The information content in the radiation recorded in observations allows astronomers to derive the properties of celestial objects. The ranges of the values of these properties are found to be “astronomically” large. Luminosities are derived from measured fluxes and distances. The solar luminosity, 3.8 × 1026 W, is a benchmark reference; that of a bright quasar is 1013 times larger. The mass of the moon, earth, or of a galaxy can be determined by tracking the motion of one or more orbiting objects. The sun's mass, 1.99 × 1030 kg, is also a standard reference; the (Milky Way) Galaxy is > 1011 times more massive. The virial theorem is used to obtain the masses of clusters of galaxies. Temperatures can be defined for thermal sources, wherein the matter and radiation are in, or approximately in, thermal equilibrium. The temperatures of a hot gas may be determined in a variety of ways that may yield different values. Thus astronomers refer to kinetic, color, effective, excitation, and ionization temperatures. The last is obtained from spectral observations with the aid of the Saha equation.

The distance to a celestial object is not an intrinsic property but it is required to find intrinsic quantities. Ancient astronomers used geometry to learn the earth size and distance to the moon. The mean earth–sun distance is defined as the astronomical unit (AU). 1.00 AU =1.496 × 1011 m. […]