Our Milky Way galaxy has 59 known companion galaxies. All but two of those (the LMC and SMC) were discovered in the 20th or 21st centuries. The Draco dwarf spheroidal galaxy was discovered in 1954 by Albert George Wilson by studying photographic plates from the National Geographic Society's Palomar Observatory Sky Survey. This galaxy has the highest known dark matter concentrated object.

This is my first dwarf spheroidal galaxy target and first Milky Way companion as well. As such I've been doing a fair bit of reading to learn more. What follows are some facts about these fascinating objects.

A dwarf spheroidal galaxy (dSph) is a term in astronomy applied to small, low-luminosity galaxies with very little dust and an older stellar population. They are found in the Local Group as companions to the Milky Way and to systems that are companions to the Andromeda Galaxy (M31). While similar to dwarf elliptical galaxies in appearance and properties such as little to no gas or dust or recent star formation, they are approximately spheroidal in shape and generally have lower luminosity.

Despite the radii of dSphs being much larger than those of globular clusters, they are much more difficult to find due to their low luminosities and surface brightnesses. Dwarf spheroidal galaxies have a large range of luminosities, and known dwarf spheroidal galaxies span several orders of magnitude of luminosity. Their luminosities are so low that Ursa Minor, Carina, and Draco, the known dwarf spheroidal galaxies with the lowest luminosities, have mass-to-light ratios (M/L) greater than that of the Milky Way. Dwarf spheroidals also have little to no gas with no obvious signs of recent star formation. When it comes to the Local Group, dSphs are primarily found near the Milky Way and M31.

it is not universally agreed upon how to differentiate between a dwarf spheroidal galaxy and a star cluster; however, many astronomers decide this depending on the object's dynamics: if it seems to have more dark matter, then it is likely that it is a dwarf spheroidal galaxy rather than a faint star cluster. In the current predominantly accepted Lambda cold dark matter cosmological model, the presence of dark matter is often cited as a reason to classify dwarf spheroidal galaxies as a different class of object from globular clusters, which show little to no signs of dark matter. Because of the extremely large amounts of dark matter in dwarf spheroidal galaxies, they may deserve the title "most dark matter-dominated galaxies."