A spiral galaxy is a flat galaxy with spiraling arms (spiral arms). One of three galaxy classes described by Edwin Hubble along with elliptical galaxies and lenticular galaxies. The galaxy classification designations for spiral galaxies are SA or SB followed by a letter, a,b,c or d, indicating how tight are the arms. SB is for barred, i.e., a barred galaxy, a spiral galaxy whose center appears like a bar between two opposing arms rather than just a circular mass of stars.
Spiral galaxies are generally younger than elliptical galaxies with more interstellar gas and dust, and are generally forming stars.
The dynamics that produces the galaxy's spiral structure (i.e., the spiral arms) is not immediately apparent: if the arm structure simply consists of the position of the stars as they orbit around the galaxy, the arms would too-soon wrap far tighter than what is evident from populations. Theories are under study including the concept that the visible bright arms consist of young, bright stars resulting from rotating "fronts" of star formation. N-body simulations have resulted in a spiral structure.
The stars' orbits are not simple circles, and the density of stars in the plane is such that the dynamics is somewhat like a disk rather than an independent bodies orbiting the center. Efforts to characterizing the orbits in a useful way include an epicycle frequency (toward and away from the center) and vertical oscillation frequency (up and down from the plane).
The Large Magellanic Cloud and Small Magellanic Cloud were earlier considered irregular but are now sometimes cited as single-armed spiral galaxies.