What is a Spiral Galaxy?

About Spiral Galaxy:

• Spiral galaxies are twisted collections of stars and gas that often have beautiful shapes and are made up of hot, young stars. 
• Most of the galaxies that scientists have discovered so far are spiral galaxies, as opposed to the other two main categories of galaxy shapes—elliptical and irregular.
• Approximately 60% of all galaxies are thought to be spiral galaxies.
• The Milky Way, the galaxy that includes Earth and our solar system, is an example of a spiral galaxy.
• Structure:
  • Most spiral galaxies contain a central bulge surrounded by a flat, rotating disk of stars. 
  • The bulge in the centre is made up of older, dimmer stars and is thought to contain a supermassive black hole. 
  • Approximately two-thirds of spiral galaxies also contain a bar structure through their centre, as does the Milky Way. 
  • The disk of stars orbiting the bulge separates into arms that circle the galaxy.
  • These spiral arms contain a wealth of gas and dust and younger stars that shine brightly before their quick demise.
• Spiral galaxies are thought to evolve into elliptical galaxies as the spirals get older.