Doppler shifts as two stars orbit one another. The absolute and apparent brightness of the star gives the star's distance from Earth. The study of the way in which atoms absorb and emit light electromagnetic radiation. Spectroscopy allows astronomers to determine the chemical composition of stars. The fastest possible speed, according to the currently known laws of physics.
Electromagnetic radiation exists in the form of waves or photons moving at the speed of light.
Spiral density waves can lead to the appearance of rings. A proposed explanation for the existence of galactic spiral arms, in which coiled waves of gas compression move through the galactic disk, triggering star formation. Any object with an easily recognizable appearance and known luminosity, which can be used in estimating distances. A glowing ball of gas held together by its own gravity and powered by nuclear fusion in its core. Stars in clusters are useful to aid our understanding of stellar evolution because they are all roughly the same age and chemical composition, and lie at roughly the same distance from the Earth.