Time evolution of density profile

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SUMMARY

The discussion focuses on the analytical determination of the mass density profile ρ(r) of a spherically symmetric ball of gas as it evolves over time, starting from an initial profile ρ0(r). The participants emphasize the importance of considering gravitational collapse and the trajectories of thin shells of test particles. By treating the distance between these shells as small compared to the overall radius, they derive the relationship between mass distribution and volume scaling, ultimately concluding that density is inversely proportional to volume during the collapse process.

PREREQUISITES
  • Understanding of gravitational collapse in astrophysics
  • Familiarity with spherically symmetric mass distributions
  • Knowledge of differential equations and their application in physics
  • Basic principles of fluid dynamics related to density profiles
NEXT STEPS
  • Study the equations governing gravitational collapse in astrophysical contexts
  • Learn about the derivation of free-fall collapse time in astrophysics
  • Explore the mathematical modeling of density profiles in fluid dynamics
  • Investigate the implications of mass conservation in dynamic systems
USEFUL FOR

Astronomers, astrophysicists, and students studying gravitational dynamics and fluid mechanics will benefit from this discussion, particularly those interested in the time evolution of mass density profiles in collapsing gas spheres.

throneoo
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Is it possible to work out analytically how the mass density profile ρ(r) of a ball of gas (spherically symmetric) evolve with time given the initial profile ρ0(r)? The assumption here is that the particles collapse only under the influence of gravity. I thought of this question in the process of deriving the free-fall collapse time. I know the trajectory of each spherical shell yet I don't know how to determine the mass distribution at any given moment during the collapse (quantitatively)
 
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It sounds like you are talking free fall, so imagine two thin shells of test particles, and solve their trajectory. Treat the distance between the shells as small compared to the radius. Find the distance between the shells as a function of time. The mass between the shells stays constant, and the volume between the shells scales in proportion to r2 dr, where dr(t) is what you figure out when you figure out r(t) for the two shells. The density is inverse to the volume.
 
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