The reality is the star is/was always there...it’s not like it suddenly appeared.
It’s unfortunate we can’t know what that star was, so we could maybe calculate it’s distance from us, and therefore it’s age, etc. it could prove interesting to do such an analysis.
Perhaps it was some supernova not seen elsewhere for its brevity. Although I don’t know of any supernova that would be so short in duration. Or maybe some other civilization saw it but didn’t record it for some reason.
Most likely it’s another miracle akin to the sun stopping in the sky (hence the earth’s rotation stopping) as is spoken of elsewhere in Scripture (and of course at Fatima). These events certainly aren’t recorded worldwide so they are, for whatever reason, chosen by God to only appear to those He has chosen to witness them, for what reason remains ultimately a mystery.