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While there are many types of spiral as shown in Figure 17d, it is the equations for fluid dynamics (the Navier-Stokes Equations) that dictates a particular one. There is a subtle difference for adopting spherical coordinates in the explicit expression for u and thus selects a different type of spiral (see "Formulas"). | |
Figure 17d Types of Spiral |
In spherical coordinates the Navier-Stokes Equations can be expressed as : |
Figure 17e Spherical Coordinates and Formulas [view large image] |
This angular motion has no source or sink (for the fluid) just swiveling round and round. It is the radial component of Eq.(23c) that generates or drains the fluid as described in an example later. |
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Figure 17f Fermat's Spiral |
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Figure 17g Hurricane |
Figure 17h Barred Spiral |
These different cases are shown in the diagrams below [view large image] : |
Figure 17i SgrA* Spirals |
In barred spirals, there is a bar of stars runs through the central bulge. The arms of barred spirals usually start at the end of the bar. It is suggested that galactic bars develop when stellar orbits in a spiral galaxy become unstable and deviate from a circular path. The tiny elongations in the stars' orbits grow and get locked into place, forming a bar. The bar becomes even more pronounced as it collects more and more stars in elliptical orbits. Eventually, a high fraction of the stars in the galaxy's inner region join the bar. This process is said to be demonstrated repeatedly with computer-based simulations (see "Instability"). | |
Figure 17j Milky Way Bar [view large image] |
Figure 17j shows the structure of the inner Milky Way including : the bulge, the long bar (origins of the spiral arms at its ends), an additional galactic bar, and the "3-kpc arms" - the near one is inside the Norman arm, both 3-kpc arms are devoid of stars. |
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Figure 17k Barred Spiral |
to power the change of stellar orbits into elongated shape. |
Figure 17l Profile of Cyclone |
The constant "B" is estimated by reading off the variation of r ~ 23 km corresponding to turning the spiral by 3600, i.e., 2X3.1416 in radian from Figure 17m,b. Substituting the data to Eq.(23j) yields B = r2/2 ~ 42 km2. Then the radial velocity at the eye's wall can be calculated via Eq.(23g) : ur = [B/(re)2](ru)re ~ 20 km/h. | |
Figure 17m [view large image] Profile of Hurricane Katrina |
Comparing to ~ 10 km/h at the center (see cyclone). |
compressed gas triggers star formation and helps to explain why we see the concentration of bright young stars and clusters in the spiral arms. Figure 17n is a schematic diagram to illustrate the action of a density wave, which causes stars and interstellar gas and dust to bunch up temporarily when the two different types (the high density wave and the spiral of gas and dust) of rotations coincide (in phase), with the spiral arm being the result of a temporary compression of material (Figure 17o). | ||
Figure 17n Density Wave Theory |
Figure 17o Density Wave Illustration [view large image] |
Figure 17p Spiral Arm |
which has a maximum at r = 1/k. In comparison to the Kermat's spiral with no F, B = A + r2/2, which has no maximum. |
Figure 17q Angular Velocity of Spiral Arm [view large image] |
Figure 17q plots u as function of r for k = 0.3, 0.4, 0.5. These curves (except the dubious case for k = 0.3) are similar to the total angular velocity in Figure 17n (the yellow curve) by the density wave theory. |
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Figure 17s Coriolis Force |
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Figure 17t Super-Cluster Perseus [view large image] |
Figure 17u MKSpiral [view large image] |