Galaxy
A massive, gravitationally bound system consisting of stars, stellar remnants, interstellar gas, dust, and dark matter.
Governing formula
M_galaxy = v² r / G (includes Dark Matter)
SI unit
Solar Masses (M_☉) and Kiloparsecs (kpc)
In depth
Categorized by Edwin Hubble's tuning fork diagram into spiral, elliptical, lenticular, and irregular galaxies. Range from dwarf galaxies with 10⁷ stars to giant ellipticals containing 10¹⁴ stars.
Examples in the real world
The Milky Way, a barred spiral galaxy containing ~200–400 billion stars spanning 100,000 light-years.