Stellan became a Swedish name in 1597, when Stellan Otto von Mörner immigrated from Brandenburg. So Stellan is in fact German to begin with (and has probably nothing to do with the female name Stella). Though it is not used in Germany today and is considered a Swedish name there. [noted -ed]
My name is Stellan and have researched the name before. I can not remember where I got the info though. I was informed that it means 'bright star' e.g. interstellar, constellation etc. It also happens to be in the middle of the name for the Spanish language Castellano. I'm half Spanish myself but my father who named me did not realise, he's the Spanish one! He named me after the 1971 world champion table tennis player called Stellan Bengtsson. I live in Bristol, England and have never met anyone else called Stellan in my life. I know though that it is much more common in Sweden. Cheers.