Physics Question on Stars

Question

A new star is discovered to be approximately 1 light year from Earth. When measured on Earth it has intensity of 1.34x10^-1 Wb/m. 

a) How long does the light ray take to reach us from this star?

b) What would be the intensity of the star if we were 1.0x10^6 m from it?

Answer 1

a) It is kinda obvious, 1 year :)     A light-year is the distance light travels in one year.

 

b) The original distance to the star is D = V x t , (3 x 10^8 m/s) x (3.154 10^7 s) = 9.5 x 10^15 m when the intensity is 1.34 x 10^-1 Wb/m. 

 

Then part b says we are closer since 1.0 x 10^6 is smaller than the original distance, thus the intensity should increase proportionally to the square of the ratio between the distances due to inverse square law. 

 

So the ratio is 9.5 x 10^15 m / 1.0 x 10^6 m = 9.5 x 10^9 , then take the square = (9.5 x 10^9)^2 = 9.03 x 10^19 (Rounded) (Important! The ratio does not have a unit)

 

Lastly, multiply the original intensity with this ratio squared (1.34 x 10^-1) x (9.03 x 10^19) = 12.1 x 10^18 Wb/m