'hijastro' & 'filiaster'

Both words mean 'stepson', and the Spanish is derived from the Latin. But why '-aster'? What does a star have to do with the meaning of 'step-' in English?

I asked this question to a classics professor at Cal State Long Beach. He replied that 'filiaster' has nothing to do with the noun 'aster', that the -er is just a masculine marker, and that the -ast- must be a bound Latin morpheme meaning 'step-'.

I wonder if anyone here agrees or disagrees with my friend the classics professor.