惺 is rarely used alone. It appears in set phrases: 惺惺相惜 (kindred spirits appreciate each other), 假惺惺 (feigning sincerity), and 惺忪 (drowsy, half-awake eyes).
Heart radical on the left signals an inner state — here mental clarity and alertness. The same radical drives 怕 fear, 情 feeling, 怀 cherish; anything to do with mind or mood lives in this family.
Right side supplies the sound and a vivid image: xīng → xīng (no shift). A waking, alert mind sparkles like a star — the metaphor is preserved in modern compounds like 惺忪 (just-waking, drowsy-eyed) and 惺惺相惜 (clear minds appreciate each other).
