Not to be overly picky, but the three stars visible thing is when Shabbat ends on Saturday night, not when it begins.

The start of Shabbat strictly speaking is when the sun dips below the horizon and is commemorated by the lighting of two shabbat candles.

The last time for lighting those candles is 18 minutes prior to the astronomical event of sun-below-horizon, which under rabbinical interpretation is the true beginning of shabbat (that is, the last minute during which "work" -- in the technical, term of art sense -- may be performed without sanction).

It's a key event in Jewish ritual and not surprisingly there's a ton of commentary and meaning-making that has attached to how, when, where, and why Shabbat observance is undertaken. It is said that "More than the Jews have kept Shabbat, Shabbat has kept the Jews."