hɪz mˈoʊst sɪɡnˈɪfɪkənt kˌɑːntɹɪbjˈuːʃən wʌz tə ðə stˈʌdi ʌv ˈɑːɹ ænd ˈɛŋklˈæs stˈɑːɹz bᵻlˈɔŋɪŋ tə ðə kˈuːl ɹˈɛd dwˈɔːɹf tˈaɪp lˈeɪɾɚ ɹᵻklˈæsᵻfˌaɪd æz kˈɑːɹbən stˈɑːɹz.
His most significant contribution was to the study of R- and N-class stars belonging to the cool red dwarf type (later reclassified as Carbon stars).