## Color and Amounts of LightThe physical property that magnitude actually measures is radiant flux - the amount of light that arrives in a given area on Earth in a given time. Since color is measured by magnitude, a star's color also depends on how much light arrives at Earth. Radiant flux is the physical basis for color. The definition of magnitude m in terms of radiant flux F is: m = -log The star Vega in the northern hemisphere constellation Lyra is used as the standard
for the magnitude system, so F This does The negative sign in the definition ensures that brighter stars have smaller magnitudes. So if Earth receives less light from a certain star than from Vega (through a given filter), that star's magnitude will be positive. If Earth receives more light from a certain star than from Vega, that star's magnitude will be negative.
Remember that magnitude is a logarithmic quantity; a magnitude four star emits 2.51
A rule of logarithmic quantities, true for a logarithm with any base number
(whether 10, e, or 2.51), is that log (x) - log (y) = log (x/y). Color is a difference in
magnitudes; g-r is the difference between a star's green magnitude and the same star's red
magnitude. Since magnitude is the log
In other words, a star with G-R = 0.8 emits 2.08 times as much red-wavelength light as green-wavelength light. Click here for another example. ## What is Color?So far you've looked at stars and calculated their astronomical colors. But what exactly is this thing we've been calling "color"? What is it about a star that makes it red or blue or yellow? Click Next to find out. | |||