Download Brightness Luminosity and Inverse Square Law

Why do the stars look as bright as they do?
the nature box
Why do the stars look as
bright as they do?
Consider two light bulbs.
30 Watts
120 Watts
30 Watts vs. 120 Watts
If you wanted the bulbs to appear
equally bright………how much farther
away would you place the 120 Watt
bulb?
Nope, not 4 times further. TWO TIMES
FURTHER AWAY.
Brightness follows the inverse
square law!
The Inverse Square
Law!
Apparent brightness
(b) changes with the
inverse square of the distance (d).
b α 1/d2
At 2x the distance, brightness is 1/22 or 1/4th
At 3x the distance, brightness is 1/32 or 1/9th
How would the brightness change if you
increased your distance from a star by 10
times?
brightness “b” and Luminosity “L”
Luminosity is the Power output of a star
in Watts
(1 Watt = 1 Joule/second)
brightness as viewed from a particular
location is the Power received per unit
area, in W/m2
b=
L
4πd2
How to use the inverse square law for
brightness of stars…
We know that the apparent brightness “b” of our
Sun as viewed from Earth is 1362 W/m2
We also know that Saturn is 9.7 times further away
from our Sun than the Earth.
If b α 1/d2 , then as viewed from Saturn, the sun
would appear 1/(9.7)2 or 1/94th as bright.
The brightness would then be 1362 W/m2(1/94) =
14.5 W/m2
Exit slip!
We know our “Solar Constant” (the apparent
brightness of our Sun from Earth) is 1362 W/m2
Neptune is 30 times the distance from our Sun
than is the Earth.
Calculate, using the inverse square law, the
apparent brightness of the Sun as viewed from
Neptune.