This is a computer simulation of the formation of a galaxy like our Milky Way. A galaxy is a huge collection of hundreds of billions of stars immersed in vast amounts of gas and dark matter. There are hundreds of billions of galaxies within the observable universe. Observational astronomers take pictures of these galaxies to learn what they're like, where they came from, and what that means to us in the Milky Way. Galaxies last for many billions of years, so unfortunately we only get to see a tiny snapshot of their life - even if we look at every telescope image taken in the past hundred years. If we were able to take telescope photos millions of years apart, we would actually be able to see these galaxies moving - they spin around, smash into other galaxies and turn into all sorts of interesting shapes. That's where computer simulations are useful - by using powerful supercomputers like the Cerberus cluster at Saint Mary's University, we can see what it would look like to watch a galaxy for billions of years.
The galaxy in this simulation forms from a big ball of gas. There are two important forces in this simulation - gravity and pressure. Gravity is always attractive and tries to squish the gas as much as it can. Pressure is always repulsive - it tries to make the gas spread out. Hotter gases have higher pressure. At first the ball of gas is in "hydrostatic equilibrium" - this means that the gas is hot enough that the pressure exactly balances out the gravity, and the gas ball doesn't move. Then the gas starts to cool, and the pressure goes down. Gravity is now stronger than pressure, and the gas-ball starts to collapse. In this simulation, the gas first collapses into little clumps, and these clumps collide to form a disc of gas - similar to what the Milky Way may have formed from.
-Simulation performed by David Williamson and Rob Thacker (June 2010).
