Star dust

C. C. Patterson. Letter to Edmund S. Muskie. Oct. 7, 1965. in Caltech PP, Box 16.13. Source for the letter about preparing to testily surprise to medicine in 1920s Kehoe is out of date, believes in star-dust and sharp line. 

Stardust February 7, 1999, saw the start of NASA s Stardust mission the cometary probe, the first mission to collect cosmic dust and return the sample to Earth, has a time-of-flight mass spectrometer (CIDA, Cometary and Interstellar Dust Analyser) on board. This analyses the ions which are formed when cosmic dust particles hit the instrument s surface. In June 2004, the probe reached its goal, the comet 8 IPAVild 2, getting as close as 236 km The CIDA instrument, which was developed at the Max Planck Institute for Extraterrestrial Physics in Garching (near Munich), studied both cometary dust and interstellar star dust. 

Earth. According to Ott (1993), the SiC grains can be regarded as star dust , prob-... 

The Allende, Murchison, Murray and Orgueil meteorites are particularly highly prized for research into stellar grains, since several kilograms of this material have been identified in each of them. This is sufficient to be able to take samples of the order of 1 g without damaging the source. Such samples can then be subjected to compositional analysis. But how can we extract these stellar jewels, measuring at most 1 /rm in diameter, from the matrix in which they are embedded The best way of finding a needle in a haystack is to bum the hay. Cosmochemists employ basically the same method when they use chemical processes to isolate star dust trapped in meteoritic stone. They may then analyse... 

Each known type of grain is made from a particularly refractory form of material. Themselves born in extreme heat conditions, these grains survived the formation of the Solar System without the least difficulty. They have been able to carry down the isotopic composition of their source quite intact, throughout the whole prehistory of the Sun. But their message has not yet been perfectly decoded. The story of this star dust will therefore be continued, especially as it is radioactive and can be identified by its gamma emissions. 

A We also developed laser-desorption laser-ionization mass spectrometry for the analysis of adsorbates on surfaces, such as interplanetary dust particles and meteoritic samples. We use one laser to rapidly heat the sample and evaporate molecules from the surface. A second laser intercepts the rising plume of molecules and ionizes those that absorb that color of light. We then weigh the ions using a mass spectrometer. We have analyzed graphite particles extracted from meteorites and found polycyclic aromatic molecules (PAHs). The PAHs have to isotope ratios that match closely the graphite grains, which are believed to be the remnants of the star dust from which our solar system condensed some 4.5 billion years ago. These are the first interstellar molecules observed directly in the laboratory. 

Genuine star dust is preserved in meteorites. Most of the presolar grains comes from RG and from 0-rich and C-rich AGB stars. Dust from supemovae and novae has also been found. Elemental, isotopic, and structural analyses of this star dust gives details on stellar nucleosynthesis and dust formation conditions in the circumstellar environments. 

In the dense interstellar medium characteristic of sites of star fonuation, for example, scattering of visible/UV light by sub-micron-sized dust grains makes molecular clouds optically opaque and lowers their internal temperature to only a few tens of Kelvin. The thenual radiation from such objects therefore peaks in the FIR and only becomes optically thin at even longer wavelengths. Rotational motions of small molecules and rovibrational transitions of larger species and clusters thus provide, in many cases, the only or the most powerfiil probes of the dense, cold gas and dust of the interstellar medium. 

The cleaning cycles are usually controlled by a timing device which deactivates the section being cleaned. The dusts removed during cleaning are collected in a hopper at the bottom of the baghouse and then removed, through an air lock or star valve, to a bin for ultimate disposal. 

The entire observable universe, of which the Earth is a veiy tiny part, contains matter m the form of stars, planets, and other objects scattered in space, such as particles ol dust, molecules, protons, and electrons. In addition to containing matter, space also is filled with energy, part of it in the form of microwave radiation. 

In addition to ordinai y matter, scientists have evidence for the existence in the universe of dark matter. Some of the dark matter is ordinai y matter, such as dust in outer space and planets going around other stars. Astronomers cannot see ordinai y dark matter because any light coming from such matter is too faint to be observed in telescopes. However, most of the dark matter in the universe is believed not to be ordinary matter. At the present time it is not known what this mysterious dark matter is, or what it is made of. Scientists know that this dark matter exists because it exerts a gravitational force on stars (which are made of ordinary matter), causing the stars to move faster than they otherwise would. According to present estimates, there is perhaps five times as much dark matter in the universe as ordinary matter. 

Figure 25. First light image of the Keck LGS AO system. The lens-like nebula at upper left is a disk of dust and gas surrounding the young star HK Tau B, The star is hidden from direct view, seen only in light reflected off the upper and lower surfaces of the disk.

The darkness associated with dense interstellar clouds is caused by dust particles of size =0.1 microns, which are a common ingredient in interstellar and circum-stellar space, taking up perhaps 1% of the mass of interstellar clouds with a fractional number density of 10-12. These particles both scatter and absorb external visible and ultraviolet radiation from stars, protecting molecules in dense clouds from direct photodissociation via external starlight. They are rather less protective in the infrared, and are quite transparent in the microwave.6 The chemical nature of the dust particles is not easy to ascertain compared with the chemical nature of the interstellar gas broad spectral features in the infrared have been interpreted in terms of core-mantle particles, with the cores consisting of two populations, one of silicates and one of carbonaceous, possibly graphitic material. The mantles, which appear to be restricted to dense clouds, are probably a mixture of ices such as water, carbon monoxide, and methanol.7... 

The first question to ask about the formation of interstellar molecules is where the formation occurs. There are two possibilities the molecules are formed within the clouds themselves or they are formed elsewhere. As an alternative to local formation, one possibility is that the molecules are synthesized in the expanding envelopes of old stars, previously referred to as circumstellar clouds. Both molecules and dust particles are known to form in such objects, and molecular development is especially efficient in those objects that are carbon-rich (elemental C > elemental O) such as the well-studied source IRC+10216.12 Chemical models of carbon-rich envelopes show that acetylene is produced under high-temperature thermodynamic equilibrium conditions and that as the material cools and flows out of the star, a chemistry somewhat akin to an acetylene discharge takes place, perhaps even forming molecules as complex as PAHs.13,14 As to the contribution of such chemistry to the interstellar medium, however, all but the very large species will be photodissociated rapidly by the radiation field present in interstellar space once the molecules are blown out of the protective cocoon of the stellar envelope in which they are formed. Consequently, the material flowing out into space will consist mainly of atoms, dust particles, and possibly PAHs that are relatively immune to radiation because of their size and stability. It is therefore necessary for the observed interstellar molecules to be produced locally. 

What is the ultimate fate of the molecular material formed in the envelopes of carbon-rich stars as it heads out into space The dust grains will be processed only slowly by the interstellar radiation held and survive almost intact until they become part of an interstellar cloud. The survival of individual PAHs depends on their size the larger ones withstand radiation much better than do the smaller ones.115 By survival we are referring to the aromatic skeleton the interstellar radiation field will efficiently break H bonds and cause ionization so that unsaturated, ionized PAHs are likely to dominate those found in the diffuse interstellar medium. Such species have been suggested as a source of the DIBs.118,123 Small molecules photodissociate in the interstellar radiation field before the material becomes part of an interstellar cloud. 

The dark clouds were responsible for the discovery of ISM, as they absorb the light from stars which lies behind these clouds of interstellar matter. It is difficult to obtain reliable information on the dust particles. They are probably about 0.1 pm in diameter, consisting of a silicate nucleus and an envelope of compounds containing the elements C, O and N, which, with H and He, are the main elements present in interstellar space. There are only two sources of information for more exact characterisation of the dust particles ... 

Interstellar dust is also important for the formation and development of stars. Although the dust particle component is only a minor one in ISM, it acts as a cooling agent for collapsing clouds, thus preventing the buildup of an effective thermodynamic counterpressure. 

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