You've run into one of the more unique features of Swedish phonology, namely the so-called "sj-sound" (Swedish: "sje-ljudet"). The rules governing it are unfortunately rather opaque. To begin with, there are multiple different ways of spelling it (65 variants according to an article from Sveriges Radio - see link below). Word-initially, the variants you will see the most often include the letter "s" ("sj", "sh", "sk", "skj", "stj", "sch"). In the middle of words, however, it is not quite as clear-cut. Aside from the variants mentioned earlier, oftentimes "ti" (as in "station") will use this sound, though the pronunciation varies by dialect. Depending on the dialect of Swedish, the "sj-sound" may be produced (almost) like the "tj-sound" (Swedish: "tje-ljudet"), which is very similar to the sound produced by the digraph "sh" in English. If, for example, I asked my mother, who is from Norrland, to pronounce the word "stjärna" from your example, I would be hard-pressed to hear a difference from her pronunciation of the word "kärna" ("core"). She, however, insists that there is a difference. In my own dialect (Gothenburg), we make a distinct difference between "stjärna" (sj-sound) and "kärna" (tj-sound). Regarding phonetic rules governing when it occurs, I must admit that I am very uncertain. Wikipedia suggests (see link below), that it occurs before front vowels. However, if I am not mistaken, the Swedish long "a" is an open back rounded vowel, and the word "sjal" ("shawl") definitely uses the sj-sound, thereby seemingly violating that constraint. My best advice would be to learn it on a case-by-case basis, and learning to recognize the most common spelling variants. Hope that helps to answer your question. For further reading, I suggest: https://en.wikipedia.org/wiki/Sj-sound http://sverigesradio.se/sida/artikel.aspx?programid=411&artikel=3963073