Astrophysics is the study of the physics of stars and stellar systems and interstellar materials. The laws of physics are applied to astronomical systems to help us understand how these systems came into being, how they work with the other systems and how they cease. The Laws of Newton give an accurate model for the dynamic system of stars. This is the known as the study of stellar dynamics.
Newton not only studied dynamics but also calculus in order to further understand the way the planets moved. He came up with a mathematical model that still holds today. His model is still fundamental to the study of star clusters, galaxies and the solar system.
Newton’s equation still holds as a good start to model many of the astrophysical systems such as the solar systems, star clusters and galaxies. The reason that Newton’s equation still holds today is due to gravity. Gravity is a long-range force in the universe that has an attraction force in everything in the universe. Gravity was in Newton’s early equation and therefore makes the equation valid for a large amount of astrophysics studies.
In stellar dynamics, gravity plays a role as a long-range force. Electromagnetism is also a long-range force but is not considered important in the large scale because the positive and negative charges cancel each other out.
There are short-range forces like gas pressure that may have some effect on small scales like inside of stars. Within large scales, pressure is not an issue.
With some middle size stellar systems, gas pressure can pay a role when there are big gas clouds that exert pressure and magnetic fields are exerted. These have some bearing in the middle size stellar systems but in gigantic or small systems, gas pressure is usually not considered.
Having gravity a constant in our universe makes it easy for simulation and for relatively simple equations to be used in astrophysics. Generally complicated math equations and physics principles are not necessary when used in plasma astrophysics, radioactive transfer or nuclear astrophysics.
Stellar dynamics is the study of the effect of Newton’s equation in astrophysical environments. Newton had studied the motion of the planets. Celestial mechanics is the study of coming up with analytical approximations of the motion of the planets.
Our solar system is a very predictable and consistent system. The planets move around the sun in the same direction and in an ecliptic pattern. The planets are spaced apart with virtually no close encounters possible.
With our galaxies and other star clusters, close encounters are possible. There is not a set direction that stars can move. Stars can move in any direction. Analytic approximations are more difficult to determine with the less constant system of stars. For some galaxies, analytic as well as semi-numerical models are used. But to be more precise, numerical simulations are used. Strong computers have added significantly to this study.
There are two subfields of stellar dynamics namely, collisional and collisionless stellar dynamics. Collisional stellar dynamics is the aspect of gravitational meetings with pairs of stars. Sometime people tend to think that collisional stellar dynamics refers to the stars actually colliding with each other, but that is not accurate.
In very dense systems of stars, you can have physical collision of stars but collision stellar dynamics is referring to the role that gravity plays with stars. In dense systems of stars, motion is dominated by the gravitational forces put out by the very large number of stars. This is known as a collisionless stellar dynamic.
The long-term effects of close star encounters involves a slow diffusion of ‘heat” from inside the star to the edge of the star. Heat is transported through the close encounter of pairs of stars. There is actually little heat exchanged during the close encounter.
Global clusters are the embodiment of idealizations of collision stellar dynamics. They are almost spherical because they don’t rotate very much. They are pretty much isolated from influences of the galactic disk.
To get an idea of a globular cluster, imagine that you are at its very core. The stars that would be normally seen in the night sky would be brought closer by a factor of 102 and each star would become brighter by a factor of 104. The brightest star would be comparable to the light of the full moon. You could not directly look at the stars because they would be too bright.