What does the dreaded "E" word mean, anyway

Natural History, Feb, 2000 by Stephen Jay Gould

Ironically, astronomy undeniably trumps biology in faithfulness to the etymology and the vernacular definition of "evolution"--even though the term now holds far wider currency under the radically altered definition of the biological sciences. In fact, astronomers have been so true to the original definition that they confine "evolution" to historical sequences of predictable unfolding and resolutely shun the word when describing cosmic changes exhibiting the key features of biological evolution--unpredictability and lack of inherent directionality.

As an illustration of this astronomical usage, consider the most standard and conventional of all sources--the Encyclopaedia Britannica article "Stars and Star Clusters" (15th edition, 1990 printing). The section entitled "Star Formation and Evolution" begins by analogizing stellar "evolution" to a preprogrammed life cycle, with the degree of evolution defined as the position along the predictable trajectory:

   Throughout the Milky Way Galaxy ... astronomers haw' discovered stars that
   are well evolved or even approaching extinction, or both, as well as
   occasional stars that must be very young or still in the process of
   formation. Evolutionary effects on these stars are not negligible.

The fully predictable and linear sequence of stages in a stellar lifetime (evolution, to astronomers) records the consequences of a defining physical process in the construction and history of stars: the conversion of mass to energy by nuclear reactions deep within stars, leading to the transformation of hydrogen into helium.

   The spread of luminosities and colors of stars within the main sequence can
   be understood as a consequence of evolution.... As the stars evolve, they
   adjust to the increase in the helium-to-hydrogen ratio in their cores....
   When the core fuel is exhausted, the internal structure of the star changes
   rapidly; it quickly leaves the main sequence and moves towards the region
   of giants and supergiants.

The same basic sequence unfolds through stellar lives, but the rate of change (evolution, to astronomers) varies as a predictable consequence of differences in mass:

   Like the rate of formation of a star, the subsequent rate of evolution on
   the main sequence is proportional to the mass of the star; the greater the
   mass, the more rapid the evolution.

More complex factors may determine variation in some stages of the life cycle, but the basic directionality (evolution, to astronomers) does not alter, and predictability from natural law remains precise and complete:

   The great spread in luminosities and colors of giant, supergiant, and
   subgiant stars is also understood to result from evolutionary events. When
   a star leaves the main sequence, its future evolution is precisely
   determined by its mass, rate of rotation (or angular momentum), chemical
   composition, and whether or not it is a member of a close binary system.

In the most revealing verbal clue of all, the discourse of this particular scientific culture seems to shun the word "evolution" when historical sequences become too meandering, too nondirectional, or too complex to explain as simple consequences of controlling laws--even though the end result may be markedly different from the beginning state, thus illustrating significant change through time. For example, the same Britannica article on stellar evolution notes that one can often reach conclusions about the origin of a star or a planet from the relative abundance of chemical elements in its present composition.