Having shown, with Miller, that stars are chemically related to the Sun, Huggins next wanted "to ascertain whether this similarity of plan observable among the stars, and uniting them with our own sun into one great group, extended to the distinct and remarkable class of bodies known as nebulae." He expected they would differ from stars more in their temperature and density than in what they were made of, and through lab experiments with Miller knew the emission
line spectrum of a glowing gas was very different from a star's continuous spectrum.