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. 
• In a spiral galaxy, the stars, gas, and dust are gathered in spiral arms that spread outward from the galaxy's center.  
• 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 center 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 center, 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.

• The majority of spiral galaxies rotate in the sense that the arms trail the direction of the spin. 

• The visible portion of spiral galaxies contains only a small fraction of the total mass of the galaxy, and that spiral galaxies are surrounded by an extensive halo consisting mostly of dark matter.
• Spiral galaxies are thought to evolve into elliptical galaxies as the spirals get older.