In this series, we are looking at astronomy jargon. You will feel a little weird after today's topic.
There are spirals in the universe, with their arms stretching for tens of thousands of lightyears. The ellipticals are demanding respect for their vast populations of stars.
There are irregular galaxies. The weird people. The odd ones. The ones that defy description and categorization end up in a category because they don't fit.
The irregulars would be dropped off on the Island of Misfit Galaxies.
Most irregular galaxies are small, with a tenth of the mass of the Milky Way. There are no more than a few kiloparsec across the smallest irregulars. They usually have no structure at all. They make up a quarter of all known galaxies. They have very little active star formation and appear dimmer than regular galaxies of the same mass.
There are three different categories of irregulars. There are three different categories: a) mostly irregular but with a little bit of structure, b) totally and completely irregular, and c) both irregular and very small.
That is it. There are three categories.
Irregulars are younger than ellipticals so they may represent cast-off in the normal process of how galaxies evolve.
