•Apparent Brightness: How bright star appears to us
•Intrinsic Brightness: “Inherent” –
corrected for distance
•How does brightness change with
distance?
–Flux = energy per unit time per unit
area: joule/sec/m2 = watts/m2
•Example:
100 watt light bulb (assume this is 100 W of light energy)
spread over 5 m2 desk gives 20 Watts/m2
–Sun’s flux at the Earth
•Luminosity = 3.8 ´1026 Watts
•It has spread out over sphere of radius 1 AU
= 1.5 ´ 1011 m
–Surface area of sphere = 4 p R2 =
2.8 ´1023 m2
•FSun =
3.8 ´1026 Watts / 2.8´1023 m2 = 1,357 W/m2
•
–Inverse Square Law: Flux falls of as 1/distance2
•Double distance – flux drops by
4
•Triple distance – flux drops by 9