Why do Scandinavian Christmas menorahs -- and St. Lucia's crown -- have seven candles?

I began to get interested in Saint Lucia’s day many years ago—but this tradition involving seven I do not understand. There are traditions about who to remember or say a prayer for, but I doubt these are authentic.

Although it is true that the Hebrew/Jewish menorah has seven candles, the Hanukkiah does not. And some church altars have a pair of candelabras with seven candles—I don’t know if this is for the same reason, or what this is about.

Feel free to share anything you know or think. I am looking for as much information as I can find. However, to uncover the real reason would be must appreciated.