Use the same formula to find the volume of the hollow inner part. Subtract the inner volume from the outer volume to find the volume that emits light. Multiply this density by the volume to get the mass in the Ring Nebula in kilograms. The entire mass of this nebula will become a part of the interstellar medium. There are about 700 planetary nebulae in our galaxy.
New stars form at the rate of about one solar mass per year.