The modern character is a clean phono-semantic compound: 星 = 日 + 生, the sun radical 日 above as the meaning component (a celestial point of light) and 生 (shēng) below lending the sound. The early history is more crowded with light. Oracle-bone and seal forms wrote multiple star-dots, small squares or circles scattered like a constellation, often three of them, the same idea preserved in 晶 (jīng), the three-sun character that originally depicted starlight and now means "crystal" and "sparkling." The phonetic 生 was added to pin down the reading, and the dots gradually consolidated into a single 日.

There is a quiet poetry in the surviving anatomy: 生 is the sprout-breaking-ground character, life emerging, so the settled form reads to the modern eye as light born above growth. That is folk etymology rather than philology, but it is exactly the kind of accidental image that makes a character memorable: the sky's lights over the growing world. The choice of 日 as radical also files the stars, correctly, in the sun's own drawer: the dictionary knew they were the same kind of thing long before astrophysics agreed.

星 extends from astronomical bodies to anything star-like: points of light (火星, sparks flying off a grinder), scattered small things (零星, "scattered bits"), and prominent people (明星, the bright stars of stage and screen). The measure word for stars is 颗 (kē), the classifier for small round things, so a wish is made on 一颗星.

Traditional China counted days in ten-day 旬 (xún) cycles and by the sexagenary stems and branches; the seven-day week was an import. It arrived during the Tang dynasty with Buddhist and Central Asian astronomy as the cycle of the 七曜 (qīyào), the seven luminaries: sun, moon, and the five visible planets, one presiding over each day, the same scheme behind Sunday, Monday, and (via Norse substitution) the rest of the English week. Japan borrowed that system wholesale and still uses it: Japanese Monday is 月曜日, moon-day. Chinese let the luminary names lapse and, when the modern calendar was standardized in the last years of the Qing, adopted 星期 ("star period") as the official word for the week, numbering the days instead of naming them: 星期一 (Monday, "star-period one") through 星期六, with Sunday as 星期天 or the more formal 星期日.

Two rival week-words survive in daily speech, each carrying its history. 礼拜 (lǐbài, "worship") came in with Christian and Muslim observance, so 礼拜天 is Sunday by way of church; it remains common and colloquial, especially in the south. 周 (zhōu, "cycle") is the compact written choice: 周一 through 周日, 周末 for the weekend, 下周 for next week. A learner needs all three passively and can output whichever feels natural; the numbers do the real work, and only Sunday breaks the pattern, in every one of the three systems.