Biological and Stellar Evolution
Astronomers and Biologists have Different Meanings of Evolution
Astronomers and biologists both frequently use the term evolution, but don't mean exactly the same thing.
The term evolution simply means change. It usually refers to a slow change rather than a rapid change. In this context it is used in many fields other than science.
In biology Darwin's theory of evolution applies the word in a specific context. In biological evolution, the slow change refers to the changes that take place in a species or the evolution from one species to another. The change in this case is slow because it takes place over very many generations.
Astronomers also use the term evolution to refer to a slow change, but in the astronomical context it is a different kind of change. One of the contexts in which astronomers use the term evolution is referring to stellar evolution.
Rather than referring to changes over many generations of stars, stellar evolution refers to the slow changes that take place in the life cycle of an individual star. (Notice that astronomers often refer to the life cycle of stars, but that does not mean stars are alive in a biological sense.) Stars form gradually from clouds of dust and gas which eventually ignites in a nuclear fusion chain reaction. When they run out of nuclear fuel they die out - more like a fire dying out than an organic death. In the process stars undergo many changes in their size, energy output, temperature, and other properties. This process takes an unimaginably long time. In the case of the Sun and similar stars, we are talking billions of years. So these changes that take place in individual stars are properly described by the term stellar evolution.
In the biological theory of evolution, biologists refer to the changes taking place over many generations of organisms. In astronomy, stellar evolution refers to the changes taking place in an individual star as it is formed, uses its fuel, and dies out. If biologists used the term evolution in the same way astronomers refer to stellar evolution, it would refer to being born, maturing, growing old, and dying in an individual organism.
Scientists also use the word evolution in contexts other than biological or stellar evolution. The evolution of a planet, Earth for example, refers to the changes that take place on the planet over geological time. Mountains forming and weathering away is an example of the evolutionary changes that can take place on a planet.
On the largest scale astronomers refer to the evolution of the entire universe. In the big bang theory, the entire universe has evolved, meaning changed, in the approximately 15 billion years since its creation.
Scientists of all types often take common words and use them with very precise meanings that may differ from the everyday meaning of the word. In many cases the meaning will also differ among different areas of science.
