What does this number measure?
Approximate luminous extent given by the source. The outlines are diffuse: size depends on the region and wavelengths observed.
Compare the stated dimensions
The comparison uses the number in each fact sheet: altitude, height, diameter, length or distance, depending on the reference. Two values in metres do not necessarily describe the same property.
Source and limitations
Catalogue value: a source is provided, but the measurement has not been individually audited in this project.
Whirlpool Galaxy: interpretation of M51 and its companion; measured extent of the main disc, approximate luminous outlines.