Expert: Jayendra Upadhye - 12/1/2006

Question
Hello!

I want to ask you something about how to calculate the distance of anddromeda from Milky Way.

In the Great Debate, Shapley compared two novae. S Andromedae in the Andromeda Nebula, with a peak magnitude of 9.0, and Nova Persei in the Milky Way with a peak magnitude of 6.2. Assuming that these two novae had the same absolute magnitude, Shapley concluded that S Andromedae is 3.5 times more distant than Nova Persei.
If we know that the radius of the Milky Way is 10 kpc, how can we find the distance to the Andromeda Nebula?

Thank you in advance,
Sharon

Answer
Hi,
One need not go into details of the inverse square relationship of Actual intensity to apparent magnitude, over diatance, as we are already told here that S.Andromedae is 3.5 times farther than Nova persei.
The radius of the milky way is immaterial here unless one says nova persie lies at the exyremity of the milky way ..
then it becomes a straight forward extrapolation (rule of 3's)...if 1 is to 10 kpc then 3.5 is to 35kpc!
Your distance then would be 35 kiloparsecs.
If that was a school question i must say i was highly intrigued!
Jayen