Stars come in many types. The nearest star is at a distance of about 4 light years. From the ground, we can only measure parallax distances out to a few hundred light years. With the Hipparcos satellite, astronomers extended the range to a few thousand light years. We measure the observed brightness of stars by the magnitude scale.
Smaller magnitude means brighter.
A star's luminosity is its true power. To find the luminosity, we must measure both brightness and distance. Stars are classified by Spectral Type , which is a measure of surface temperature. We can infer the radius of a star from its luminosity and surface temperature. To do this, we use the Stefan-Boltzmann Law , which is determined from experiments on Earth.