Why is it difficult to measure the distances to planetary nebulae?

Estimating the distance to a planetary nebula is difficult primarily because these objects are not a homogeneous class; they exhibit a wide variety of shapes, masses, and physical properties.1

Several factors contribute to this uncertainty:
* Structural Complexity: There is significant uncertainty regarding the amount of ionizing radiation that escapes from the nebula, as well as the presence of hot, low-density material that fills part of the nebula's volume without emitting appreciable radiation.1
* Lack of Direct Methods: Because they lack uniform characteristics, astronomers cannot easily apply standard distance-measurement techniques. Instead, they must rely on statistical methods calibrated by objects with especially favorable properties, such as those associated with stellar clusters or stars with known properties. These statistical estimates often carry errors of up to 30 percent.1

Once a reliable distance is determined, astronomers can calculate the true size of the nebula using its angular size, which in turn helps determine the nebula's age since ejection.1