Learn French: Le noyau est entouré d'une substance bizarre. - The nucleus is surrounded by a weird substance.

In the passive voice, we normally use par to introduce the agent doing the action. However, for verbs that describe a state, condition, or emotion rather than a specific physical action, we use de. Since being surrounded (entouré) describes a continuous state, it takes de.

Would the past participle entouré change if the subject was feminine or plural?

Yes. In the passive voice, the past participle acts like an adjective and must agree with the subject in gender and number. If the subject was feminine, it would be entourée, and if plural, entourés or entourées.

Does the word noyau always translate to "nucleus"?

Not always! While it means "nucleus" in biology or physics, in everyday French un noyau usually refers to the pit or stone of a fruit, like a peach or a cherry. It can also refer to the "core" or "heart" of a group of people.

Why does the adjective bizarre come after substance instead of before it?

Most descriptive adjectives in French come after the noun. Only a specific group of common, usually short adjectives (often remembered by the acronym BAGS for Beauty, Age, Goodness, and Size) go before the noun. Bizarre doesn't fit those categories, so it follows the normal rule.