It is difficult to determine the original galaxy forms of the systems discussed in this chapter. Compared to other galaxy types, they exhibit a much larger range of possible forms.

THE MORPHOLOGY OF IRREGULAR GALAXIES AND INTERACTING SYSTEMS

In this chapter, irregular galaxies and interacting galaxy systems are considered together. Based on their morphology, which in both cases is characterized by asymmetry and irregularities, the two cases are usually distinguished in astronomy. The examples discussed here, however, demonstrate that one often cannot sharply separate irregular and interacting galaxies. In fact, one often has to take into account combinations of events and interdependencies in the astrophysical interpretation of the visible forms.

About three to four per cent of galaxies cannot be classified as elliptical or spiral since they are lacking the basic structures in their appearance. For example, often a plane of symmetry or clearly defined centre is lacking, which sometimes leads to the observation of collections of large-scale star-forming regions, loose H II regions and individual dust filaments. Their masses are in the range 108–1010 solar masses and at a few thousand to 30 000 light years, their diameters are very small. The term “irregular galaxy” should not be confused with “peculiar”. The latter is used as a qualifier to the standard galaxy types, for example to indicate that a spiral structure exists but is perturbed, but still allows the original classification to be recognized. In the case of irregular galaxies, however, there is no sharp dividing line between perturbation and typical structure, especially since the irregular forms can also be interpreted with respect to the diverseness of their histories.

Interactions with other galaxies are mainly responsible for the appearance of irregular galaxies. There is a broad spectrum of possible interactions. These range from weak perturbations in the gravitational field of two approaching galaxies to stronger tidal interactions which can cause a flow of material to a collision of galaxies which can even end in a merger and produce a new galaxy.

THE GREEK TRADITION

Human beings have long looked up at the sky and pondered its mysteries. Evidence of the long struggle to understand its secrets may be seen in remnants of cultures around the world: the great Stonehenge monument in England, the structures and the writings of the Maya and Aztecs, and the medicine wheels of the Native Americans. However, our modern scientific view of the universe traces its beginnings to the ancient Greek tradition of natural philosophy. Pythagoras (ca. 550 b.c.) first demonstrated the fundamental relationship between numbers and nature through his study of musical intervals and through his investigation of the geometry of the right angle. The Greeks continued their study of the universe for hundreds of years using the natural language of mathematics employed by Pythagoras. The modern discipline of astronomy depends heavily on a mathematical formulation of its physical theories, following the process begun by the ancient Greeks.

In an initial investigation of the night sky, perhaps its most obvious feature to a careful observer is the fact that it is constantly changing. Not only do the stars move steadily from east to west during the course of a night, but different stars are visible in the evening sky, depending upon the season. Of course the Moon also changes, both in its position in the sky and in its phase. More subtle and more complex are the movements of the planets, or “wandering stars.”

The Geocentric Universe

Plato (ca. 350 b.c.) suggested that to understand the motions of the heavens, one must first begin with a set of workable assumptions, or hypotheses. It seemed obvious that the stars of the night sky revolved about a fixed Earth and that the heavens ought to obey the purest possible form of motion. Plato therefore proposed that celestial bodies should move about Earth with a uniform (or constant) speed and follow a circular motion with Earth at the center of that motion. This concept of a geocentric universe was a natural consequence of the apparently unchanging relationship of the stars to one another in fixed constellations.

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).