Finding the diameter of a molecular cloud using period of intensity variation of hydroxyl maser So I have this problem (which I  translated to English as accurately as could):


  When studying a compact molecular cloud with radiotelescope, at a wavelength of 18 cm, hydroxyl (OH) maser(ic) radiation was registered. Radiation intensity changes at about 5 min cycle. Evaluate the maximum diameter of the cloud.


Actually we learned very little about molecular clouds at high school and I have no idea how to solve this problem. 

If the intensity change cycle is wave period, I can calculate wave velocity, which would be very low (as well as the frequency). I'm not asking for the answer, some hints would be enough. Thanks.
The principle that you apply is that the properties of a body cannot change faster than it the time it takes for light to cross the body.  So pulsars can be very small, which we know from the high frequency of their variation.  

A body which varies with a period of 5 minutes can't be larger than about 5 light-minutes across.