How Hubble Class Galaxies are Formed
Almost all observed galaxies can be placed into one of the four basic Hubble classes – lenticular, spiral, elliptical or irregular. It was formerly thought that galaxies evolved through the Hubble classes, from elliptical to spiral. Now, it is believed that a galaxy’s class depends on the environment in which it formed, and the mergers and interactions with other galaxies that it has undergone during its life.
According to the hierarchical scenario, the dense central bulge of the disk galaxies (lenticulars and spirals) is the consequence of the merger of clusters of stars under their mutual gravitational attraction. Accretion of further gas and dust produces the thin rotating disk surrounding the bulge.
The origin of the arms of spiral galaxies is not well understood. Mathematical models suggest that, in a smooth rotating disk of matter, any small variations in density can become exaggerated and self-propagating. The result, known as a "spiral density wave", sweeps around the disk indefinitely, compressing the matter ahead of it as it progresses. Applied to a disk galaxy, this model produces spiral arms containing many stars, with fewer stars in the spaces between the arms – as seen in astronomers’ observations.
Elliptical galaxies are thought to be produced by mergers between galaxies. Theories indicate that mergers always produce an elliptical galaxy, even when both of the merging galaxies are disks. The stars in a disk galaxy all orbit the galactic centre in the same direction, but during a merger these orbits become disrupted by gravitational forces. The result is a galaxy full of stars orbiting their common center of gravity in random directions; that is, an elliptical galaxy. Through repeated mergers, elliptical galaxies can become extremely large. M87 in the Virgo Cluster is thought to contain 200 times the mass of the Milky Way (a typical spiral galaxy) and extends up to 500,000 light-years across, compared with the Milky Way's 100,000 light-years.
Irregular galaxies are thought to arise through interactions between other types of galaxy. Such encounters are not close enough to result in a merger, but the gravitational forces involved can cause massive distortions of the galaxies' shape, sometimes eliminating all traces of their original Hubble class.