Galax­ies are by defin­i­tion vast rotat­ing sys­tems of stars, gas and dust. They are the build­ing blocks of the uni­verse. Galax­ies range in size from dwarf types, only a few thou­sand light years across to great spir­al and ellipt­ic­al galax­ies span­ning sev­er­al hun­dred thou­sand light years across. They con­tain any­where from a few mil­lion to as many as one tril­lion stars.

The earth is loc­ated with­in a large spir­al arm (the Ori­on arm) in a spir­al galaxy we call the “Milky Way”. The closest galaxy sim­il­ar to our own is the Andromeda Galaxy (M31).

[descrip­tion from Rob Gendler’s primer]