Evolution Of Nebulae Research Articles

Mineralogy of chondritic interplanetary dust particles

From a mineralogical survey of approximately 30 chondritic micrometeorites collected from the lower stratosphere and studied in detail using current electron microscopy techniques, it is concluded that these particles represent a unique group of extraterrestrial materials. These micrometeorites differ significantly in form and texture from components of carbonaceous chondrites and contain some mineral assemblages which do not occur in any meteorite class. Electron microscope investigations of chondritic micrometeorites have established that these materials (1) are extraterrestrial in origin, (2) existed in space as small objects, (3) endured minimal alteration by planetary processes since formation, and (4) can suffer minimal pulse heating (<600°C) on entering earth's atmosphere. The probable sources for chondritic interplanetary dust particles (IDPs) are cometary and asteroidal debris and, perhaps to a lesser extent, interstellar regions. These sources have not been conclusively linked to any specific mineralogical subset of IDP, although the chondritic porous (CP) aggregate is considered of likely cometary origin. Chondritic IDPs occur in two predominant mineral assemblages: (1) carbonaceous phases and phyllosilicates and (2) carbonaceous phases and nesosilicates or inosilicates, although particles with both types of silicate assemblages are observed. Olivines, pyroxenes, layer silicates, and carbon‐rich phases are the most commonly occurring minerals in many chondritic IDPs. Other phases often observed in variable proportions include sulphides, spinels, metals, metal carbides, carbonates, and minor amounts of sulphates and phosphates. Individual mineral grain sizes range from micrometers (primarily pyroxenes and olivines) to nanometers, with the predominant size for all phases less than 100 nm. Specific mineral characteristics for particular chondritic IDPs provide an indication of processes which may have occurred prior to collection in the earth's stratosphere. For example, pyroxene mineralogy in some chondritic aggregates is consistent with condensation from a vapor phase and, we consider, with condensation in a turbulent solar nebula at relatively low temperatures (<1000°C). Carbonaceous phases present in other CP aggregates have been used to imply low‐temperature formation processes such as Fischer‐Tropsch synthesis (∼530°C) or carbonization and graphitization (∼315°C). Alteration processes have been implicated in the formation of some layer silicates in CP aggregates and may have involved hydrocryogenic alteration at <0°C. In general, interpretations of transformation processes on submicrometer‐size minerals in chondritic IDPs are consistent with formation at a radius equivalent to the asteroid belt or greater during the later stages of solar nebula evolution using currently available models.

Atomic hydrogen in planetary nebulae

The authors searched for neutral atomic hydrogen associated with 22 planetary nebulae and three evolved stars in the 21 cm line at the Arecibo Observatory. Objects whose radial velocities permitted discrimination from Galactic H I were chosen for observation. Hydrogen was detected in absorption from IC 4997. From the measurements new low limits are derived to the mass of atomic hydrogen associated with the undetected nebulae. Radio continuum observations were also made of several of the nebulae at 12.6 cm. The authors reexamine previous measurements of H I in planetary nebulae, and present the data on a consistent footing. The question of planetary nebula distances is considered at length. Finally, implications of the H I measurements for nebular evolution are discussed and it is suggested that atomic hydrogen seen in absorption was expelled from the progenitor star during the final 1000 yr prior to the onset of ionization.

Evolution and Gas Dynamics of Planetary Nebulae

Since the last IAU Symposium on Planetary Nebulae at Tatranska-Lomnica, a number of significant theoretical and observational advances have been made in the study of nebular dynamics. The following review emphasizes the evolution of nebulae after they have been ejected from the central star and have become optically thin to most radiation.

Model Planetary Nebulae.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Model Planetary Nebulae. Mathews, William G. Abstract The dynamics of gaseous shells of ionized hydrogen are studied. The gas is heated photoelectrically and cooled by [0 III] radiation. The gas density, temperature, and velocity are calculated as functions of time and space for two models. The projected appearance of the shell in H~ radiation, the H~ line profile, the total HP luminosity, and the emission measure are also calculated with each model. The models fail to resemble the observations unless they are supported from within. This support is assumed to be produced by continued mass loss. The central star evolves during the nebular evolution. Publication: The Astronomical Journal Pub Date: 1966 DOI: 10.1086/110163 Bibcode: 1966AJ.....71T.169M full text sources ADS |

The evolution of extragalactic nebulae and the origin of metagalactic radio emission

The evolution of extragalactic nebulae and the origin of metagalactic radio emission

The evolution of extragalatic nebulae and the origin of metagalactic radio emission

The evolution of extragalatic nebulae and the origin of metagalactic radio emission

The physics of the expanding universe

certainly because the evolution of nebulae cannot be traced in sufficient detail to interpret the measures of objects that are consistently younger the further we look. So long as those other theories suffer from this debility, and whether they are in fact right or wrong, they are in the category of the satisfying scheme rather than the useful scientific theory. The future observations of nebulae, which will be concerned with detailed measures of quantities and numbers, would remain merely a filrther collection of data in no way surprising or important except if those detailed quantities can show that some great generalization is true or false.

Surface Photometry of Southern Ellipitical Nebulea: (Second Paper)

After the investigation described in Paper I there remained seven nebulae south of declination –26° and brighter than integrated magnitude 12.0, which have been classified as elliptical. These objects have now been investigated photometrically. The true elliptical nebulae are four in number: three objects previously so classified are spirals. The four true ellipticals conform closely to the type of intensity distribution proposed by Hubble. The nucleus of the spiral nebula, NGC 6744, also conforms to this rule. Some possible consequences of this fact for our ideas on nebular evolution are discussed.

On a New Cluster of Nebulae in Pisces

Introduction. - Clusters of nebulae represent the largest aggregations of matter known. It may be expected that an investigation of their distribution in space, of their composition and physical properties will throw new light on such problems as the determination of nebular masses [1], on the red shift of light from nebulae and on the evolution of stars and nebulae as well as the evolution of the universe as a whole. For these reasons it was thought desirable to undertake a more concentrated study of clusters of nebulae than hitherto attempted. The first step of this program consists in securing more complete data on the number and the distribution of groups of nebulae containing two, three or more members up to clusters of many hundred nebulae. The new 18-inch Schmidt telescope of the California Institute of Technology promises to be an ideal instrument for the investigation of groupings of nebulae brighter than the sixteenth magnitude.