Galaxy Collision Simulator Instructions
Quick start – try these two icon buttons – try pressing the Rewind Simulation icon (in pink) to restart the current simulation and then press the Switch Camera icon to watch it from three different vantage points (click the Switch Camera icon multiple times to move sequentially between the 3 views). Each press of the Rewind Simulation icon alternates between a “Setup Path” view and a full “Star and Dust” simulation view. The “Setup Path” view schematically calculates where the red galaxy will go (showing a red disk with the dotted path and orientation indicators). The more detailed “Star and Dust” simulation view uses the latest parameters changed in the “Setup Path” view and then rewinds the simulation to use them. Each Rewind press alternates between these setup and full star view styles and restarts the collision simulation with the current parameters.
Use the right mouse and middle mouse buttons to rotate and zoom each view as desired. Each simulation has 3 camera views tracking the galaxies – one follows the green galaxy center, one follows red, and one deep camera shows both from a long distance away. You can reorient and zoom the cameras by dragging with the mouse right and center button or you can use arrow keys and zoom with the angle bracket keys <,>.
Once you are done viewing the current setup from multiple camera positions and want to experiment with the red galaxy’s speed, mass and starting position, click on the Rewind Simulation button once again to get back to the Setup Path (dotted line) view and use the sliders at the upper left to change the starting conditions for the red galaxy (most of these setup slider only appear in this Setup Path view). Change the parameters as desired then press the Rewind Simulation icon once again to run the full Star and Dust simulation view with those new parameters.
Changing the starting position and velocities of the red galaxy relative to the green galaxy controls the path of the collision or creates a glancing encounter or maybe just a small gravitational interaction instead.
Try changing the relative mass of the red galaxy to be less than or more than the green galaxy. Which galaxy keeps its shape better in these different cases? What happens when they are about the same?
In either simulation view you can use the Simulation Speed slider or the keyboard numbers between 1 and 8 to control the simulation rate depending on the performance of your computer. Toggling the Space bar temporarily pauses and un-pauses the simulation (Note: a speed of 1 is extremely slow and may also appear paused).
Note: These applications run on Chrome, Edge, recent Safari or Firefox browsers on Mac and PC (but not Windows Explorer).
What is a Galaxy?
The stars and dust clouds found in galaxies are held together by their mutual gravity as they orbit around a common center – in this simulation, two galaxies are about to collide. They are artificially colored in red and green to make them more visible as they do so.
The word “collision” is a bit of a misnomer however. Since space is really big and the matter in galaxies is spread very thin, this means that actual collisions between individual stars or planets are quite unlikely.
What Happens in a Galaxy Collision?
In a galaxy collision, large galaxies absorb smaller galaxies, tearing them apart and incorporating their stars. But when the galaxies are similar in size and mass – like the Milky Way and Andromeda – the close encounter destroys the spiral arm structure. The two groups of stars eventually become a giant elliptical galaxy with no real structure.
Such collisions also trigger new star formation. When the galaxies collide, it causes vast clouds of hydrogen to collect and become compressed through shock waves, which can trigger a series of gravitational collapses of dust and gas, resulting in millions of new stars and eventually new solar systems like our own. In this simulation, new star creation shows up as brief flashes of bright yellow stars created where dust cloud shockwaves compress near galactic disk crossings.
A galaxy collision also causes a galaxy to “age” prematurely, since some of its gas is converted into stars shortly after a collision using up its future star-making materials. After this period of fast star formation, galaxies run out of fuel. The youngest hottest stars explode as supernovae leaving the older, cooler red stars with much longer lives. This is why giant elliptical galaxies, the results of galaxy collisions, have so many old red stars and very little active new star formation.
Interactions between galaxies are quite common given the billions of galaxies within our view, and especially between giant and smaller satellite galaxies. This is often the result of galaxies drifting too close to one another, to the point where the gravity of the satellite galaxy will attract one of the giant galaxy’s spiral arms.
In other cases, the path of the satellite galaxy may cause it to gravitationally intersect with the giant galaxy. Collisions may lead to mergers, assuming that neither galaxy has enough momentum to keep going after the collision has taken place. If one of the colliding galaxies is much larger than the other, it will remain largely intact and retain its shape, while the smaller galaxy will be stripped apart and become part of the larger galaxy (you can experiment with this in our simulation, by changing the galactic mass slider in the Setup view).
Some stars will also be thrown out of the galaxy and left in spiraling trails; a few may be destroyed as they crash into the merging supermassive black holes found in many galactic centers. The delicate spiral structure of both galaxies will be destroyed as they become a single, giant, elliptical galaxy. But as cataclysmic as this sounds, this sort of process is a very natural part of galactic evolution over billions of years.
Galaxy Collisions Are Happening Right Now
Such collisions are relatively common, and Andromeda is believed to have collided with at least one other galaxy in the past. Several dwarf galaxies (such as the Sagittarius Dwarf Spheroidal Galaxy) are currently colliding with the Milky Way and merging with it. New evidence from the GAIA space probe and advanced simulations like this even point to the possibility that our own sun and solar system (about 4.6 billion years old) may have been created as a result of the very first collision with Sagittarius DSG about 5-6 billion years ago. Parts of the smaller Sagittarius DSG continue to orbit around and through our Milky Way galaxy disk, causing new star formation and an elongated trail of stars each time it passes by.
Read a summary of this research here: Galactic Triggered Solar Formation
It is currently estimated that a galaxy like our milky way has between 100 billion and 400 billion stars so in this simulation each one of the ~10,000 red or green stars you see here represents approximately 10 to 40 million stars in a real galaxy!