They observe astronomical objects such as stars, planets, moons, comets and galaxies – in either observational (by analyzing the data) or theoretical astronomy.
ðeɪ əbzˈɜːv ɐstɹənˈɑːmɪkəl ˈɑːbdʒɛkts sˈʌtʃ ɐz stˈɑːɹz, plˈænɪts, mˈuːnz, kˈɑːmɪts ænd ɡˈælɐksiz ɪn ˈiːðɚɹ ɑːbzɚvˈeɪʃənəl baɪ ˈænɐlˌaɪzɪŋ ðə dˈeɪɾə ɔːɹ θiəɹˈɛɾɪkəl ɐstɹˈɑːnəmi.