Saturday, February 14, 2009

Darwin blogging: Evolution in Astrophysics


In honor of Charles Darwin's 200th birthday a few days ago (February 12th) I thought it might be fun to consider the concept of evolution as it appears in astrophysics. Several rather fundamental concepts in modern astrophysics have the word evolution in them, and we're not even talking about astrobiology either.

What are these concepts and why (and how) are we using the word evolution to describe them?

Lets start by making it clear that evolution, when used in astrophysics, is largely used in the sense "change with time" and perhaps, at a stretch, "descent with modification." We don't mean "change in gene frequencies within a population of organisms" or natural selection.

There are three major core concepts in modern astrophysics that bear the name evolution:

  1. Stellar Evolution: The set of physical changes and stages a star (usually a single star) undergoes throughout its lifetime. This includes its formation, ignition of nuclear hydrogen burning at the start of its Main Sequence life, the Main Sequence, and post Main Sequence life as core hydrogen burning ceases. For some classes of stars there will be mass loss via stellar winds or expulsive processes, and some stars end up as white dwarfs, neutron stars, or even black holes. The details depend on the initial mass of the star, its initial elemental composition (often termed chemical composition), whether its part of a binary stellar system (even more complicated than single star stellar evolution), and in some cases its environment (stellar evolution in dense star clusters has additional complications, including stellar collisions).
  2. Chemical Evolution: The changes in the chemical composition of the Universe (in particular stars and galaxies, but also interstellar and intergalactic gas) as a result of nucleosynthesis within stars, and the expulsion of newly created elements in stellar winds, novae, and supernovae. Supernovae do more than just expel elements but also explosively synthesize elements that normal stellar nuclear fusion can not produce. The expelled elements usually end up in the interstellar medium (ISM), from which subsequent generations of stars form. The details of stellar evolution depend on the initial composition of the stars, so chemical evolution affects stellar evolution, which in turn drives chemical evolution, and so on...
  3. Galaxy Evolution: The processes by which galaxies form and change with time, which are influenced by their environment within the inter-galactic medium (IGM) and neighboring galaxies, their gas content (fuel to create new stars with), the history of star formation within them (in turn affecting their own chemical evolution), and the presence and role of and Active Galactic Nuclei. Galaxies certainly grow and change with time, and to a limit extent events can change some of them one from one type galaxy to another (e.g. mergers of spiral galaxies can eventually create elliptical galaxies).
This is a very superficial outline of stellar, chemical and galaxy evolution, and when played out 13.5 billion years you can imagine the interconnections between all these processes can generate a fair bit of complexity. Nevertheless, this complexity falls far short of the complexity and interconnectedness of biology, and none of these processes approach the biological concept of evolution. So why call them evolution?

One could try to argue that astrophysicists are simply using the older meaning of evolution as change or progression. But that explanation seems contrived, as all of these astrophysical concepts were developed well after Darwin's "Origin of Species" came out in 1859. Indeed, the leading British (*) physicist Lord Kelvin (William Thomson) caused Darwin and his supporters much anguish as Kevin's (incorrect) theory for the age of the sun allowed only 20 million years for the age of the Solar System, seemingly too little time for evolution to have happened.

More recognizably modern theories of stellar structure, nucleosynthesis and stellar evolution only followed the development of quantum mechanics in the first decades of the 20th century. The big names in the development of these astrophysical theories are Eddington, Jeans, Milne, Chandrasekhar (**), Bethe, Gamow, Fowler and the (rather tragic) Fred Hoyle. In short, the foundation of what is now termed stellar evolutionary theory and chemical evolution was developed between ~1920 an ~1960.

The concept of Galaxies as separate entities also date from about this time. The famous Shapley - Curtis debate happened in 1920, so the issue had been brewing for a little while before that (Virginia Trimble has a nice discussion of the history, background and outcome of this debate). Hubble's work, demonstrating the reality of galaxies other than our own, and the expanding Universe, also date to the 1920's.

I would suspect that the reason astrophysicists use the word evolution to describe many of the core concepts of modern astrophysics has a lot to do with the great influence and power biological evolution has a scientific concept. In short, the biological theory of evolution is such a elegant, beautiful and powerful bit of science that we astrophysicists are happy, even eager, to associate the word with the theories we consider to be some of the most fundamental, powerful, elegant and important aspects of our own science.

This is of course pure speculation on my part. But I would like to think that naming these theories stellar evolution, chemical evolution and galaxy evolution is (perhaps unconsciously) the tribute we astrophysicists pay to the great naturalist and scientist Charles Darwin.

(*) Kelvin was an Ulster Scot. Ireland was at that time still part of the U.K.
(**) The Chandra X-ray Observatory (which I use a lot) and the Chandra Postdoctorall Fellowship that I had are all named after Chandrasekhar.

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