What's the difference between aster, stella, sidus and astrum in Latin?

Question

aster, stella, sidus and astrum are are all nearly means or related to stars. But are there any key differences between these words ?

Answer 1

Aster and astrum are borrowings from Greek ἀστήρ and ἄστρον. The former is also a name given to e.g. a plant and some type of clay, but you can find it used for any given star, too. The latter is the more common of the two when it refers to stars, and in the plural usually refers to the heavens as a whole (as it does in Greek, too).

Stella and sidus are thus the native Latin words. Stella is actually cognate with aster, and is the normal word for a star, while sidus more properly means "constellation." The sidus Pleiadum, for example, translates to "the constellation of the Pleiades."

For whatever reason, both words can mean the other. The close association between an individual star, a constellation, and the heavens as a whole means you quite often get some synecdoche (i.e. "star" instead of "constellation") and metonymy (i.e. "constellations" instead of "all the sky") going on when they're actually used.

While that can cause some confusion, usually it's made clear from context.

Answer 2

Of these words, stella appears to be the most technical and prosaic one, standing for the lights in the night sky, but usually not referring to the heavens as a whole, and not to the effects that the stars are believed to have on human beings. Astrum is used in a similar way, but astra for “the heavens” or great heights does occur.

On the other hand, sidus (constellation; often used in the plural) is often used to talk about the influence of the stars on human fate or health (e.g. faustum sidus).

Here is what Döderlein's Hand-book of Latin Synonymes (1875) says:

Stella (dimin. of ἀστήρ) means any one of the innumerable individual stars, like ἀστήρ; astrum (ἄστρον), any one of the greater bright heavenly bodies, the sun, moon, and principal stars, with their peculiar names, like ἄστρον; sidus (εἶδος), a complication of stars, a constellation, and, by affinity of the notion with number and magnitude, a great star, like τέρας, τείρεα. Astrum and stella denote the stars more in a mere physical relation, as bright heavenly bodies; sidus, more in an astronomical and astrological relation, as portentous and influencing human affairs. Sen. Helv. 9. Dum ortus siderum, occasus intervallaque, et causas investigare velocius meandi vel tardius spectare tot per noctem stellas micantes liceat. (iv. 409.)

Answer 3

"Stella" is just a star

"Sidus" is like a constellation mostly in plural

"Astrum" is a star, a constellationt, poetically

"Aster" is also a star, but it's when the star has a name and they call it by its name.