Editing Stellar dynamics (section)

== Connection with fluid dynamics ==
Stellar dynamics also has connections to the field of plasma physics.<ref>{{Cite web | title=Archived copy | url=https://cds.cern.ch/record/1053485/files/p37.pdf | archive-url=https://web.archive.org/web/20200702151018/http://cds.cern.ch/record/1053485/files/p37.pdf | access-date=2024-12-15 | archive-date=2020-07-02}}</ref> The two fields underwent significant development during a similar time period in the early 20th century, and both borrow mathematical formalism originally developed in the field of [[fluid mechanics]].

In accretion disks and stellar surfaces, the dense plasma or gas particles collide very frequently, and collisions result in equipartition and perhaps viscosity under magnetic field. We see various sizes for accretion disks and stellar atmosphere, both made of enormous number of microscopic particle mass, 
<math> (L/V,M/N) </math>
*<math> \sim (10^{-8}\text{pc}/500\text{km/s},1M_\odot/10^{55}=m_{p}) </math> at stellar surfaces,
*<math>\sim (10^{-4}{\text{pc}}/10{\text{km/s}},0.1M_{\odot }/10^{54}\sim m_{p}) </math> around Sun-like stars or km-sized stellar black holes, 
*<math> \sim (10^{-1}{\text{pc}}/100{\text{km/s}},10M_{\odot }/10^{56}\sim m_{p}) </math> around million solar mass black holes (about AU-sized) in centres of galaxies.

The system crossing time scale is long in stellar dynamics, where it is handy to note that

<math> 1000\text{pc}/1\text{km/s} = 1000 \text{Myr} = \text{HubbleTime}/14.  </math>

The long timescale means that, unlike gas particles in accretion disks, stars in galaxy disks very rarely see a collision in their stellar lifetime. However, galaxies collide occasionally in galaxy clusters, and stars have close encounters occasionally in star clusters.

As a rule of thumb, the typical scales concerned (see the Upper Portion of P.C.Budassi's Logarithmic Map of the Universe) are <math> (L/V,M/N) </math>
*<math> \sim (\mathrm{10pc/10km/s},1000M_\odot/1000) </math> for M13 Star Cluster,  
*<math> \sim (\mathrm{100kpc/100km/s}, 10^{11}M_{\odot }/10^{11}) </math> for M31 Disk Galaxy, 
*<math> \sim (\mathrm{10Mpc/1000km/s},10^{14}M_{\odot }/10^{77}=m_\nu) </math> for neutrinos in the Bullet Clusters, which is a merging system of ''N'' = 1000 galaxies.