Galaxies are the basic structural units of the universe. A galaxy is a stellar group containing hundreds of million to hundreds of billion of stars. It also contains considerable quantities of interstellar matter (gas, dust...).

The classification of galaxies, according to E.P. Hubble, is dependant of their shape whether they are elliptic, lenticular, spiral or of irregular shape.

Etymology : The word galaxy is from the Greek galaxies or kyklos galaktikos which means 'milky circle'.

Whirlpool galaxy

Other deep sky objects:

  • Star clusters (open clusters and globular clusters)
  • Diffuse nebula (emission nebula and reflection nebula)
  • Planetary nebula
  • Dark nebula
  • Supernova remnants