In some fashions of expression, stars do not “evolve,” they change through stages. If you count the pre-stellar material, then that is the stuff that “evolves,” in the the traditional sense of the term. Yet, if we call that evolution, then the caterpiller to butterfly metamorphosis would have to be called an evolution.
A common picture of the stage or process that begins a star is actually an instructive fiction. We don’t know for certain how each star has begun. We do have some somewhat plausible ‘educated guesses,” though. But don’t let the word “guess” concern you. In an early astronomy course I had the professor was speaking of the star we commonly call Rigel. He said that he identified over seventy “assumptions” that had to be made in order to estimate the distance to that star.
There are essentially two basic processes in understanding how stars develop. One is to look at our nearest star, the sun, and make comparisons to what we see elsewhere. The other is to gather basic information about the stars we can identify in the sky (not all bright spots are stars) and sort them by common features.
The two most common features are color and brightness. More than once, however, that information has been a bit misleading. Some stars behave differently. When a star is at one level of brightness one night, but another at a different time, the variableness puts some stars into a different set of stars. Most, though, fit in a series commonly called the main sequence. Our star is in the middle of the main sequence, as they are commonly arrayed. There are, however, as with the variable stars, some oddities that need more information.
Something close to a hundred stars have distances that have been directly measured. At first, it was thought that direct measurement was a hopeless prospect. Most stars in the sky are too far away, especially in the earlier days of modern astronomy when measures were considerably more crude and imprecise than today. But there were some few that were close enough that one could observe the location of a star, relative to the position of the astronomer on earth at that moment, but then essentially six months later the astronomer is on the opposite side of our orbit around the sun, so at an opportune time another sighting measurement is made and compared to the first. We have measured that the earth is some 93-94 million miles from the sun, so there is a range someplace in the neighborhood of 180 million miles from the first measure to the second. That information, applying some basic geometry, and we have an idea of the distance to the nearer of stars. Still, especially in days before calculators and computers made mathematics easy, the numbers were, well, “astronomical”-unimaginably huge for most people.
In the process, we had people who measured and calculated how fast light traveled. Then when comparing how far light would travel in the course of a whole year, the calculation gave astronomers a useful yardstick for measuring distances to stars-light years. When comparing the sorted information about stars that are measurably close, we get a range of characteristics that help us make assumptions about the more distant stars.
Another interesting thing happens when a prism, or prism-like, effect is made to the light of the stars. The spectrum of the light tells things about the stars that becomes useful for still other ideas or theories of how stars work. In the process of that scrutiny, some ideas have been made about the aging process of stars. Fred Hoyle, the same man that gave us the term “big bang” as a derogatory expression against a theory he detested, also gave us the basics of the formula for how stars “burn” the elements to generate the energy that makes them luminous. Since that time, there have been many refinements to the fusion fire sequencing, including some that circumvent the cycle, calling into question other assumptions that have been made for various phenomena. For the most part, though, stars that are seen to be mostly hydrogen are assumed to be young, while stars that are demonstrating certain heavier proportions of other elements then they must be older. At some points the guesses are that a stage change takes place when one fusion sequence changes to another.
Sometimes the stage change happens explosively. A cataclysmic explosion, commonly called a supernova, rips away large masses of the star’s outer layers, expelling them in sometimes marvelously colorful displays called a nebula. Sometimes what remains is a small star called a dwarf, other times there is something called a neutron star, and in still other events a black hole results. What makes the difference? Usually it is the mass of the original star. So formulas have been devised that tell us the kinds of mass sizes do one or the others. Meanwhile, in the process of expelling that mass, heavier elements are fused and flung into the expanse of space.
Not only do we have the measure of enormous distances, but enormous time scales have to be employed. Essentially two dozen billion years is the general estimate of the age of the universe since the “big bang” (the notion that Hoyle and others used to think was that the universe was steady and eternal, continuously static). Here is where the “evolutionary” metaphor comes in. In order for what we see today to have developed from what we assume began it all. In public discussion, scientists of this day tend not to speak of God creating it all, but don’t dismiss the idea for there are scientists who still accept it. Hoyle was poohing the Big Bang because it sounded so close to some notions of the Creation account in the Bible. Just as biological evolutionists, an enormous length of time is necessary to make the gradual change assumptions. How do we make that assumption? It is the time decay of heavy radioactive elements. For instance, some of the uranium isotopes detected in our solar system indicate they might have derived from a shorter-lived plutonium. Similarly, other isotopes of other elements, and their estimated quantities, help recheck the numbers. We therein determined that our solar system is someplace close to four or five billion years old.
Because of these assumptions and comparisons, we have notions about the future development of our own sun. It will eventually cool and swell (swallowing up Mercury, Venus, and the Earth), then at some point collapse, and then explosively expel a large share of its mass. What is left then is a dwarf. Some, such as Oliver Manuel (until recently a chemistry professor at the University of Missouri, Rolla), has reason to believe that our sun already has gone through this stage, but a neutron star resulted and began to reacquire gasses from the gravitational attraction. It gets complicated, but he has some good reasons.
As it is, we don’t really fully know how stars “evolve,” but we have made some very solid sets of assumptions that tend to work well. Still, hardly a week goes by that some announcement is made that something doesn’t behave just right. In recent weeks there were some questions as to whether we have a good understanding of how massive a neutron star can be. The “typical” neutron star was at one time assumed to be a slow and static body, ala our sun’s slow turning as a convenient example of what stars do, but when pulsars were discovered, they realized that some neutron stars have hot spots, or jets of matter spewing out, and we happen to be within the sweep of those spots of spewings as they spin around like a search light. Once again, the numbers were astounding, because that assumption meant that some of these pulsar/neutron stars would have to be spinning incredibly fast, in many cases, several times per second (as in the whole star is turning on its axis dozens or even hundreds of times each second).
The universe gets stranger still-the discussion of “dark matter” and “dark energy” get really weird, and for that we are really truly guessing, but that is another story.
