Question

A quasar has a brightness that corresponds to apparent magnitude 13.6 (Unit 55.4).

a. If it is 1.18 billion pc away, how many times brighter than the Sun is it?

The quasar is ______× 10¹¹ times brighter than the Sun.

b. How does this compare to the brightness of the Milky Way? Assume that the Milky Way contains about 10¹¹ stars.

The quasar is _______ times brighter than the Milky Way.

Answer 1

The relation between Apparent (Ap), Absolute magnitude (Ab) and distance (d) is given as

Hence,

Ap-Ab+5=5log(d) where log is to the base 10

So for the Quasar the absolute brightness is

13.6-Ab+5=5log(1.18 x 10⁹ )

Ab=13.6+5-5log(1.18 x 10⁹ )=13.6+5-45.5=-26.9

Now,

Ab of Sun is +4.83

Now Luminosity ratio between the objects is given as

B) If we assume that the Milky way contains 10¹¹ Sun like stars then the Luminosity of Milky way is 10¹¹ L_S.

Hence, Quasar is 49 times brighter than the Milky Way,