Energy and Stellar Evolution
Conservation of Energy tells us Stars Must Evolve
How do we know stars must evolve? The fundamental law of conservation of energy, when applied to stars, tells us that they must run out of fuel and therefore evolve.
Stars Evolve
Enjoying the beauty of a dark starry night, we tend to think stars are eternal. We see the same pattern of stars in our old age as in our childhood, and this pattern is essentially the same as seen by our distant ancestors and descendants. Stellar lifetimes are so long that on human time scales stars seem eternal, but they are not really eternal. Like humans, stars die out (not in a biological sense), and new stars must form to replace them. Over its lifetime, a star goes through many changes over very long time spans: stellar evolution.
We see stars in various stages, but do not live long enough to watch stars evolve to the next stage. Deducing stellar lifecycles is like deducing human lifecycles from a thirty second video clip of a group of people of various ages. If the people also have pets in the video, the confusion between different life stages and different species mounts.
Some stellar differences are indeed different types of stars, but not all. Stars do evolve and eventually die out. How do we know?
The Simple Observation
The answer is surprisingly simple, yet profound. The simplest observation that we can make about stars is that we can see them. Stars emit light - a form of energy. This energy must come from someplace within the star by some process. Even without the details, we know that the energy supply is finite. As the fuel runs out, the star evolves and eventually dies out, just as exhausting the fuel dims a fire. Hence, stars must evolve. (We can also apply this reasoning to understand that Earth's energy supplies must eventually run out.)
The Profound Part
This rather intuitive argument is based on a very fundamental law of physics: the law of conservation of matter and energy. This law states that although matter and energy can interchange (via Einstein's famous equation: E=mc squared) the total amount of matter and energy in the universe remains constant. It changes form, but is not created or destroyed. Never! Ever! Nature allows no exceptions to fundamental laws.
We humans must however allow the possibility that we do not completely understand nature's laws and change our statements of these laws as we learn more. For example prior to Einstein's famous equation, conservation of matter and energy were separate laws. Then we learned that matter and energy were interchangeable and combined two laws into one.
Our sometimes poor understanding of nature's laws does not diminish their power to help us understand nature. The assumptions that nature allows no exceptions to her fundamental laws and that the same laws apply everywhere in the universe have far reaching consequences. Telling us that stars, including our own Sun, must eventually run out of fuel and die is merely one example of their predictive power. Notice that we don't have to know anything at all about the process by which stars generate their energy. The law applies anyway. That is part of the power of fundamental laws. Applying a few simple laws of physics to a wide range of phenomena tells us much about the universe around us.
It seems obvious that the same fundamental laws of physics apply everywhere in the universe, but the idea took us a couple thousand years to figure out. At the dawn of science, the ancient Greeks thought that different laws applied on Earth and in the heavens. They did not consider the laws of science universal. This misunderstanding persisted until the seventeenth century. Newton's understanding of gravity was called the law of universal gravitation. It no longer seems like a major breakthrough, but Newton thought his laws of motion and gravity were the same everywhere in the universe. Bucking a couple millennia of scientific tradition, Newton applied the same laws to both motions on the earth and motions of heavenly bodies. Finally we could discover laws of physics by experiments on Earth then apply their power to help us understand the rest of the universe, such as stars. It often takes a genius to see simple ideas.
Enjoying a starry night reveals both the powerful simplicity and the deep complexity of nature.
