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What does the dreaded "E" word mean, anyway
Natural History, Feb, 2000 by Stephen Jay Gould
When astronomers talk about the evolution of a star, they clearly do not have a variational theory like Darwin's in mind. Stars do not change through time because mama and papa stars generate broods of varying daughter stars, followed by the differential survival of daughters best adapted to their particular region of the cosmos. Rather, theories of stellar "evolution" could not be more relentlessly transformational in positing a definite and predictable sequence of changes unfolding as simple consequences of physical laws. (No biological process operates in exactly the same manner, but the life cycle of an organism certainly works better than the evolution of a species as a source of analogy.)
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.
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