportions of the elements鈥攁nd
the younger the stars are, the more heavy elements they contain.

The same is true for isotopes: the lightest isotope of silicon, silicon-28,
forms relatively early in galactic evolution and its heavier isotopes,
silicon-29 and silicon-30, increase in abundance over time. The silicon isotopes
show that SiC in meteorites must have come from at least several tens of stars
with very different apparent ages.

But the silicon isotopes pose a puzzle. Compared with the Sun, most of the
grains have relatively more of the heavy silicon isotopes. So this simple
picture of the evolution of the Galaxy suggests they formed from stars that were
younger than the Sun. Yet the grains existed when the Sun was formed, so they
must be older.

There are at least two ways out of this conundrum. Chemical evolution is not
a smooth, uniform process throughout the Galaxy, and the Sun could have formed
in a local region that had a slightly unusual composition. Alternatively, we
know from astronomical observations that different regions of the Galaxy have
evolved differently according to their distance from the galactic centre. Close
to the centre, where the density of matter is greatest, stars form faster. So
the central region has evolved more quickly than in the more rarefied outer
reaches, where the Sun formed. In 1998, Donald Clayton of Clemson University in
South Carolina suggested that the SiC grains might be a hitchhiker鈥檚 guide to
the Galaxy, telling us about the movement of stars before the Sun formed. Highly
evolved stars from the central regions of the Galaxy may have moved outwards,
seeding our own pre-solar cloud with SiC.

Since the discovery of SiC, many other kinds of pre-solar grain have been
reported, but researchers believe the menu is far from complete. SiC is produced
towards the end of a red giant鈥檚 life, but what grains are made earlier on? The
grains that form at this stage are mostly destroyed in the brutal acid
treatments used to isolate the known pre-solar grains a process that Anders once
likened to burning down the haystack to find the needle. To date, the only
pre-solar grains found to contain oxygen are the acid-resistant minerals
corundum (the composition of sapphires and rubies) spinel, hibonite and rutile,
which can also be precious or semi-precious stones.

Nearly all these oxygen-rich grains have oxygen isotope compositions that fit
with the notion that they too came from red giant stars. They can鈥檛 have come
from a single star鈥攖he isotope ratios vary too much. And the strange thing
is that they don鈥檛 seem to have come from the same stars as the SiC grains.
Using Gallino鈥檚 models of stellar evolution, the SiC seems to come from
chemically more evolved stars than the oxide grains. Yet the same stars that
produced SiC should have also produced oxide grains. Where are these grains? Are
they missing, or are our models wrong? For now, no one knows.

What we do know from the SiC and oxide grains is that at least some of our
terrestrial elements must have come from a variety of red giants. But there is
more to the story of our origins than this. Some grains have been found that
appear to have formed from the cloud of debris produced by a supernova
explosion. Around 1 per cent of SiC grains, called grains X, have isotope
compositions that are completely different to the red giant majority.

For example, they are very enriched in carbon-12, rather than carbon-13, a
signature astrophysicists expect of a supernova. Other possible supernova grains
that have been identified include graphite, corundum and silicon nitride. The
grains best fit predictions for a so-called type II supernova. This occurs in
massive stars more than 10 times as heavy as our Sun when their cores collapse
to form an ultra-dense neutron star, or even a black hole. The collapse releases
an enormous amount of energy, and produces a shockwave that blows off most of
the star.

Which brings us back to the diamonds. Though they were the first of the
grains to be discovered, they have proved to be the toughest pre-solar nuts to
crack. Unlike SiC and most of the other minerals, the diamonds are far too small
to analyse individually. And yet they are the most abundant pre-solar mineral in
meteorites. They may also be the most important, at least in terms of their
sheer mass in the Galaxy. Astronomers Lou Allamandola, Scott Sandford and
colleagues at NASA鈥檚 Ames Research Center in Moffet Field, California, suggested
in 1992 that the large clouds of gas and dust floating between stars may contain
vast amounts of nanodiamonds. If they are right, up to 20 per cent of the carbon
may be in the form of diamonds, a staggering amount even by comparison with the
carats salted away in vaults around the world. Where did all these diamonds come
from?

One in a million

Although only one cosmic diamond in a million contains even a single atom of
xenon, this element still gives us the best clues we have to the origin of these
gems. No known single nucleosynthetic process can form Xe-HL. Xe-L, which
represents enrichment in the lightest isotopes, is formed in places where there
are large numbers of free protons. Xe-H, the heaviest, is formed during a
process involving reactions with neutrons over a very short timescale. Both of
these energetic events are associated with supernovae, but they occur in
different regions of the star. Last year, researchers at Britain鈥檚 Open
University, led by Sasha Verchovsky, and in the US, led by Alex Meshik at
Washington University in St Louis, managed to separate some of the Xe-L from the
Xe-H, confirming that they came from different parts of a supernova.

However, if all the diamonds came from supernovae, we would expect to see
other strange isotopic signatures, including evidence that they contained
numerous highly radioactive isotopes when they formed. Uli Ott from the Max
Planck Institute for Chemistry in Mainz, Germany, and his colleagues have been
searching for these signatures for the past few years. In 1998, they reported a
very unusual isotopic composition for the element tellurium, which is also
consistent with a supernova origin, but effects in other elements remain
elusive.

Supernovae probably produced only a small proportion of the diamonds, with
the majority coming from somewhere else entirely. One possibility is that they
originated in the same kind of stars鈥攃arbon-rich red giants鈥攖hat
produce the SiC. In 1997, Hugh Hill from NASA鈥檚 God-dard Space Flight Center in
Greenbelt, Maryland, and his colleagues studied the infrared spectra of diamonds
in meteorites and realised that they looked a lot like spectra that had already
been seen around red giant stars. This suggests that red giants may also have
contributed diamonds to our Solar System.

All these tiny gems are helping us to understand how stars and the Galaxy
work. But more than that, they have told us something about where we all came
from. Out in the Galaxy are black holes or neutron stars, the remnants of giant
stars whose atmospheres were blown off in dramatic explosions and went on to
form you and me. We also owe our existence to red giants that wandered the
Galaxy before finally yielding their elements in puffs of stellar wind. We are
all, in the end, stardust.