Since contemporary astronomy focuses on understanding physical processes in the universe, the data collected usually do not match what the human eye would perceive. For example, our eyes detect radiation over a range bounded on one end by blue light (high energies) and the other by red light (low energies). One can use the instruments and filters on telescopes to replicate this range. If this is done, natural color images can be constructed. However, in astrophysics a filter is more often used to isolate particular energies of radiation. For example, astronomers will use a filter to isolate gas of a specific temperature from other sources of light. Gas of a second temperature could also be imaged by using filter which isolates a different energy range. Despite the difference in the energy of the radiation passed by these 2 filters, it may be the case that both gases appear to be the same color of light to the human eye! So if only these sort of filtered observations exist, then only composite color images are useful. Therefore each filtered image is assigned a distinct color in order to illustrate the difference in temperatures existing in the astronomical object.

The Hubble Space Telescope archive only contained data collected using filters that isolated heated gas of different temperatures and chemical composition. Therefore our version of the Bubble Nebula is a chromatic but not natural color image.

Copyright T. Bonev, K. Jockers, and T. Credner

Astronomers T. Bonev, K. Jockers, and T. Credner at Max-Planck-Institut fur Aeronomie have also made a composite color image of the Bubble Nebula from their observations of the radiation from hot gases.

Copyright IAC/RGO/Malin

However, David Malin at the Anglo-Australian Observatory has used observations that correspond to the human eye in order to create a natural color image of the Bubble Nebula. Note that to the human eye the different temperature gases appear predominantly rose colored.