Complex sentence: Lenticular galaxies are disc galaxies (like spiral galaxies) which have used up or lost their interstellar matter, therefore, very little star formation is thought to occur and they consist mainly of aging stars (like elliptical galaxies).
Split into simpler sentences: Lenticular galaxies are disc galaxies (like spiral galaxies) which have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. As a result, they consist mainly of aging stars (like elliptical galaxies).