Dwarf Galaxy

A classification of small galaxies with low mass and small populations of stars. While a major galaxy like the Milky Way might contain hundreds of billions of stars, dwarf galaxies tend to contain fewer than a billion, and sometimes no more than a few thousand constituent stars. Where dwarf galaxies exist within greater galaxy clusters, they tend to be caught in the gravitational attraction of more massive galaxies, and are drawn into orbit as satellite galaxies, or distorted by the forces exerted by their more massive neighbours.

Types of Dwarf Galaxy

Dwarf galaxies are commonly (but by no means exclusively) elliptical in form, showing little distinct structure or star formation (and thus they tend to be populated by older stars). True dwarf ellipticals (categorised as 'dE' type) are distinct from the fainter dwarf spheroidal galaxies (or 'dSph' type). Though superficially similar to dwarf ellipticals, dwarf spheroidals tend to be smaller and fainter, and may represent a distinct galactic class of their own.

Other dwarf galaxies are irregular in form, and are categorised as 'dI' or 'dIrr'. These dwarf irregulars are often typified by significant levels of dark matter, and are rich in gases but low in metallicity (that is, in elements heavier than helium). Two prominent examples of dwarf irregulars the are Magellanic Clouds that orbit the Milky Way Galaxy, though these are not in fact fully irregular, and appear to be highly distorted spirals. Dwarf galaxies of this type are classed as 'Magellanic spirals'. True dwarf spiral galaxies ('dS-type') do also exist, though they are less common that the other types, and their spiral forms tend to be rather less well defined than in more massive spiral galaxies.

Especially common in the central regions of large galaxy clusters, ultra-compact dwarf galaxies (or UCDs) are extraordinarly dense dwarf galaxies. Typically containing millions of stars, these ultra-compact galaxies span just a few hundred light years in diameter, small enough (on a galactic scale) that they are comparable in size with the largest globular clusters. The origins of these densely packed galaxies is still debated, but one possibility is that they represent the nuclei of more massive galaxies, whose outer regions have been stripped away by the gravitational forces within the cluster.

Local Dwarf Galaxies

Dwarf galaxies tend to be considerably more common than their larger, more massive counterparts, and this is illustrated by the composition of the Local Group of galaxies. That group contains three major spiral galaxies: the Milky Way, the Andromeda Galaxy and the Triangulum Galaxy, but it also contains dozens of dwarf galaxies, mainly of the spheroidal classification. Of these, the vast majority have been drawn into orbit by one or other of the larger galaxies, mostly becoming satellites of either the Andromeda Galaxy or the Milky Way. The Milky Way itself is estimated to have approximately twenty dwarf galaxies in orbit around it.

Some of these satellites have done more than simply orbit the Milky Way, but have actually collided, or at least interacted, with it in the past. The Sagittarius Dwarf Spheroidal Galaxy, in particular, appears to have impacted with the main spiral of the Milky Way several times over its history, and is on course for a further such collision in the future (indeed, it is speculated that this effect may have contributed to the Galaxy's spiral structure). The Omega Centauri cluster may also represent such a historical interaction, as it appears to represent the core of an ancient dwarf galaxy whose outer stars have been absorbed into the Milky Way's own main structure.