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Projects: Stardust@homeInterstellar Dust: The Hunt for the Building Blocks of the Universeby Amir Alexander "We are stardust" --Joni Mitchell. Light-years from any star or planet, in the vast empty stretches that separate one star from the next, space is not quite as empty as it seems. Even in those dark regions, where nothingness prevails, something, nonetheless, is. It is hard to detect, and easy to miss, but astronomers still know that it's there, because it makes trouble. It gets in the way of our observations of distant objects, distorts what we see of others by absorbing certain wavelengths of radiation, and blurs our view of neighboring stars. When we look towards the center of our galaxy, it gets even worse – an impenetrable screen seems to descend between us and that star-rich region, blocking even the brightest stars from our view. Something is definitely out there in that space between the stars, dense in the center of the galaxy, thinner but noticeable in the outlying regions, always interfering with our orderly observations.
Astronomers are a resourceful lot, however, and not much given to brooding over what they might have observed if this troublesome interstellar substance did not get in the way. Instead they focus on the positive, and see what they can learn from the interference itself about this strange substance. And this, it turns out, is quite a lot. First they concluded that this "stuff" is made of tiny grains, because only such particles would reflect and deflect the light in such a way. They also found that the particles affected the different parts of the spectrum of visible light differently – blue light was scattered more than red light, causing objects to appear "redder" than they would otherwise. This meant that the particles were truly miniscule – smaller than the wavelength of visible light, and therefore affecting the shortest wavelengths more than the longer ones. By closely monitoring the precise frequencies of the spectrum that were lost in the passage the passage through empty space, astronomers were actually able to pinpoint the size of different types of these grains, all ranging from less than 1/100th of a micron to 10 microns in size, with each micron representing one millionth of a meter. For comparison, a particle of smoke is about 1/10th of a micron in size. Together, these particles form that most diffuse and insubstantial of substances -- interstellar dust. For observational astronomers interstellar dust is mostly an inconvenience, but for scientists studying the origins and evolution of the universe they are much much more. In fact, these miniscule grains floating in empty space are responsible for much of the world we see around us. According to current theories, the Big Bang, which occurred around 13 billion years ago, created only the simplest and lightest elements – mostly hydrogen, some helium, and traces of lithium and beryllium. The heavier elements that make up our world were all formed later on – in the cores of burning stars. But how were such elements disseminated throughout the universe, ending up on small rocky planets such as ours? The surprising answer is, through interstellar dust. These insignificant-seeming particles flow through interstellar space, carrying with them the essential components that make stars and planets possible. From the Cores of Aging StarsInterstellar dust is formed through several different processes that take place during the lifetime of stars. Our own Sun, for example, is currently about half way through its "main sequence," which is expected to last for another 5 billion years or so. During this stage the Sun "burns" hydrogen in its core, combining hydrogen atoms into helium atoms in a process of nuclear fusion. When this process approaches completion, and a substantial part of the core is composed of helium, the pressure generated by the fusion process in the core diminishes, and the outer layers of the star begin to press in upon the core. This gravitational contraction in turn heats up the core once more, to an even higher degree than before, causing the outer envelopes to heat up and expand. At this point, our familiar Sun will grow to be a Red Giant, encompassing the orbits of Mercury and Venus. The core meanwhile will reach a temperature of 100 million degrees Kelvin, and the helium nuclei will begin fusing into carbon nuclei. At the end of the process a carbon core of the star will be surrounded by a shell where helium is still being fused into carbon, which in turn is surrounded by and envelope where hydrogen is converted into helium. Stars at this stage of their lifespan are known as "Asymptotic Giant Branch" (AGB) stars, and they are highly volatile. Their shells, which apart from hydrogen, helium, and carbon also contain portions of the heavy elements formed in the stellar core, are inherently unstable, and their pulsing creates a withering stellar wind a billion times stronger than the solar wind we know today. In a relatively short time (about 1000 years) the entire shells will be ejected into space, joining the gas and dust in the interstellar medium.
Most stars are likely to share the life sequence of our Sun. Following a prolonged "main sequence" period they will briefly flower into a "red giant," and shed much of their mass before finally fading into a small and astoundingly dense "white dwarf." But truly heavy stars, more than 8 times as massive as the Sun, have an even more dramatic fate in store. Unlike the Sun, which remains in its main sequence stage for 10 billion years, these giants race through their main sequence stage in a fraction of the time. A star of 15 solar masses, for example, goes through its main sequence stage a thousand times faster than the Sun, converting the entire store of hydrogen in their core into helium in only 10 million years! When this process is complete, the star's core contracts under the gravitational pressure of the outer layers, and heats up to a hundred million degrees. The extreme heat and pressure then initiate a new nuclear reaction in the core – the conversion of helium to carbon. Repeated cycles see the core converted into a series of increasingly heavy elements – neon, magnesium, silicon, sulphur, and eventually – iron and nickel. At this point the cycle of conversion ends: iron, unlike the lighter elements that preceded it, does not release energy when it undergoes nuclear reactions, but absorbs it. For a star this means that an iron core no longer produces the internal pressure needed to counteract the weight of the outer layers of the star, pressing in upon it. Within milliseconds, the star then collapses in upon itself, and when its center can contract no more – it produces a catastrophic explosion. This is a supernova, which when seen from Earth, appears as a bright new star in a place where none (or only a very faint star) appeared to be before. A supernova not only blasts the heavy elements formed in the star out into interstellar space, but also initiates its own nuclear reaction, producing even heavier elements. All of this, in the form of fine grains, becomes the stuff of interstellar dust. AGB stars and supernovae of this type (known technically as "type II" supernovae) are two of the most common sources of interstellar dust, though there are others as well. All of them involve the expulsion into space of grains of "heavy" elements formed in the cores of stars. Once they are out floating in the vast emptiness of space, they may join with other dust particles and interstellar gas, and become a swirling cloud of debris. It was precisely such a cloud of gas and dust that condensed to form our Sun and planets, just as similar clouds condensed to form other "second generation" stars. These seemingly insignificant particles of interstellar dust are, in other words, fundamental building blocks of the universe.
The Hunt BeginsThis being the case, it is hardly surprising that scientists have been eager to get a hold of interstellar dust grains, believing that their study could shed much light on some of the "Big" questions of science: where did we come from, how did our world become what it is, and where is it heading? Unfortunately, until the spacecraft Stardust returned with its samples on January 15 of this year, it was not clear where any could be found. In fact, it was not even clear how any particle, if found, could even be identified as a grain interstellar dust.They did have one good lead to go on: when interstellar dust grains form in the cores of stars and are then ejected into space, they carry with them the signature of the event that produced them. Depending on the nature of this event, the proportions of the different isotopes of any given elements are somewhat different. Isotopes are different varieties of the same type of atom, all completely identical in their chemical behavior, but differing slightly in their atomic weight. The most common isotope of oxygen, for example, has an atomic weight of 16, but there are also rarer isotopes with weights of 17 and 18, caused by additional neutrons in their nuclei. According to existing models, the proportions between the different isotopes in a dust grain formed in a supernova will differ from the proportions of the same isotopes in a grain formed in an AGB star, which in turn will differ from isotopic proportions in grains formed in other ways. This, scientists reasoned, should help in identifying these elusive particles if they are, in some way, located. As long as scientists had no good way to measure isotopic proportions, the whole question was, well, "academic." But in the 1950s the first sophisticated mass spectrometers were developed, capable of measuring isotopes in small samples. At first, scientists set their sites on meteorites, looking to see if these object from outer space had their own unique isotopic signature. It quickly became clear, however, that the isotopic composition of meteorites differed not at all from that of the Earth itself. These solar system objects, it seemed, came from the same atomic "soup" that gave birth to the Sun, the Earth, and the nine familiar planets. Far from yielding unusual isotopic distributions, the asteroids provided powerful evidence that the entire solar system was formed of a single relatively homogenous cloud of gas and dust billions of years ago. Burning Down the HaystackFor the next 30 years or so, despite intense effort, nobody was able to identify individual grains of ancient interstellar dust. But in the 1980's scientists at the University of Chicago, led by Ed Anders, and at Washington University in Saint Louis, led by Ernst Zinner, did finally manage to extract grains of highly unusual isotopic composition from meteorites. They used highly corrosive chemical agents to isolate grains that seemed like they could not possibly have come from the relatively homogenous dusty cloud that gave rise to the solar system. Composed of micro diamonds, aluminum oxide, and silicon carbide, the isotopic distribution of these grains bore the telltale marks of their birth: ancient supernovas and red giant stars, that shone brightly and then flickered out, billions of years ago, before the birth of the solar system.The work of Zinner, Anders and their colleagues was a landmark in the study of interstellar dust particles. For the first time, grains from distant stars were available for study by scientists on Earth, and could be compared with the solar system element scientists know so well. Nevertheless, there were serious limitations that had to be kept in mind when studying these unusual dust grains. First there was the matter of the process used to extract the grains from the meteorites in which they were found. The procedure involved the use of extremely corrosive chemical agents, which destroyed most of the rock in order to preserve the interstellar particles within. The process is so extreme that Anders referred to it as "burning down the haystack to find the needle." It is quite possible that just as the chemicals corrode the meteorite, they also destroy some interstellar grains that are not recognized as such. A second, related, problem is that in all likelihood the samples collected in this manner are not at all representative of "normal" interstellar dust particles, and are, in fact, highly unusual. This is because the procedure only recognizes grains as being of interstellar origin when their isotopic proportions are extremely different from the proportions in Earthly minerals. If the proportions in an interstellar dust particle are only moderately different, or actually similar, to what one finds on Earth, this method would never identify the particle as being a grain of interstellar dust. The end result is that by definition, the samples produced through Zinner's chemical process are very atypical, and highly skewed. The only way to obtain a true and unbiased sample of interstellar dust particles is to go to where they are – in space – and collect them there.
Pristine particles of StardustThat is precisely what Stardust did. Two previous missions, Ulysses and Galileo, had already detected the flow of dust particles into the solar system. Scientists could tell it came from interstellar space because it was not affected by the presence of any planet, and because it flowed from precisely the same direction as neutral interstellar gas, which had been detected before. It was, in other words, a stream of interstellar dust flowing right at our doorstep, and Stardust was sent out to collect. On two occasions during its 7 billion mile journey, between February and May of 2000 and again from August to December of 2002, Stardust passed through the dust stream and spread out its collector to the interstellar flow. The stream is so thin, however, that scientists believe that even with seven months of exposure, Stardust probably captured only a few dozen grains of interstellar dust.But this, really, is all that scientists need. With a pristine and unbiased sample of interstellar dust they can truly find out if and by how much the elements of our own world differ from others throughout the galaxy. They can study these relatively recent building blocks of the universe, and compare them to those ancient particles frozen in time for the last 4.5 billion years – since the birth of the solar system. Has the galaxy changed? Is it evolving in a particular direction? Or, to put it in a less scientific but more evocative way – where did we come from and where are we going? The precious particles on Stardust do not have all the answers to these eternal questions. But, scientists believe, they may well provide us with some important clues. |
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