# Flux, Luminosity, Distance, Size

1. May 5, 2007

### kingwinner

1) You observed two galaxy clusters. You have determined the distance to Cluster A (by Cephied variables) to be 50 Mpc. You find the galaxies in Cluster B to be on average 16 times fainter than those in Cluster A. How far away is Cluster B? How does the (apparent) average size of galaxies in Cluster B compared to that of Cluster A?
[Using the formula flux=luminosity/(4*pi*d^2), I have that the answer to the first part is 200 Mpc, but I don't know how to do the second part (in red), can someone please help me?]

2) The scale factor of the universe R(t) is equal to 1 (by definition) at the current epoch. This question compares the universe at a redshift of 0.8 to today.
If 2 galaxies were in each other's gravitational potential, and orbiting each other with an orbital of 0.5 Mpc at z=0.8. Assuming the orbit is stable, how far apart are they today?

[can someone give me some hints? I really have no clue how to calculate this...]

Thanks a lot!

Last edited: May 6, 2007
2. May 6, 2007

### kingwinner

If the object is farther, the object's apparent size should be smaller....but what is the exact relationship?

3. May 6, 2007