星
星 – Xing1 – Heavenly Body, Small Amount
星期 – Xing1 Qi1 – Week
You also name each weekday with 星期 and an offset.
星期一 – Xing1 Qi1 Yi1 – Week(day) One — Monday
…
星期六 – Xing1 Qi1 Liu4 – Week(day) Six — Saturday (Jenny sometimes sings a cute song about “星期六”)
Sunday is a special case, 星期天 – Xing1 Qi1 Tian1. 天 means Heaven, Day, Sky.
星 is also used in the names of lots of stars, planets, and other random stuff in the sky. Here’s the entire solar system:
太陽 – Tai4 Yang2 – Too Bright (this is the Yang of Yin and Yang) — Sun
水星 – Shui3 Xing1 – Water Planet — Mercury
金星 – Jin1 Xing1 – Gold Planet — Venus
地球 – Di4 Qiu2 – Ground Ball — Earth
月亮 – Yue4 Liang4 – Moon Bright — The Moon
火星 – Huo3 Xing1 – Fire Planet — Mars
木星 – Mu4 Xing1 – Wood Planet — Jupiter
土星 – Tu3 Xing1 – Earth (dirt) Planet — Saturn
天王星 – Tian1 Wang2 Xing1 – Heaven King Planet — Uranus
海王星 – Hai3 Wang2 Xing1 – Sea King Planet — Neptune
冥王星 – Ming2 Wang2 Xing1 – Hell King Planet — Pluto
For more 星 words, try my dictionary.