2 syllables: cas, tor. Stress on cas.
KAS-tur
/ˈkæstɚ/
castor is pronounced KAS-tur (/ˈkæstɚ/). It has two syllables (cas-tor), with the stress on "cas". Castor has several senses: an animal product (castor oil) from the beaver’s castor glands and, in astronomy, the brighter of the two main stars in Gemini. In botany and chemistry, ‘castor’ can refer to a castor oil plant or to materials derived from it. The term also appears in historical and mythical contexts, and in various cross-disciplinary references. The core pronunciation remains /ˈkæs.tər/ with stress on the first syllable in common usage.
Castor has several senses: an animal product (castor oil) from the beaver’s castor glands and, in astronomy, the brighter of the two main stars in Gemini. In botany and chemistry, ‘castor’ can refer to a castor oil plant or to materials derived from it. The term also appears in historical and mythical contexts, and in various cross-disciplinary references. The core pronunciation remains /ˈkæs.tər/ with stress on the first syllable in common usage.
"The beaver’s castor glands produce a natural oil used centuries ago."
"In Gemini, Castor is traditionally paired with Pollux as the brighter twin star."
"Castor oil is often used in cosmetics and as a lubricant in industrial applications."
Usual pronunciation is /ˈkæs.tər/ (CAST-er) with primary stress on the first syllable. The first vowel is short æ as in cat, the second is schwa or a light /ə/ depending on accent. In careful speech, you may hear /ˈkæs.tər/ with a subtle reduction of the second syllable. If you’re discussing the star, the pronunciation remains the same, though context makes it clear you mean the astral Castor, not the oil.
Common errors: 1) Pronouncing it as /ˈkæs.tɔːr/ with a full rhotic-vowel in the second syllable; correct is /ˈkæs.tər/ with a schwa-like second syllable. 2) Slurring to two identical syllables (/ˈkæs.kæs.tər/); keep a light, quick second syllable. 3) Not reducing the second syllable in rapid speech, making it sound like ‘CAST-OR’ with a hard 'or'. Tip: keep the second syllable short and unstressed, like a quick ‘ter’.
US tends toward /ˈkæs.tər/ with a clear /ə/. UK often retains /ˈkɑː.stə/ or /ˈkæst.ə/ depending on speaker, but most mainstream UK usage aligns with /ˈkæs.tə/—short vowels and a lighter second syllable. Australian pronunciation similar to US but with a more centralized /ə/ and less rhoticity. In all: stress on first syllable; second syllable reduced; final consonant often a soft /ə/ or /ə/.
The challenge lies in the short, reduced second syllable and the delicate vowel in the second position. Beginners often overemphasize the second syllable or substitute /ɔː/ or /ɜː/ for /ər/. Mastery requires producing a quick, relaxed schwa or /ə/ in the second syllable and ensuring the first syllable has the crisp /æ/ or /æ/ vowel. Focus on keeping the sequence /ˈkæs.tər/ with minimal vowel length in the final syllable.
Yes—its second syllable typically reduces to a weak vowel, which can trip non-native speakers who expect a clear ‘ter’ or ‘tor’. The key is a clipped, unstressed /ər/ or /ə/ after a strong /æ/ on the first syllable. In careful speech, you’ll hear /ˈkæs.tər/ with a nearly silent or very short second vowel; in faster speech, it can nearly disappear. Mastery comes from balancing volume between syllables and storing the first syllable with crisp consonants.
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Castor comes from Latin castor, castoris meaning beaver, allied to Greek kastor, meaning beaver. The word extended to refer to castoreum, the secretion of beaver castor sacs used in perfumery and medicine. The Latin form was borrowed into English by the 15th century, initially to denote the beaver itself and castoreum, and by the 17th–18th centuries it broadened to refer to castor oil derived from Ricinus communis seeds, reflecting the oils’ resinous, sticky quality. The astronomical Castor, however, derives from Latin Castor and Pollux, naming the two bright stars in Gemini; Castor is tied to the idea of a strong, shining companion. In modern usage, the word persists in both zoological/oil contexts and as a proper name in astronomy and botany, with the pronunciation stabilized around the stressed first syllable /ˈkæs.tər/ in US/UK English.
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💡 These words have similar meanings to "castor" and can often be used interchangeably.
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Words that rhyme with "castor"
-ter sounds
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