Pulling information from user input
If you decide to implement this without using a class you can simply write:

def get_starlist():
    ...
def get_inputkey():
    ...

if __name__ == "__main__":
    get_starlist()
    get_input_key()
    define_lists_and_arrays()  
    determine_normalization_point()
    create_fobs_fobserr()
    search_DUSTY_models()
    calculate_chi2()
    delete_null_components()
    dust_whittet_sub()


But if you find that some of your functions are sharing data a lot you may want to attach them to a class, or pass instances of a class (objects) between the different functions. It's hard to say more without knowing what you're trying to do.