Irregular and Peculiar Galaxies

Irregular Galaxies  are galaxies that have no regular structure, unlike spirals and ellipticals, and lack any discernible nucleus or spiral structure. Irregulars may once have been other types of galaxies, but have since lost their structure due to disorders in gravitational pull. Irregular galaxies contain bountiful gas and dust, facilitating the formation of stars. About 5% of all galaxies are irregular.

Dwarf Irregulars are small galaxies with low metallicity and high amounts of gas, suggesting that these are some of the youngest galaxies and may resemble the first galaxies in the universe.

Peculiar Galaxies  are, like Irregulars, galaxies that are unusual in their shape, size or composition. They are the result of the interactions between two galaxies, either gravitational or tidal forces from afar, or through collisions and mergers. They exhibit the same properties as irregular galaxies.

Example of Irregular Galaxies include NGC 1427A and the Small Magellanic Cloud, and examples of Peculiar Galaxies include the Antennae Galaxies, Mayall’s Object and NGC 520.

Classification
Using Hubble’s Classification system, irregulars are classified as either Irr I or Irr II. Irr I galaxies are irregulars with some semblance of spiral or elliptical structure, while Irr II galaxies are completely random with regards to their morphology. Dwarf irregulars are denoted with either dIrr I or dIrr II, as above.

Peculiars are designated with a ‘p’ or ‘pec’.

Properties
The stellar density of an irregular or peculiar galaxy, due to the lack of any regular structure, is left to the discretion of the worldbuilder.

High levels of radiation as well as rapid star formation could make habitability difficult in irregular and peculiar galaxies, but other than these concerns it is entirely possible for stars within an irregular or peculiar galaxy to meet the conditions for planet formation and for systems to remain undisturbed long enough for life to evolve.

As for the shape of the galaxy and, consequently, the appearance of the night sky, the worldbuilder has complete creative freedom.

Worldbuilding in Practice
"ISG 14 is a dwarf irregular galaxy (dIrr I) which orbits the elliptical galaxy Inti in the Thebes Galaxy Cluster. At this point astronomers cannot be sure, but guess that it was once a dwarf spiral galaxy which was then warped beyond recognition by immense tidal forces from Inti."