•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