How did gravity build astronomical objects that rotate?

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Discussion Overview

The discussion centers on the formation of rotating astronomical objects through gravitational processes, exploring how a nearly homogeneous primordial state evolved into a heterogeneous universe with varying angular momentum. Participants examine the roles of gravitational condensation, collisions, turbulence, and shear in this context.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that gravitational condensation alone does not fully explain the formation of rotating structures, proposing that fluid shear plays a significant role in the process.
  • Another participant emphasizes that gravity amplifies non-homogeneities, leading to denser regions and the formation of planets, comets, and asteroids from primordial turbulence.
  • A different viewpoint posits that collisions during gravitational collapse lead to a preferential direction of spin, which is amplified as particles spiral into a gravity well.
  • Some participants argue that the initial distribution and velocities of particles are not isotropic, which influences the final direction of rotation.
  • Others contend that a preferred direction of rotation can emerge randomly from chaotic collisions, regardless of initial conditions.
  • One participant questions the applicability of the "collisions between infalling lumps" explanation for the rotation of the sun and spiral galaxies, suggesting that fluid dynamics may be more relevant.
  • Another participant references a computer simulation of galaxy formation to support the idea that molecular collisions contribute to the acquisition of spin in primordial clouds.
  • There is a request for clarification on whether Keplerian shear contributes to the storage of angular momentum in these processes.

Areas of Agreement / Disagreement

Participants express a range of views on the mechanisms behind the formation of rotating astronomical objects, with some agreeing on the importance of collisions and turbulence, while others challenge or refine these ideas. No consensus is reached regarding the specific roles of Keplerian shear and fluid dynamics in this context.

Contextual Notes

Participants note that the discussion involves complex interactions and assumptions about initial conditions, particle distributions, and the nature of gravitational collapse, which remain unresolved.

  • #31


Codex34 said:
Can gravity form non-axial rotations such that any stars, planets and moons do not rotate in planar orbits but orbit in any orbital direction they wish?
. There's a bit of confusion here. In an empty model universe the orbits for two masses attracted to each other by gravity are always "planar" and "axial; relative motion then lies in a plane defined by two lines: one joining the two masses and the other along their relative velocity as seen by any observer (assuming the two lines to not be collinear). You might say that a lump traveling in any direction "it wishes" orbits a central attracting mass it encounters in a plane determined by what this "wish" was when they first met and began perceptibly to gravitate, but this is a bit wooly, don't you think?

Chronos said:
Random axial rotation would be a logical conclusion. Have you examples to the contrary? Shear only affects the process of collapse.
Random axial rotation would be logical only if one took a large enough average over many independently-formed systems. Logical in the case of planetary systems formed by gravitational collapse in different parts of a galaxy, yes. But not in planet formation in say, our solar system. Henry Ford was wrong: history is not bunk, and planetary formation initiated by gravitational shear in a single rotating disc is a process with a shared history.

Don't most of our planets rotate (roughly -- ignore their seasons) in the same sense (clockwise or anticlockwise) if viewed from a distance, along the ecliptic axis? With their moons? I'll check. If so, that's your example. Just remember, shear and gravity go together, like a horse and carriage.
 
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  • #32
I came across http://www.sjsu.edu/faculty/watkins/solarspin.htm that could provide an example?

Thayer Watkins said:
One of the most remarkable features of our solar system is that nearly all of the revolutions and rotations are in the same direction. From a point high above the north pole of the solar system the planets are revolving about the sun and rotating about their axes in a counterclockwise direction. This holds true also for the asteroids. If the planets and asteroids were formed from merely random accretions the would be an even mixture of the directions of revolution and rotation. The sun itself also rotates in a counterclockwise direction. The satellites of the planets also generally revolve and rotate in a counterclockwise direction. Of the thirty something satellites only six do not do so; they are said to have retrograde motion. Of the six exceptions five are outer satellites likely to be captured asteroids.

The Exceptions
Venus and possibly Uranus are the exceptions to the counterclockwise rotations of the planets. Venus travels around the sun once every 225 Earth days but it rotates clockwise once every 243 days. This pecular combination gives it a day with respect to the sun of 117 Earth days. Uranus is tilted on its side about 90° so its direction of rotation is ambiguous. Its angle of inclination is usually given as 98° which would mean that its direction of rotation is not retrograde. If its direction of rotation is presumed retrograde then its angle of inclination would be 82°.
 

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