National Astronomical Observatory of Japan

Formation of a Spiral Galaxy ver. 3


We live in the Milky Way Galaxy. Based on various observations, it is thought that this is a spiral-shaped disk galaxy. And many galaxies of this type have been observed in the Universe. How are these spiral disk galaxies formed?

This video introduces computer simulations of galaxy formation based on the currently accepted hierarchical formation scenario, in which small galaxies form first and then merge to create larger galaxies.

From a Cluster of Small Galaxies to a Single Large Galaxy

Right after the beginning of the Universe, matter is distributed almost uniformly in space. But because small density fluctuations exist, matter is gradually collected by the gravity from parts with higher density. Before long, the gas density becomes high enough and stars are born, forming small galaxies. These small galaxies merge with each other, creating large galaxies. In the process of merging, a disk is formed and stars are born in this disk. As small galaxies pass around the periphery of this disk, the effects of their gravity create a spiral structure in the disk. Spiral galaxies like our Milky Way Galaxy are thought to be formed in this manner.

Video Data

Computer UsedGRAPE-5
Number of Particles1,000,000 dark matter; 1,000,000 baryon
Phenomenon Time Scale~13.5 billion years
Phenomenon Spatial Scale~300,000 light-year
Calculations byTakayuki R. Saitoh (with NAOJ when the calculations were performed, now at Tokyo Institute of Technology)
CreditTakayuki R. Saitoh (simulation); Takaaki Takeda, Sorahiko Nukatani (visualization); Four-Dimensional Digital Universe Project, NAOJ

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