osmic dust grains in our Shrouds of the Night (Figure 23) - how large are they? Do such dust grains need to be comparatively large to effectively obscure distant starlight? The answer is decidedly: no! How highly effective is fog in obscuring homes in a valley, yet its particle sizes are truly minuscule.
In the warmer dust clouds (whose temperature is about 60 degrees above absolute zero, or minus 213 degrees Centigrade), the silicate grains are on the order of 1/100th of a micron in size (recall that one micron is a thousandth of a millimeter), whereas in very cold dust clouds (20 degrees above absolute zero, or minus 253 degrees Centigrade), the sizes are typically 1/10th of one micron, so they are minute indeed. A comparison may be helpful.
An ordinary snowflake - one of billions which may gently drift downward during winter - is 25 000 times larger - snowflakes are typically of the order of 2500 microns across. A snow-flake is a veritable immensity compared to one grain of cold cosmic dust. Furthermore, the nucleus at the center of a snowflake measures one micron; the aerosols in the atmosphere around which a snowflake crystallizes measures 1/10th of a micron. Cold grains in dusty Shrouds of the Night, 1/10th of a micron in size, are truly like aerosols - the mists of a fog.
Cosmic dust pioneer Mayo Greenberg devoted his entire life to the study of these minute laboratories in space. The legendary Greenberg never developed theories to explain existing observations; rather, he developed theoretical models and predicted scores of observations. One may think of other great scientists who followed similar patterns; the deflection of starlight around our Sun was predicted by Einstein before it was observed. Greenberg was a mentor to one of us, David, for many years, extending the warmest of hospitality at his home Chemical Factories in Leiden. Greenberg predicted that cold cosmic dust would pervade space, at a time when 43
his contemporaries regarded dust as a "nuisance;" a topic of small relevance, to be avoided. The giant Greenberg thought otherwise.
Greenberg always likened minute dust particles to those of smoke particles; Leiden astronomers Jan Oort and Henk van de Hulst also referred to cosmic smoke, and for good reason. The cold dust particles are approximately the same size as smoke particles exhaled by a cigarette smoker.
Stand in a room and look at a light-bulb. Next, fill the room with smoke. The dimming of the light-bulb is immediately apparent. The dimming of light in a smoke-filled room is caused by exactly the same phenomenon as the dimming of the light of distant stars. Each particle of smoke, whether in a home on Earth or lurking in interstellar space, diverts the light which hits it. It does so either by scattering it off in other directions, or by absorbing it, or by a combination of both. Our sky is blue because of the scattering of short-wavelength blue photons from sunlight by particles of gas in our atmosphere.
The starting point for the production of interstellar dust grains is in the atmospheres of cool, older stars. Minute rock-like particles called silicates, are blown from the atmospheres of these stars into surrounding space. These "submicron" particles (smaller than one micron) are blown away from their parent stars, and there, drop down in the coldness of space to temperatures as low as minus 253 degrees Centigrade - sometimes colder!
Herein lies the marvel of the story. These grains are so cold that any ion, molecule or atom from surrounding gases will simply stick to them. Cold dust grains have mantles of ice around them; this was not merely a theoretical postulate but proven to exist by Greenberg, who carefully conducted experiments in his laboratories in the United States and in the Netherlands.
The chemical changes which may occur on the surfaces of these ice mantles are profound. Just as water vapor forms frost on the inside of windows on a cold winter's day, these tiny silicate grains first accrete mantles of frozen water, methane, ammonia, carbon monoxide and many other molecules. When the ice mantles on grains of cosmic dust are exposed to the ultraviolet Chemical Factories radiation from neighboring stars, simple molecules gradually convert into more complex ones. 45
Eventually, many pre-biotic molecules (such as amino acids) are produced. Chemical factories much smaller than a snowflake? Of such are our Shrouds of the Night made.
Common molecules include carbon monoxide and ammonia. These dusty regions have an average density of one hundred to one thousand particles per cubic centimeter of space, whereas the average number of such particles in the vicinity of our Sun is only about one per cubic centimeter. In the dense cores of molecular clouds, the number of particles may be in excess of ten thousand to one million particles per cubic centimeter. The masses of giant molecular clouds range from ten thousand to a few million times the mass of our Sun; these clouds are enormous indeed.
Greenberg was renowned for his famous "yellow stuff" - the complex organic materials produced in the ice mantles of cosmic dust grains. He would produce ice mixtures in laboratories on Earth, and then study the changes in chemistry which occur as these substances are exposed to ultraviolet photons. Ultraviolet lamps were used instead of hot stars! During the lifetime of an interstellar grain of dust, the grain mantles containing organic material are actually subjected to substantial ultraviolet light, leading to chemical and physical modifications of these "first generation" organics as simulated in the laboratory.
The ERA (Exobiology Radiation Assembly) platform on the EURECA satellite offered the opportunity to expose Greenberg's laboratory samples to long-term radiation from the Sun. (The Sun may not be a typical representation of the radiation fields encountered by grains in space, but it provides a significant advantage over any laboratory experiments - not only in long-term exposure, but also in that such an experiment permits simultaneous bombardment of many different samples.)
Greenberg and colleagues prepared this "yellow stuff" by subjecting samples of ice (water, carbon monoxide, ammonia, methane, methyl alcohol and acetylene) to radiation from ultraviolet lamps for periods of between twenty-two and sixty-eight hours. The samples were next deposited onto small blocks of aluminum at a temperature of ten degrees above absolute zero and mounted in the ERA sample carrier. The sealed sample carrier was taken into space Shrouds of the Night on the ERA platform of the EURECA satellite, which remained at an altitude of 500 km; all
46 of the samples were subjected to illumination by the full solar spectrum for a total of four months. Finally, the sample carrier was sealed in space and returned back to Earth - and finally, the University of Leiden. The solar radiation had indeed changed the organic "yellow stuff" to organic "brown stuff," and Greenberg and colleagues could examine the chemical changes using an instrument known as a spectrometer.
Nearby giant molecular complexes may cover considerable portions of a constellation, such as the Orion Molecular Cloud, or the Taurus Molecular Cloud. These local giant molecular clouds are arrayed in a ring. This ring includes many bright stars, and is known to astronomers as Gould's Belt (named after Benjamin Gould, who noted this "circle" in the heavens). Writing from Argentina in 1874, Gould penned these words:
I desire to mention a fact which early attracted and repeatedly compelled my attention during my residence in South America ... [A]great circle or zone of bright stars seems to gird the sky, intersecting with the Milky Way at the Southern Cross, and manifest at all seasons.
Actually, Sir John Herschel had already traced the southern section of Gould's Belt working from the Cape in South Africa. In his observations published in 1847, Herschel described:
the zone of large stars which is marked out by the brilliant constellation of Orion, the bright stars of Canis Major, and almost all the more conspicuous stars of Argo [modern Puppis, Vela, and Carina] - the Cross - the Centaur, Lupus, and Scorpio.
It was Gould who had discerned the belt around the entire sky, and the enigmatic structure bears his name. Early studies of this extensive belt, inclined at some 10-20 degrees to the plane of our Milky Way, were conducted by astronomers Charlier (1916), Seares (1928) and Bok (1931). We recently again gazed upon this extraordinary structure by eye from outside our offices at Mt Stromlo; the air was crisp, the atmosphere steady, and the sight: it was breathtaking to view the Scorpio-Centaurus aggregates of bright stars. We could rekindle the excitement which must have been experienced as photons from this belt reached the eyes Chemical Factories of Herschel, Gould and others. 47
The Gould Belt contains many fiery hot blue stars in the neighborhood of the Sun, such as the Cassiopeia-Taurus associations which include the Pleiades star cluster. It is believed to be part of the local spiral arm of our Galaxy to which the Sun belongs; in other words, our Sun is presently crossing the Gould Belt in its orbit about the center of our Milky Way. The age of the Gould Belt is estimated to lie between forty-five and ninety million years. Molecular cloud complexes such as those in Aquila, Cepheus, Cassiopeia, Perseus and Vela all appear to be part of the Gould Belt.
In giant molecular clouds, dust grains may coagulate ("stick together"): the largest dust grains will lurk in the interiors of the more massive and colder dust clouds, where shielding from the rays of nearby stars is highly effective. The temperatures of dust grains in these environs may be very cold indeed - possibly only a few degrees above absolute zero.
Carbon based star dust is not at all far removed from our everyday existence, for it is from these very dusty shrouds that the starry hosts are born - and it is indeed the stuff of which every reader is made.
Professor Owen Gingerich at Harvard University wonderfully links his thoughts to a story of the three little bears: "Clearly we live in a Universe with a history, a very long history, and things are being worked out over unimaginably long ages. We live in an incredibly vast cosmos, something that goes hand-in-hand with a long history. Stars and galaxies have formed, and elements are coming forth from the great stellar cauldrons. Like the little bear's porridge, the elements are just right, the environment is fit for life, and slowly life forms have populated the earth."
Galaxies look supremely different once their masks of cosmic dust are penetrated ... it is the New View of Galaxies ...
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