It depends on the context. If you are out looking at the night sky talking about some 星座，you would want to say "constellation." For example, the Big Dipper is a constellation, or there is the Big Dipper constellation. Only twelve constellations can be called "star signs" or "zodiac signs," these are the 十二星座. Even though is the same word in Chinese, it is important to use the right word in English, because the Big Dipper is a constellation but is not a star sign. Leo is both a constellation and a star sign.