PX158 - D1 - luminosity and flux

luminosity

• power of an object
• units: watts ($W$)
• luminosity of sun $(L_{\odot })=3.83\times {10}^{26}W$

flux

• power per unit area as seen by an observer:

where, $d=$ distance to object

• eg: the flux of the sun from earth
  $L_{\odot }=3.83\times {10}^{26}W$
  $d=1AU=1.5\times {10}^{11}m$
  $$\therefore f = \frac{L_{\odot}}{4\pi d^{2}} = 1350 , Wm^{-2}$$
• eg: the flux from a sun-like star at $10pc$
  $L=L_{\odot }$
  $d=10pc=10\times 3.05\times {10}^{16}m$
  $$\therefore f \approx 3\times10^{-10},Wm^{-2}$$