Astrophysicists working in California and Zurich have created a virtual Milky Way galaxy using a sophisticated supercomputer simulation that took an astonishing eight months to run.

Even without the need for all that computing time, it’s not easy to create a spiral galaxy. Previous attempts yielded awkward results, but the astrophysicists working on this simulation were able to account for important processes — like supernova winds that push hydrogen gas out of a galaxy and shut down star formation — to produce a galaxy that has the right proportions.

The main challenge was to create the galaxy in the context of current cosmology, which says that the universe is mostly made up of stuff we can’t even see — dark matter and dark energy. Cosmologists (physicists who study the overall structure and evolution of the universe) have calculated what percent of the total ‘stuff’ of the universe is comprised of each major component — visible matter, dark matter, and dark energy. Astrophysicists who study galaxy formation were then tasked with figuring out how to create a Milky Way-like galaxy given these proportions. Here is the model of the universe they were given:

Data from sky surveys, such as the Sloan Digital Sky Survey, show that the universe appears on large scales to be comprised of giant sheets and chains surrounding enormous voids. Galaxies are the visible building blocks of this cosmic web-like structure. Dark matter is posited to be the main gravitational component in creating these sheets and chains, drawing hydrogen gas in to eventually create galaxies. Notice in the simulation how big globs slammed into the galaxy from all directions as it was forming — those were flows of cold hydrogen gas and smaller galaxies crashing into the nascent spiral galaxy.

The following is an excellent (and incredibly beautiful) series of simulations showing the large-scale structure of the universe and how this cosmic web likely formed:

Note: Gpc = gigaparsecs (a billion parsecs); Mpc = megaparsecs (a million parsecs); one parsec = approximately three light-years. (The ‘h’ is a parameter for the Hubble constant, which basically says how fast the universe is expanding. Its value is approximately 1.) The Milky Way is about 100,000 light-years across. The initial scale of the Millenium simulation is therefore HUGE. It’s not until the sim zooms in to the smallest scales that you can discern individual galaxies.

This most recent supercomputer simulation of the Milky Way-like spiral galaxy is a step forward, because it demonstrates that it’s possible to create such a galaxy given the known laws of physics and what we understand about the overall structure of the universe.