Galaxies and the Milky Way

Galaxies can be broadly classified by appearance into categories such as: spiral; elliptical; irregular. The Milky Way is a barred spiral galaxy. It contains: stars; stellar systems; interstellar gas; dust; dark matter. Our Solar System is located within the Milky Way, not at its center. Observations of distant galaxies are extremely important because their: positions; distances; spectra; redshifts; * motions provide information about the history and expansion of the universe.

For a simplified circular-orbit estimate: **v²r ≈ GM/r² therefore: v² ≈ GM/r** This simplified Newtonian relation helps introduce how orbital motion can reveal gravitational mass.