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Hubble Galaxy Classification            
The Hubble 'tuning fork' classification of galaxy morphological structure. It was devised by Edwin Hubble in 1925.            
Image copyright F. Sulehna
           

Galaxies are massive conglomerations of stars, dust, gas, nebulae and the mysterious dark matter. There are four main types, spiral, barred spiral, elliptical and irregular. There are other types of galaxies such as starburst, Seyfert, dwarf, peculiar and colliding/interacting. Altogether, a galaxy contains billions of stars and there are trillions of galaxies distributed across the vast ocean of the Universe.

Just like stars, galaxies are usually arranged in galaxy clusters and there are many superclusters. The structure of these superclusters is filled with voids and bubbles.

 
                           
 
                         
  Spiral galaxies have a large central bulge and arms of stars and dust that originate outwards. Some spirals have only two spiral arms whereas others can have four or five. Galaxies with small cores have large open spiral arms and ones with large cores are similar in appearance to ellipticals as they have tightly wound spiral arms. Most spiral galaxies have an outer halo that contains globular clusters and old stars.
                         
Whirlpool Galaxy    

Barred spiral galaxies are offshoots of spiral galaxies, but instead of having a central bulge, they have a bar and the spiral arms emanate from the ends of the bar. Recently the Spitzer Space Telescope discovered that the Milky Way is a barred spiral galaxy. At the moment it is not clearly understood how the bar is formed but might be due to gravitic tidal forces.

Two thirds of all spiral galaxies are barred spirals and after a certain amount of time, the bars disappear.

       
The Whirlpool Galaxy has a classic spiral structure. It is also interacting with a galaxy at the top. The spiral arms have many star clouds and nebulae.
Image copyright R. Gendler
         
Barred Spiral NGC 1300  
A type SBc galaxy that is 70 million light years away. The reason it looks larger than the above galaxy is because it was imaged by the Hubble Space Telescope.    
Image copyright NASA/ESA/Hubble Heritage (STSci/AURA)
   

Elliptical galaxies have no star formation or any visible features. Most of them have thousands of globular clusters at the edges and all the other stars in ellipticals are old.

Ellipticals might be formed when two galaxies merge and all the gas, dust and material is stripped from one galaxy leaving a tiny part of the galaxy. This is further reinforced by the fact that most ellipticals seem to be part of either a compact group or galaxy clusters.

Irregular galaxies are ones that don't have any spiral or elliptical structure. Most of them were originally spirals that got distorted by the gravity of other nearby galaxies.

Galaxies can crash into other galaxies and the two galaxies eventually morph into one single massive galaxy. This is how galaxies might have formed at the beginning of galactic evolution. The smaller galaxy involved in a collision merger has its gas and dust stripped away and is added to the larger galaxy.

In fact, the Andromeda Galaxy will collide with the Milky Way in 2.5 billion years time and the collision might possibly send Earth hurtling into a black hole.

   
Crashing galaxies, the Antennae                      
The Antennae Galaxy is two galaxies that are crashing into each other. The collision has caused massive star formation to occur and there are many super star clusters as well.                    
Image copyright NASA/ESA/Hubble Heritage (STSci/AURA)
   
Andromeda Galaxy
The Andromeda Galaxy is a classic spiral galaxy that is relatively close at 2.5 million light years away.          
Image copyright R. Gendler