Question

Another star cluster ("star cluster 2") lies 7.4 Mega-parsecs from the starship, and has an apparent radius of 6.3 arc seconds. You would like to know how big it is. Is it the same size as galaxies in our universe (i.e. tens of kilo-parsecs) or only as big as a star cluster in our universe (a few parsecs).

What is the physical radius of star cluster 2? (in kilo-parsecs)

Answer 1

since arc length s = rθ, just plug in the numbers:

7.4*10^3 * 6.3 * 1/3600 * pi/180 = 0.226 Kpc