An active galaxy is measured to be 100 Mpc away.How many million years ago did the light we see now leave this galaxy?

Question

Quizplus · Accepted Answer

We can use the equation t=d/v, where t is the time taken for light to travel a distance d at a speed v. 
Here, d = 100 Mpc = 100 million parsecs and we know that the speed of light, v = 3 x 10^8 m/s. 
However, we need to convert Mpc to meters by using the conversion factor 1 Mpc = 3.086 x 10^22 m. 
So, d = 100 x 3.086 x 10^22 m = 3.086 x 10^24 m. 
Using the equation now, t = d/v = (3.086 x 10^24 m) / (3 x 10^8 m/s) = 10.287 x 10^15 s. 
We can convert this to million years by dividing by the number of seconds in a year and then by 1 million. 
So, t = 10.287 x 10^15 s / (3.154 x 10^7 s/yr) / 1 million = 326 million years. 
Therefore, the answer is C: 326.

An Active Galaxy Is Measured to Be 100 Mpc Away