The study of galaxy formation and evolution focuses on the formation of the first galaxies, the way galaxies change over time, and the process that have generated the variety of structures observed in nearby galaxies. Galaxy formation is triggered with the small fluctuations in the initial density distribution. Edwin Hubble created the first galaxy classification scheme known as the Hubble tuning-fork diagram, which classify galaxies into ellipticals, spirals, and barred spirals. The disk galaxies likely formed first, and then evolved into elliptical galaxies through galaxy mergers. 

 

   Elliptical galaxies are among some of the largest known thus far. Their stars are on orbits that are randomly oriented within the galaxy. Mergers between large galaxies are regarded as violent. On the other hand, the disk galaxies evolve in isolated environment. The secular evolution process due to the bar or spiral arms play important roles in the shaping of the disk morphology.