Objects of interest include planets, moons, stars, nebulae, galaxies, and comets. Galaxies were initially discovered telescopically and were known as "spiral nebulae". The nebula is one of the largest diffuse nebulae in our skies. He further suggested that other distant "nebulae" might be other galaxies. Spectroscopic observations show that all planetary nebulae are expanding. Protoplanetary disks are present in both nebulae. Type P objects are stars within planetary nebulae and type Q objects are novae. He also discovered several interstellar nebulae and some double stars. The central stars of planetary nebulae are very hot. The majority of gaseous emission nebulae are formed of neutral hydrogen. Still other nebulae form as planetary nebulae. They can be observed as dark clouds silhouetted against bright emission nebulae or background stars. The distances to planetary nebulae are generally poorly determined. Like many nearby planetary nebulae, the Dumbbell contains knots. Isolated small dark nebulae are called Bok globules.