A Journey Through the Milky Way

Our home galaxy, the Milky Way, is believed to be a typical barred-spiral galaxy, composed of the stellar disk, gaseous disk, central barred bulge, and dark matter halo. But observations alone cannot determine clearly how our galaxy looks from the outside, or its evolutionary history. This is mainly due to uncertainties in Solar System’s location within the disk itself and difficulties in determining the distances to sources when mapping Galactic structures.
This movie displays the various structures of the simulated Milky Way Galaxy, based on fundamental physical processes such as the gravitational interactions between stars and gas; the dynamics and evolution of the gas; star formation from the gas clouds; and energetic feedback from the stars.

This is the simulated “Milky Way.” Using a telescope he built himself, Galileo Galilei as the first to recognize that the Milk Way spreading across the night sky is composed of countless stars. In addition to the stars, now it is known that there are red-colored “diffuse nebulae” in the Milky Way. Furthermore, we can see a dark belt structure that cuts across the Milky Way. This is the so-called “dark lane.”
Let’s zoom-in on the Milky Way. Then we can see that the dark lane is composed of many cloud-like objects, the so-called “dark clouds.” The high-density nature of these clouds blocks background starlight and makes the clouds dark. Most atoms in the dark clouds such as hydrogen, carbon, and oxygen exist in the form of molecules. Thus, dark clouds are often called “molecular clouds” as well. Stars and star clusters form inside molecular clouds.
Next we step back from the Milky Way. We can see that most of the stars and gas clouds are distributed in the form of a flat “disk.” Furthermore, we can see a central bar-like structure and spectacular double spiral arms extending from the ends of the bar. This is the true nature of the “Milky Way” we see in the night sky: the “Milky Way Galaxy.”
Now we run a time evolution to see how stars move inside the Milky Way Galaxy. We can see that the stars move around the center, as well as up and down relative to the Galactic disk, likes dolphins near the surface of the sea. It is believed that the Solar System also moves like this.
Moving back inside the Galactic disk, we see that young star clusters born inside molecular clouds emit strong ultraviolet light. This light ionizes the surrounding gas and forms red-colored nebulae, i.e. H II regions. Then, these H II regions expand and destroy the parent molecular clouds.
Countless stars comprise the Milky Way we see in the night sky. It contains various Galactic structures like the bar and the spiral. In addition the process whereby star clusters form within the molecular clouds and then emit energy which illuminates and dissipates the surrounding gas is repeated over and over.

Details of Numerical Simulation

PurposeTo reveal the differences in the motions of stars and gas, as well as the formation processes of molecular clouds in various Galactic structures (such as spiral arms and the bar) in the Milky Way Galaxy
Calculation ModelASURA code which implements the Tree-GRAPE method for the gravity calculations, and smoothed particle hydrodynamics (SPH) for the gas dynamics. Physical processes such as radiative cooling; supernovae and Hii region heating; far-ultraviolet heating; molecule formation; and star formation are also included.
Number of Particles: baryons: 10 – 100 million
dark matter: fixed external potential
ComputerCray XC30 "ATERUI" (CfCA, NAOJ)
Time Scale~ 100 – 1000 million years
Spatial Scale~ 500,000 light-years
ResearchersJunichi Baba(now at NAOJ; Tokyo Institute of Technology and Ehime University at the time of the simulations)
ReferenceJ. Baba: "Short-term dynamical evolution of grand-design spirals in barred galaxies", 2015, MNRAS, Volume 454, Issue 3, pp. 2954-2964
J. Baba, K. Morokuma-Matsui, & T. R. Saitoh: "Eventful Evolution of Giant Molecular Clouds in Dynamically Evolving Spiral Arms", 2017, MNRAS, Volume 464, pp.246-263
The simulation results visualized in this movie were supported in part by MEXT SPIRE Field 5 and JICFuS.

Details of Visualization

This movie was visualized and rendered using the “Unity” game engine. Not only the distribution and color of the stars, but also the H II regions and dark nebulae are visualized simultaneously. H II regions are areas where ionized hydrogen gas emits red light. So the 10,000 degree gas is reproduced here by giving it a red color. Also the dark nebulae, which appear black because they block the light from background stars, are rendered based on the density of the molecular gas. Because this video was made as an all-round stereoscopic image, you can view it with a head mounted display (HMD).

YouTube for VR

Web Browsers: You can aim the movie point-of-view in any direction you prefer by dragging the mouse over the YouTube screen.
Smartphones and Tablets: Using the YouTube App (iOS, Android), all directions can be seen by facing your device in the chosen direction. With VR viewers like Google Cardboard, you can enjoy the full effect of stereoscopic vision in all directions.
※Please note that there are some cases in which this video cannot be watched depending on your environment.


  • Simulation: Junichi Baba
  • Visualization: Hirotaka Nakayama
  • Four-Dimensional Digital Universe Project, NAOJ


MWJourney_360p_mov.zip (.mov, zip file : 379.2 MB)

MWJourney_360p_wmv.zip (.wmv, zip file : 35.8 MB)


MWJourney_720p_mov.zip (.mov, zip file : 994.3 MB)

MWJourney_720p_wmv.zip (.wmv, zip file : 137.8 MB)


MWJourney_1080p_mov.zip (.mov, zip file : 1.95 GB)

MWJourney_1080p_wmv.zip (.wmv, zip file : 170.4 MB)

Side-by-Side Stereovision, 1080p

MWJourney_SBS_1080p_mov.zip (.mov, zip file : 2.29 GB)

MWJourney_SBS_1080p_wmv.zip (.wmv, zip file : 168.8 MB)

Panoramic Video for VR

MWJourney_pano_SBS_mp4.zip (Stereovision with Side-by-Side, .mp4, zip file : 1.7 GB)

MWJourney_pano_TB_mp4.zip (Stereovision with Top-and-Bottom,.mp4, zip file : 1.67 GB)

MWJourney_pano_mp4.zip (Non-Stereo, .wmv, zip file : 1.69 GB)

Other file formats

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  • 4096x4096 Stereo/Non-Stereo Dome Master (.mp4, .png sequence files)
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  • 2017.6 Version 1.0 was released.