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Spiral galaxies range from S0, the lenticular galaxies, to Sb, which have a bar across the nucleus, to Sc galaxies which have strong spiral arms. In total count, ellipticals amount to 13%, S0 to 22%, Sa, b, c galaxies to 61%, irregulars to 3.5%, and peculiars to 0.9%. At the center of most galaxies is a high concentration of older stars.
The interfaces between regions with sharp differences in rotation are believed to be efficient sites for the dynamo processes that generate the stellar magnetic field. There is also a complex interaction between a star's rotation distribution and its magnetic field, with the conversion of magnetic energy into kinetic energy modifying the ...
[23] [24] Additionally, detailed investigations of the rotation curves of low-surface-brightness galaxies (LSB galaxies) in the 1990s [25] and of their position on the Tully–Fisher relation [26] showed that LSB galaxies had to have dark matter haloes that are more extended and less dense than those of galaxies with high surface brightness ...
A rotating black hole can produce large amounts of energy at the expense of its rotational energy. [ 7 ] [ 8 ] This can happen through the Penrose process inside the black hole's ergosphere , in the volume outside its event horizon. [ 9 ]
The density wave theory also explains a number of other observations that have been made about spiral galaxies. For example, "the ordering of H I clouds and dust bands on the inner edges of spiral arms, the existence of young, massive stars and H II regions throughout the arms, and an abundance of old, red stars in the remainder of the disk".
The reciprocal of the rotational shear is the lap time, i.e. the time it takes for the equator to do a full lap more than the poles. The relative differential rotation rate is the ratio of the rotational shear to the rotation rate at the equator: α = Δ Ω Ω 0 {\displaystyle \alpha ={\frac {\Delta \Omega }{\Omega _{0}}}}
An encounter between two stars is defined to be strong/weak if their mutual potential energy at the closest passage is comparable/minuscule to their initial kinetic energy. Strong encounters are rare, and they are typically only considered important in dense stellar systems, e.g., a passing star can be sling-shot out by binary stars in the core ...
Modified Newtonian dynamics (MOND) is a theory that proposes a modification of Newton's second law to account for observed properties of galaxies.Its primary motivation is to explain galaxy rotation curves without invoking dark matter, and is one of the most well-known theories of this class.