The quasar 3C 273 is believed to have a luminosity of about 2.5 × 10¹³ L_e at a distance of 620 Mpc. Suppose instead that it were a normal galaxy with a luminosity 1000 times smaller. How far away would it have to be to have the same apparent brightness it has now? A)620 kpc B)6.2 Mpc C)19.6 Mpc D)620 × 10⁹ pc

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Quizplus · Accepted Answer

The apparent brightness is proportional to luminosity divided by distance squared. If the luminosity is 1000 times smaller, the distance must be √1000 times smaller to maintain the same apparent brightness. √1000 ≈ 31.6, so the new distance is 620 Mpc / 31.6 ≈ 19.6 Mpc.

The Quasar 3C 273 Is Believed to Have a Luminosity