The apparent brightness is defined by how much light we get pretty much. The formula is:
This takes into account that the light is spreading out to a sphere that has the surface given by 4πd^2.
We have that d=20 ly and 1 ly is 9.46*10^12 km or 9.46*10^15m.
Hence, d=20*9.46*10^15=189.2 *10^15 m
Now, we can substitute the d value and calculate that L=<span>8.45184*
W/m^2</span>
This takes into account that the light is spreading out to a sphere that has the surface given by 4πd^2.
We have that d=20 ly and 1 ly is 9.46*10^12 km or 9.46*10^15m.
Hence, d=20*9.46*10^15=189.2 *10^15 m
Now, we can substitute the d value and calculate that L=<span>8.45184*