Welcome to the Brightness-Luminosity Simulation. Using the inverse square law we know that flux (apparent brightness) is proportional to luminosity and inversly proporional to the distance away squared. This is shown in the following figure which displays how the light spreads out over distance as it gets farther from the source.
In the figure below, use the distance slider and the luminosity box to give the model main sequence star a distance (in parsecs) and a radius (in Solar radii). The radius us used to figure out the luminosity of a main sequence star of that radius (in solar luminosities). Examine what happens to the star in the figure. Which value has a greatest effect on the visibility of the star?