Grand Design Galaxy
A Grand Design Galaxy, also called grand design spiral galaxies, are a type of spiral galaxy that have clear spiral structure and importantly well defined and distinct spiral arms. This is in contrast to flocculent spiral galaxies, which have a clear spiral structure, but lack distinct spiral arms. Around 10% of spiral galaxies are thought to be grand design galaxies.
The structure of the spiral arms are often explained using a model known as the "density wave model". It suggests that the arms are not physical objects; stars and gas clouds move in and out of the arm and the arm itself is just a higher density region that slowly moves around the galaxy. Originally it was thought that stars further out from the galaxy's core rotate slower than those near to the centre, creating a spiral pattern. This is known as "differential rotation". However, while this does explain spiral structure, it can only work for a young universe, not one that is billions of years old. Density wave model was introduced as an alternative explanation. However, it cannot explain how such a density wave would survive for billions of years, nor why some spiral galaxies are grand design galaxies and others not.