I N T R O D U C T I O N
BACKThis page contains links to YouTube video clips showing Python animations of planets, star clusters, galaxies, and other things astronomical. The videos are MP4 files. You can find explanatory pages by following the CAPITALIZED LINKS - usually, but not always, at the top of each box.
VIEWING THE VIDEOS: The best views cover the screen with a black background. Use the full screen option if available, or zoom in to the video panel. I recommend that the viewing environment be dark.
In almost every case the objects you watch are moving under the force of gravity according to Newton's Laws of Motion. The math and physics used here are centuries old. One can only imagine Newton's amazement and delight if he were able to view what you can watch here.
The purpose of assigning colours to individual stars in STAR CLUSTERS is to distinguish stars of various masses - also, larger symbols for larger masses. Stars in ISOLATED GALAXIES and the interacting galaxy options are coloured to indicate their location in the galaxy when the animation begins.
The animations are three-dimensional. What you see is the view from a certain direction, which can be changed while the animation runs. The view can also be zoomed in or out, and translated up, down, or sideways. Such freedom can lead to confusion; I've tried to employ it - sparingly - in order to give an impression of the 3D nature of the animation. The videos made from the animations, however, allow no such freedom.
In Python animations the size of plotted symbols - representing individual stars for the most part - do not change when the view is zoomed. So individual stars in clusters and galaxies can appear to merge when the view is sufficiently zoomed out. Stars can't be shown smaller than a single pixel on the screen. In case you're wondering, the sizes of the plotted points representing stars are much larger than they would be if they were displayed in correct proportion to the huge distances spread across the video screen. Even one pixel is vastly larger than a single star within one of the star clusters or galaxies in these videos. For example, a correctly scaled version of the Sun within the star cluster video would be roughly 2x10-7 millimetres across on your screen; you'd need a powerful microscope to see it.
When many hundreds or thousands of objects are involved, the motions can be a bit jerky. Even though individual computations are rather simple, my computer is sometimes being pushed to its limit by the number of computations required. As a rule of thumb, in order to follow the motion of "n" objects which each feel the gravity of all the other objects, n2 calculations are needed for each time step; that's one million calculations per step for 1000 stars. Cutting-edge simulations described in the professional literature employ complex algorithms that streamline the process; they run on computing machinery of colossal power.
DETAILS: If enough computing power were available I could construct a galaxy using millions or billions of stars, just like real life. As mentioned above the motion of each star would be controlled by the gravity of all the other stars, and any companion galaxy would respond to the pull of every star in its neighbour. The required computations would be way beyond my resources. Instead, I generate the gravitational forces using mathematical models of spherical structures which I call bulges and dark matter (DM) halos. Neither structure is actually visible. Mass - the source of the gravitational force - is assumed to be smoothly spread within each kind of structure, and the mass density - mass per unit volume - is assumed to depend only on distance from its centre. When a halo is present it's always concentric with the bulge. Where they overlap, the mass density of the two components are added. Importantly, neither bulge nor halo is allowed to change its shape or mass distribution during the animation. Although such changes probably occur during real galaxy interactions, the computations needed to account for them would be prohibitively large. The Milky Way and most large nearby galaxies have bulges made up of millions or billions of stars, plus a central supermassive black hole in most cases. I cannot create simulated bulges using vast numbers of stars, but in some animations I include a manageable number of tracer stars to illustrate the shape and extent of the bulge. The DM halos surround the galaxies you see, as in the actual universe. We do not know what comprises them, only that it exerts a gravitational force.
MORE DETAILS: Given the above assumptions, the disk galaxies you usually see here are represented by a bulge imbedded in a larger, concentric DM halo. Tracer stars are distributed in a disk also centred on the bulge, but initially inside the halo. The animation begins with them moving in circular orbits. In some cases the disk is assumed to penetrate to the centre of the bulge; in others I have added a separate population of bulge tracers which are moving initially in random directions. Since real elliptical galaxies typically lack disks, they are represented as bulges scaled up to galaxy size, plus a surrounding DM halo. Elliptical galaxy tracer stars are assigned random initial motions.
SUMMARY: The tracer stars - usually several hundred to a few thousand - show roughly what would happen during an interaction of two galaxies. Reality, though, is more complex; it includes such messy scenarios as collisions between extended clouds of gas and dust and the resulting formation of new stars. So you shouldn't expect these simulations to accurately emulate real galaxy interactions. Given the large range of possible initial conditions and viewing angles, even coming close to the appearance of systems like The Antennae or The Mice would take perseverance and a bit of luck.
Depending on available time and motivation, I might revise the present videos or add videos of different animations. I'm still exploring the possibilities.
Want to comment? You can reach me at gaw@eastlink.ca