A dwarf spheroidal galaxy ( dSph ) is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy ( M31 ) .
A dwarf spheroidal galaxy ( dSph ) is a term in astronomy applied to small , low - luminosity galaxies with very little gas 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 ) .