Latin sidus, sideris, 'star, constellation' is sometimes compared with Greek sideros 'iron', but they're probably not related. First, that intitial /s/ in Greek is problematic. Cognate words in Latin like sol 'sun' and sal 'salt', begin with rough breathing /h/ in Greek: helios and hals respectively. Some have connected Latin sidus with Germanic words for silver and slag, but it's usually traced back to PIE *sueid- 'to glow'.