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Topic: Accretion theory

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In the News (Sun 24 Jun 18)

 The Origin Of The Moon--A New Theory   (Site not responding. Last check: 2007-10-13)
According to the active disc theory, the disc, at first, was composed of elements in the same proportion as the forming Sun, the "solar abundance," which is about 98% hydrogen and helium, and 2% heavier elements.
The cold accretion theory well explains the later stages of planetary formation, after large planetesimals had already formed with their own gravity, but it has a difficult time explaining the early stages of planet formation, since cold dust and small rocks don't have enough gravity or any material cohesiveness to attract one to the other.
The theories that suppose that the Moon spun out of the earth or formed from the Earth, such as the "giant impact" theory, or the theory that has the Earth spinning so fast that the Moon spun out of it, would require that the Moon orbit somewhere near Earth's equator.
www.hoyhoy.com /subweb/moon.htm   (2748 words)

 Jose Gracia @ Turbulent Jets
In conventional standard disk theory the "inner edge" of the accretion flows appears as an adjustable integration constant and is traditionally identified with the marginally stable orbit.
In accretion theory it is turbulent and sub-thermal, while in jet formation scenarios, it is external or of large scale and dynamically dominant over thermal pressure.
To clarify these fundamental issues of accretion disk theory and jet formation theory, it this essential to gain better understanding of the development and evolution of the magnetized turbulent flow near rotating fl holes in a magnetic field topology which is characterized by turbulence near the base and becomes increasingly smooth downstream.
www.phys.uoa.gr /~jgracia/jets_MRI.html   (1123 words)

 Astrophysicists put kibosh on alternative theory of star formation
The competitive accretion model was hatched in the late 1990s in response to problems with the gravitational collapse model, which seemed to have trouble explaining how large stars form.
In particular, the theory couldn't explain why the intense radiation from a large protostar doesn't just blow off the star's outer layers and prevent it from growing larger, even though astronomers have discovered stars that are 100 times the mass of the sun.
For example, the accretion theory predicts that brown dwarfs, which are failed stars, are thrown out of clumps and lose their encircling disks of gas and dust.
www.eurekalert.org /pub_releases/2005-11/uoc--apk111605.php   (1159 words)

 The Accretion Theory
In the accretion method of building a pyramid, a solid central core was constructed, then this was expanded outwards on all four sides by the addition of successive accretion layers (or concentric shells) of masonry ranging from five to fifteen feet in thickness.
A variation of this approach was to form each accretion layer with two kinds of blocks: an outer face formed of well-squared blocks of larger dimensions that served as both a casing for the layer as well as a retaining wall for an inner portion of roughly-shaped blocks.
The idea that the Egyptians continued building pyramids with accretion layers to the end of Dynasty VI can be traced to the 19th Century Egyptologist, Richard Lepsius, who suggested that it provided a way for a king to add to his tomb over the course of a long reign (Edwards 1993:273).
www.catchpenny.org /accretion.html   (3070 words)

 Selected results - ADAF theory   (Site not responding. Last check: 2007-10-13)
Accretion disk theory was first developed as a theory with the local heat balance, where the whole energy produced by a viscous heating was emitted to the sides of the disk.
The present status of the solution for a low-luminous optically thin accretion disk model with advection is discussed and the limits for an advection dominated accretion flows (ADAF) imposed by the presence of magnetic field are analysed.
For small accretion rates where the electron temperature is much less than the ion temperature, we show that the ohmic heating of the electrons gives a radiative efficiency that is reduced by a factor of g from that for a thin disk.
astrosun.tn.cornell.edu /us-rus/adaf.htm   (675 words)

 accretion disk atmospheres
Although with the aid of Astro-tomography the modes of stellar accretion are well-understood, the detailed physics is still missing.
Accretion results from the transfer of angular momentum outwards by an unknown viscous action between shearing material.
However a broad range of physical conditions occur within the atmosphere of any accretion disk where the cool outer disk is a very different environment to the hot inner region.
lheawww.gsfc.nasa.gov /users/still/research/disk.html   (1163 words)

 Space Studies Board
The observable layer gradually increases in surface temperature and luminosity as the central mass grows by accretion from the disk and the optical thickness of the collapsing envelope decreases.
(3) In turbulent viscosity models of the accretion disk, viscous evolution results in the spiraling of nebular material inward toward the central star, leading to stellar accretion during the protostellar evolutionary stage when inward mass flux is approximately balanced by infall of matter from the collapsing cloud and the disk is roughly in steady state.
In particular, the time scale currently inferred for accretion of their cores at their present distances is longer than the lifetime of the nebula and may be longer than the age of the solar system.
www7.nationalacademies.org /ssb/detectionch3.html   (5170 words)

 An alternative theory of the origin of the Moon
The third major competing theory to be developed, was the so-called Co-accretion theory, whereby the Earth and Moon formed in the same region but independently from the same primeval material from which the whole Solar System had formed.
The theory requires a colossal collision with a massive planet, (the size of Mars), from elsewhere in the Solar System, travelling in an unusual, probably elliptical, orbit which was nevertheless in the plane of the ecliptic, where all the other major planets revolve in closely circular orbits.
No one can say for sure how long the accretion of the Earth's mantle lasted, but during this period, vast amounts of matter were added to the planet, no doubt on a scale ranging from minute particles to enormous chunks, as judged by the visible craters remaining on planets, natural satellites, and asteroids.
www.users.bigpond.com /ernestmcfarlane/lunargenesis.html   (6523 words)

 ORIGIN OF THE SOLAR SYSTEM - 1   (Site not responding. Last check: 2007-10-13)
The theory: One day our sun burst open, and planets and moons shot out at high speeds and went to their respective places, then stopped, and started orbiting the sun, as the moons began orbiting the planets.
The theory: Planets and moons were flying around, and some were captured by our sun and began circling.
The theory: Our world collided with a small planet, and the explosion threw off rocks which became the moon, and then it began orbiting us.
www.pathlights.com /ce_encyclopedia/03-ss2.htm   (1219 words)

 Final Gallery
Most of these theories required a large atmosphere, which was not extant at the time, to slow the moon down, and still would not have accounted for its current position.
The theory that the moon left the Pacific basin in its wake is also contradictory to the plate tectonics model.
Although disproved, this theory led to the currently accepted theory, the “giant impact hypothesis.” Widely accepted as the most probable theory of moon formation, the giant impact hypothesis is also the most violent.
aerospacescholars.jsc.nasa.gov /has/Students/finalGall.cfm?id=5072   (1152 words)

 SPACE.com -- Planet Puzzle: Theorists Wrestle with How They're Built
The idea might be testable: The spiral arms that form the rings may have left a record of their existence in tiny, glassy spheres found in billion-year old meteorites.
Accretion models require a core of at least 10 Earth masses, Boss said.
Another problem is that core accretion requires several million years to make a gas giant, whereas gas disks around stars like the Sun — which feed gas giant growth — typically do not last that long.
www.space.com /scienceastronomy/mystery_monday_050307.html   (1433 words)

 Origin of the Earth
There are two theories that attempt to explain how the earth formed from clouds of matter that may have existed at the birth of the Solar system - the Hot Accretion model, and the Cold Accretion model.
An older theory about how the hot gas came about in the first place was that a rogue star orbited closely past the sun, pulled out a lot of gas from its partner during the pass due to gravity, and later escaped into space.
The next theory is that the earth collected the water over a long period of time as a result of ice-bearing comets or meteorites that impacted the surface.
home.houston.rr.com /apologia/sec9p3.htm   (1717 words)

 Black Holes - Interview with Dr. Ramesh Narayan
Accretion flow is the name given to gas that goes into orbit around a gravitating object and then slowly spirals in toward the center.
In simple terms, this is an accretion flow in which as the gas spirals in and gets very hot, it does not radiate its heat energy efficiently.
I had gone to a conference on accretion flows in Sweden—in 1993 I think—and one of the topics discussed there was a mystery that was bothering people.
www.esi-topics.com /blackholes/interviews/RameshNarayan.html   (2808 words)

 Planet Quest: News Article
In the "gravitational instability" theory, planets form during a rapid collapse of a dense cloud.
With the "core accretion" theory, planets start as small rock-ice cores that grow as they gravitationally acquire additional mass.
"This is a confirmation of the core accretion theory for planet formation and evidence that planets of this kind should exist in abundance," said Greg Henry, an astronomer at Tennessee State University, Nashville, who detected the dimming of the star by the planet with his robotic telescopes at Fairborn Observatory on Mount Hopkins, Arizona.
planetquest.jpl.nasa.gov /news/giantRockyCore.cfm   (479 words)

 [No title]   (Site not responding. Last check: 2007-10-13)
In contrast, competitive accretion theory suggests that at birth all stars are much smaller than the typical stellar mass, and that final stellar masses are determined by accretion of unbound gas from the clump only after formation of the initial "seed" star.
I present a combination of analytic theory and simulations to compute the rate of accretion of unbound gas onto a seed protostar as a function of star-forming environment.
Competitive accretion works in the environments modeled in simulations, but it will not operate under the conditions found in real star-forming clouds.
www.astro.ufl.edu /theory/abstracts/krumholz.html   (176 words)

 4.2 Accretion onto black holes
The study of relativistic accretion and fl hole astrophysics is a very active field of research, both theoretically and observationally (see, e.g., [32] and references therein).
Disk accretion theory is primarily based on the study of (viscous) stationary flows and their stability properties through linearized perturbations thereof.
For a wide range of accretion problems, the Newtonian theory of gravity is adequate for the description of the background gravitational forces (see, e.g., [98]).
relativity.livingreviews.org /Articles/lrr-2003-4/node12.html   (3924 words)

 CIW - News 991110
This theory was first proposed in 1951 but was discarded because it was not believed to lead to the formation of the large ice/rock cores thought to characterize the large planets.
One major problem with this theory is that by the time the core is large enough to accrete gas (a process taking a million years or more), there may not be any gas left in the disk.
If this is true, he writes, then "much of the attraction of the core accretion theory would be lost, because lower-mass cores might not be able to trigger gas accretion." In addition, the one-step theory might now be able to account for the formation of the more modest-sized cores newly inferred for Jupiter and Saturn.
www.carnegieinstitution.org /news_970620.html   (568 words)

 Accretion (astrophysics) - Wikipedia, the free encyclopedia
In astrophysics, the term accretion is used for at least two distinct processes.
Accretion discs are common around smaller stars or stellar remnants in a close binary, or fl holes in the centers of spiral galaxies.
Use of the term accretion disc for the protoplanetary disc thus leads to confusion over the planetary accretion process, although in many cases it may well be that both accretion processes are happening simultaneously (e.g.
en.wikipedia.org /wiki/Accretion_(astrophysics)   (257 words)

 chapter 12
Adhesion and clustering due to electric polarization is probably the most important process for initial accretion in a jet stream; it also determines the persistent clustering and particle adhesion on the lunar surface (Arrhenius and Asunmaa, 1973, 1974; Asunmaa et al., 1970; Asunmaa and Arrhenius, 1974).
Our derivation of the accretion of celestial bodies in jet streams is based on a number of simplifying assumptions: There is not, as yet, any detailed general theory of jet streams; further, the relation between volatile and less volatile substances is far from clear.
The change in the proportion of dense materials from that in the accreting planetesimals is thus mainly a secondary effect of the displacement of the light component.
www.hq.nasa.gov /pao/History/SP-345/ch12.htm   (4445 words)

 The Pyramids and Temples of Gizeh by Sir Flinders Petrie - Architectural Ideas of the Pyramid Builders
This last idea, which may be called the "theory of accretion," would show that the size of each Pyramid was solely due to a series of accidental events ; and that no foreseen design can be expected in the external dimensions, or in their relations to the inside.
The accretion theory, then, though not actually condemned by the application of these questions which are adduced in its support, is at least far from being the "one entirely satisfactory answer" to them, as it has been claimed to be.
Now, if the accretion theory were true, it ought to be of the greatest value when applied to the largest Pyramids ; for these are the most difficult to account for, and their extraordinary size is the main feature appealed to in support of the theory.
www.touregypt.net /petrie/c18.html   (4271 words)

 New theory of how planets form finds havens of stability amid turbulence
This need for speed causes problems for any theory with a leisurely approach to forming planets, such as the core accretion theory that was the standard model until recently.
The theory that gravitational instabilities by themselves can form gas giant planets was first proposed more than 50 years ago.
According to the gravitational instability theory, spiral arms form in a gas disk and then break up into clumps that are in different orbits.
newsinfo.iu.edu /news/page/normal/1859.html   (1014 words)

 chapter 13
Their results are not in quantitative agreement with each other, but they all show that statistical accretion should give a spin that on the average is of the same absolute magnitude as in Giuli's case, but directed at random.
Hence during most of the accretion the condition of infinitely small grains is satisfied, which means that the inclination of the equatorial plane towards the orbital plane should be small.
The bodies which had a late runaway accretion (Saturn, Mars, and the Moon) have typically obtained 75 percent of their mass by accretion of a small number of bodies of relatively large size (statistical accretion).
history.nasa.gov /SP-345/ch13.htm   (3282 words)

The “core accretion” theories see the shape and orbits of a prototype Solar System as forming by somewhat the same mechanics, but more “weight” is given to the dust and rocky debris components for planet formation, and there is more discussion of what size the rocky components might be, whether “stardust” or asteroid.
Whether we end up preferring one theory or another to explain a particular heavenly body, it would seem as if we are debating the size and natural state of the building blocks.
Gaseous accretion assumes that coalescing gas clouds were responsible for formation of the planets as we know them today, so the “rocks” must have come later, if ever.
www.summitlake.com /WRITING/Stardust.html   (4122 words)

 Accretion Theory.
The accretion theory: happens as a cloud of gaseous material and dust contracts under the extreme forces of gravity.
Therefore, we either need to expand the accretion theory and produce more gravitational forces, or look around for an entirely new model of how solar-systems are created.
Theories here are the intellectual property right of the owners.
myweb.tiscali.co.uk /newuniverse/real_accretion.html   (1029 words)

 Astrophysicists quash alternative theory of star formation
"Competitive accretion is the big theory of star formation in Europe, and we now think it's a dead theory," said Richard Klein, an LLNL astrophysicist and adjunct professor of astronomy at UC Berkeley.
Competitive accretion theory goes on to explain that brown dwarfs — failed stars — and free-floating planets are protostars — the early phase of a star.
McKee noted that their supercomputer simulation indicates competitive accretion may work well for small clouds with very little turbulence, but these rarely, if ever, occur, and have not been observed to date.
www.eurekalert.org /pub_releases/2005-11/dlnl-aqa111705.php   (920 words)

enlarged by gravitational accretion of extraterrestrial matter until it reached spherical shape, at which point gravity could omni-directionally focus total weight of the protoplanet on its exact center.
This new H2O increases the total volume in the oceans, and the gases are either combined in solution with seawater or are released into the atmosphere, but mineral deposits deposited on the ocean bottom are subsequently covered by accretion of sediments consisting of dust (terrestrial and meteoric) and organic detritus from marine fauna and flora.
accretion of extra-terrestrial dust and meteorites, and internal expansion of the molten core
www.expanding-earth.org /page_1.htm   (1121 words)

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