Spiral arms

If spiral growth occurs due to the existence of screw dislocations, the results depend upon whether the diffusion length ijy is smaller or larger than the typical separation of the spiral arms i. In the first case the situation hardly changes from the purely kinetic situation without diffusion, but in the second case interaction between steps comes into effect [90] and phenomena such as step bunching [91] take place. We can estimate qualitatively the... 

An unknown event disturbed the equilibrium of the interstellar cloud, and it collapsed. This process may have been caused by shock waves from a supernova explosion, or by a density wave of a spiral arm of the galaxy. The gas molecules and the particles were compressed, and with increasing compression, both temperature and pressure increased. It is possible that the centrifugal forces due to the rotation of the system prevented a spherical contraction. The result was a relatively flat, rotating disc of matter, in the centre of which was the primeval sun. Analogues of the early solar system, i.e., protoplanetary discs, have been identified from the radiation emitted by T Tauri stars (Koerner, 1997). 

The mean particle density of ISM is 106 particles per cubic meter there are, however, great variations from this mean value. Between the spiral arms of the Milky Way, there are between 104 and 105 hydrogen atoms per cubic metre in the dark clouds and the HII regions, there are 10s—1010. Up to 1012—1014 hydrogen atoms per cubic metre are present in regions with OH sources and in certain infrared objects. 

Fig. 11.6 Schematic representation of the Milky Way with the galactic life zone , in which life should be possible. The centre of the galaxy is kept practically sterile by extreme radiation, while areas in which stars are formed are localized in the spiral arms...

NGC2324 A Relatively Young, Metal-Poor Open Cluster Located Beyond the Perseus Spiral Arm... 

Most of the gas in the Galaxy is contained within the disk and in particular in the spiral arms, hence in a layer only a few light-years thick. Although we cannot claim that the space between the stars is empty since the interstellar medium is actually observable, it is not far from being so. It contains on average about 1 atom cm, far less than the best laboratory vacuum. 

As is well known, the observable or accountable matter in distant spiral galaxies is insufficient for generation of sufficient gravity in the spiral arms to keep the matter in the arms from flying away [73,74]. It has been conjectured for some time that various types of exotic new matter never observed (and therefore dark ), must be responsible for the excess gravity. 

Additional models of FFMF for interstellar physics also postulated that the spiral arms of galaxies, as well as solar flares and prominences, could be constmcted of such force-free fields [17]. Similar Beltrami field structures have... 

Bob approaches a flexscreen and sketches an ellipse to represent the Milky Way Galaxy (figure 2.1). I ve drawn Mr. Plex where our Solar System is located on one of the spiral arms of the Milky Way. Like all stars, the Sun moves through... 

We only know about a few hundred of them, and they are mostly in the spiral arms of the Milky Way. ... 

Bob looks up. Stars are still being born now but more in the spiral arms than in the core. The core stars formed early in the Galaxy s history and are late in their evolution. The exploding stars, or supernovas, in the arms are kicking out heavy elements like carbon that were manufactured in the stars. ... 

Are the stars really different in the core of the Galaxy than in the spiral arms asks Miss Muxdroozol. 

Population I Spiral arms Young Luminous (e.g. our Sun) and hotter than Population II stars Rich in heavy elements... 

Bob looks up. Is that still the Milky Way Galaxy that was previously overhead He can make out a bright central region of stars. There are patches of red. The spiral arms are gone. The stars are dying, Mr. Plex. Evidently, we ve moved again further into the future. Remember, when we left Earth, our Sun was five billion years old, halfway through its life. ... 

Astronomers recognize two different types of supernovae, Type I and Type II. Bright Type I supernovae occur anywhere in a spiral galaxy, but dimmer Type II supernovae are seen only in the spiral arms. Other distinguishing characteristics are described in John Gribbin s book Stardust (New Haven, Connecticut Yale University Press, 2000). 

Fig. 5. Schematic representation of the spiral structure in the vicinity of the sun. S and P refer to density wave spiral arms, O to a material arm...

The spiral arms of the Milky Way are composed mostly of hydrogen. Except for the stars within these spiral arms, these spiral arms themselves are invisible to optical telescopes. The 21-cm line of hydrogen makes structural details of these spiral arms visible. Radio antenna tuned to this line can be swept across the disk of the Milky Way so that its spiral-armed structure can be observed. One such image constructed from hydrogen data was created by Gart Westerhout. It shows structural details of our galaxy (see Figure 17.3). ... 

A major problem for the shock model has been to find a source of powerful, pervasive, and repeatable shocks. Several have been proposed clumpy material falling into the nebula (Tanaka et al, 1998), bow shocks from planetesimals scattered by Jupiter (Hood, 1998 Weidenschillingetal., 1998), and spiral-arm instabilities in the solar nebula (Wood, 1996b). Boss infers that clumps and spiral arms could generate 10kms shocks in the asteroid belt (Chapter 1.04). lida et al. (2001) and... 

We map the spiral arm structure of the Milky Way using ordinary optical light and looking for objects commonly found in spiral arms of other galaxies. To map the largest region possible we use bright objects. These objects, known as spiral arm tracers, include O and B spec-... 

Astronomers use radio waves to map the far reaches of the Milky Way because the interstellar dust does not block radio waves as much as optical light. Spiral arms also contain interstellar gas composed mostly of hydrogen atoms. This interstellar gas is so thin (on aver-... 

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