A spiral galaxy is a galaxy consisting of a rotating disk of stars, gas and dust which has a number of arms extending outwards from the galactic centre. Some spiral galaxies have a prominent bulge around the centre, while others have a bar crossing the centre with the spiral arms extending from the ends of the bar. They tend to be fairly large and blue in color. Approxmately 77% of all known galaxies are spiral galaxies.
Some Creation scientists have argued that spiral galaxies are evidence of a young universe. Specifically, the spiral arms in the galaxy should smear out after several rotations as the inner stars orbit faster than the outer stars. This is known as the "winding problem." A young universe solves the winding problem as it means there has not been enough time for galaxies to rotate even once, the Milky Way takes around 200 million years to complete a single revolution, and so not enough time for the arms to smear out.
The bulge is a round structure, located in the centre of the galaxy, and contains large amounts of dust and gas. It has an extremely high density of stars. The large amounts of dust and gas present in the bulge make it almost impossible to explore it in visible light and other wavelengths such as radio waves and infrared must be used. Many bulges are believed to contain a supermassive black hole at their centre. Stars in the bulge tend to be yellow/red.
The disk is a flat region, usually only a thousand light years thick but perhaps a hundred thousand light years across that surrounds the bulge. The spiral arms of the galaxy are located in the disk. Dust is also present in the disk, concentrated into the spiral arms. The spiral arms tend to be very bright as they contain large numbers of very luminous blue stars, with ongoing star formation.
Spirals are classified by whether they contain a bar and how tightly wound its arms are. If a spiral galaxy contains a bar, it is labelled "SB" and simply "S" if it has no bar. The tightness of the arms is characterised by the letters a, b and c, with a meaning a galaxy with the tightest winding, c with the loosest winding and b somewhere in between. The size of the bulge decreases with this classification, with a having the largest bulge and c having the smallest. For example, a barred spiral galaxy with a large bulge and tightly wound arms would be labelled "SBa".