Barred spiral galaxies raise the question of what differentiates them from normal spiral galaxies. In this chapter, special features of barred spirals are explained, which indicate dependence of their forms on time.

THE CLASSIFICATION OF BARRED SPIRALS

Barred spiral galaxies contain, in contrast to normal spiral galaxies, a straight stellar bar which is symmetrical about the core and whose ends connect to the spiral arms. The term “bar” goes back to Edwin Hubble, who in 1936 introduced the classification “SB” for “spiral barred”, in order to distinguish between S and SB types. The classification of barred spirals is the same as that of normal spirals. For example, an SB type classified as SBa is a galaxy with a tight spiral pattern and bright nucleus. Moving to “later” SB types, the pitch angle increases and the core at the middle of the bar becomes more compact and less prominent.

In the galaxy classification introduced by Gerard de Vaucouleurs in 1959, the Hubble classification was extended by the SAB type. In this system, galaxies with weak bars are classified as SAB. In addition, de Vaucouleurs introduced the supplementary classifications (s), (r), and (rs) to describe the transition region between bar and spiral arms. This made it possible to distinguish between pure spiral patterns and galaxies with an inner ring connected to the bar region.

Bars often have a diffuse appearance and show less structure than spiral arms. Thus, there is no finer classification based on the bar. Only with the advent of modern methods of astronomical research was it determined that there are measurable differences in the bar structures. For example, the form of the bar can be more boxy or more disc-like. An important quantity for the assessment of the dynamics of a barred spiral galaxy is the axial ratio of the bar, i.e. the ratio of width to length. In the earlier Hubble types SBa to SBb, the relative length of the bar is greater than in the later types (SBc to SBd).