# Diameter of star clusters

1. Aug 5, 2010

### randa177

Maybe this is stupid question, but I am not sure how to get the diameter of the star cluster, I assume from the RA and Dec. I googled it and tried to come with a realtion between the RA , Dec and Diameter from Andrea's proposal but couldn't figure the correct answer
This is all I find (see below please)... but I don't know the distance or the angular size, just RA and Dec from SIMBAD data base
diameter = 2p × (distance to the cluste) × (clsuter's angular size in degrees)/360°

2. Sep 2, 2010

### fir3dust

hey, i can't quite remember everything, but here goes:

if u have the magnitude of the cluster u can work out the distance to the cluster. the absoulte magnitude of the cluster will (should) be on simbad.

the equation for magnitude and distance is:
http://ceres.hsc.edu/homepages/classes/astronomy/spring99/Mathematics/sec19.html

u can work out the angular size using the distance u just calculated using:
http://galacticfool.com/what-is-angular-size/

however, it should be poss to find out the cluster's angular size from a catalogue.