astrophysics - Shapes of galaxies

1. 
   

   I've heard most of galaxies are spiral or ellipsoid shaped. Is it true?

   
   


2. 
   

   If true, then why they form in such shapes?

   
   


3. 
   

   How did arms of the spiral galaxies form?

   
   

Answer

1. 
   

   Yes, Galaxies come in three main types: ellipticals, spirals, and irregulars. They is a system called Galaxy morphological classification; used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin Hubble.

   
   


2. 
   

   There isn't a clear or a known answer why they form , there are several studies and researches concerning this question. The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. It is one of the most active research areas in astrophysics.

   
   

3.Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies have tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain a great many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so remarkable. Source.