Galaxy
A vast system of billions of stars, gas and dust, held together by gravity — our own is the Milky Way.
Every star you see with the naked eye belongs to our own galaxy, the milky way. Beyond it lie billions of others, from tiny dwarfs to giants of a trillion stars. The nearest large one, the Andromeda Galaxy (M 31), is about 2.5 million light-years away — the most distant thing you can see without optics.
Through a telescope most galaxies are soft oval glows with a brighter core. Spiral arms and dust lanes need a dark sky, a bigger aperture and some practice. Astronomers sort them by shape: spirals, ellipticals, lenticulars and irregulars (see hubble type).
At the eyepiece
Galaxies are hurt most by light pollution and moonlight. Use medium power (a 2–3 mm exit pupil) and no nebula filter — galaxies shine in starlight, which those filters block.