Astrophysics - darkened sun

1. Dec 14, 2009

v_pino

1. The problem statement, all variables and given/known data
Explain, as quantitatively as you can, what would be observed from Earth if the Sun's surface instantaneously darkened and hence explain why the brightness of a body cannot significantly alter on time scales significantly less than the light travel time across it.

2. Relevant equations

Hubble's constant = H
velocity of galaxy = v
distance from earth = d
d = v / H

redshift = z
speed of light = c
z = v / c

3. The attempt at a solution

The first thing that came to my mind is that it will take ~ 8 minutes for the photons to reach the Earth, meaning that any observation from the Earth would be 8 min after the event. The Sun is still receding from the Earth. However, we will not be able to calculate the redshift.

Am I nowhere near what they are looking for in the answer?

thanks x

2. Dec 14, 2009

Stonebridge

Think about the fact that the Sun is a sphere (almost!), and that the centre of the visible sphere is nearer to you than the edge.