AST 301
Homework #9
Due Friday April 11
1. 3C48 was the first quasar discovered. It has a redshift of z = Δλ / λ = 0.37.
a) Calculate the speed at which 3C48 moves away from us by using the non-relativistic Doppler shift formula (By the Numbers 6-2).
b) Recalculate the speed at which 3C48 moves away from us by using the relativistic Doppler shift formula (By the Numbers 14-1).
2. Use Hubble's law to calculate the distance to 3C48. Use a Hubble's constant of
70 km/s/Mpc.
3. If the luminosity of 3C48 is 1013 times the luminosity of α Cen, which is about 1.3 pc from us, how does the flux (apparent brightness)
of 3C48 compare to the flux of α Cen?
4. The diameter of the pupil of your eye is about 0.2 inches. How big in diameter would a telescope have to be to make 3C48 appear
as bright as α Cen does to your naked eye?
Hint: If 3C48 were 106 times fainter than α Cen, you would need a telescope with 106 times as much collecting area as your pupil.
If the telescope has 103 times the diameter of your pupil it will have 106 times the area
(103x103). Can you use this argument to
figure out how many times bigger diameter the telescope would have to have than your pupil to have an area as many times larger as
is needed to collect as much light from 3C48 as your pupil does from α Cen?
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