Modelling Galaxy Collisions in Maya

Martin Watt

Other images and animations

The aim of this project was to model the behaviour of two spiral galaxies colliding using the dynamics tools in Maya. All the interactions are modelled using only gravitational fields (no animation paths are used.) The galaxies are modelled using particles to represent stars. The dust between the stars (the cloudy looking component in the images above) was modelled two ways, first using the 2d postprocess glow and halo shaderGlow attributes and second modelling the dust as particles and rendering using 3d cloud render types. The 2d postprocess looked better in stills but flickered during animations, the 3d process eliminated the flicker but did not look so good. The dynamics worked really well and gives very realistic looking results. In particular, the way tendrils get torn off and trail behind the galaxies in long loops is very nice.

Spiral galaxies

These are large (100 billion star) systems that are gravitationally bound. Most stars lie in an extended disk about a central spherical nucleus. The whole system rotates once every hundred million years or so. Most of the mass in the galaxies is in the form of 'dark matter' which is invisible and whose nature is unknown, but which will have a large gravitational effect which needs to be taken into account. Collisions between two such objects produce spectacular results, and are well suited to computer modelling since the basic gravitational interaction is straightforward to set up and simply requires a lot of number crunching.

Model setup

Since only gravitational forces are allowed (no animation paths), the stars need to be placed carefully with appropriate positions and velocities to ensure they stay in orbit about the centre.

This means overall there are 4 particle objects (two for the two nuclei and two for the two sets of stars) and two Newton fields (one constrained to the position of each nucleus.) Each Newton field is connected to the other 3 particle objects.

Simplifying assumptions

A full modelling job would run too slowly, so some assumptions were made to make the process easier to manage:

Rendering

The particles representing the stars are rendered using the cloud software render type. The particles representing the nuclei are also cloud types but are given a larger size. The particles are made incandescent, so all the light comes from the particles themselves with no external light sources. To do the dust, two approaches were used:

Rendering was performed on a Silicon Graphics Octane workstation.

Results

Here are some complete animations and also some still images taken from these animations. These animations are done with two equal sized galaxies, each with either 729 or 2000 particles to represent the stars. The galaxies are oriented at right angles to each other, one galaxy is stationary, the other travels directly towards it. The impact is offcentre rather than a core-core impact (which tends to completely disintegrate both objects) The idea was to have a noticeable interaction but still keep the two objects recognizable and distinct afterwards. All of these initial conditions can be modified to set up various scenarios with different levels of destruction. Since the 2000 star galaxies are more massive, the degree of interaction between the two galaxies is much greater for this case.





Links to external sites

Sites relate to modelling and producing animations of galaxy collisions and some real astronomical images of galaxy collisions.

Simulations and animations

Simulation from San Diego Supercomputer Center Biggest simulation to date - run for 750 hours on a Cray C90 using 250000 stars per galaxy, full N-body solution including collisions between dust clouds which involves hydrodynamic forces etc etc. The results were used in an IMAX film, `Cosmic Voyage' which has been nominated for an academy award. Here are some stills from the animation and a picture of the animation being created .

Another IMAX colliding galaxies movie based on an older simulation (1994) run on a Cray-2 using 25000 stars per galaxy. Wavefront Technologies is acknowledged as being a software supplier.

Dynamics of Galaxy Interactions - University of Hawaii Some nice MPEG movies of different types of interactions, for example this one

The Cosmic Explorer Uses the CAVE environment to display the results of cosmic simulations.

Real images

These were surprisingly hard to find. Here is one composite of a ground-based and Hubble Space Telescope image There is also a large and extremely spectacular version of the image here