By Robert Hazen, Ph.D., George Mason University
After years of observing, multiple models of the Sun’s life cycle and evolution have been developed. There are some major conclusions of these models, and they’re quite fascinating. They tell us not only about the distant past of the Sun, but also help us make an accurate prediction of its evolution in the future.
The Early Sun
One of the major conclusions about the Sun, based on theories and models, is that when the Sun formed, 4.5 billion years ago, it was much less brighter than it is today. Indeed, it may have only been 70 percent of its current brightness. This leads to what has been called the Early Faint-Sun Paradox: the paradox that if the Sun was that much cooler, the oceans had to be solid ice when they formed. The energy output of the Sun, over time, gradually increased.
Indeed, it’s gradually increasing today, although not at all noticeable by human standards; the Sun is moving up from the lower right to the upper left of the Hertzsprung-Russell diagram, producing a little bit more energy every 100 million years or so. Modelers estimate that the Sun’s going to continue this stable, hydrogen-burning phase for perhaps several billion more years and continue to gradually increase in brightness.
Learn more about the nebular hypothesis.
The Sun’s Expansion
All stars have to eventually run low on hydrogen fuel in their core. In the final stages of the Sun’s life, helium concentrations would build up in the core, and hydrogen burning must therefore take place farther and farther out, in concentric layers around the core. Because the hydrogen burning would be close to the Sun’s surface, it would cause the Sun to expand. There would be less pressure farther out, and so those nuclear reactions would cause the Sun to balloon outward.
When this happens, Mercury is going to be engulfed by the Sun, some billions of years down the road. The Sun would keep ballooning outward; eventually surpassing the orbit of Venus. However, the Sun’s not going to engulf Venus, because as the Sun balloons outward, its solar wind would increase, and so much mass would be flung out into space that the Sun would lose mass, so the orbit of Venus would naturally get farther and farther away from the Sun. The Earth and other planets are going to move outward a little bit, as well because the gravitational force of the Sun will be diminished.
But at some point, perhaps in a billion or two billion years, Earth will become absolutely uninhabitable because the temperature at the surface of the Earth is going to climb higher and higher as the Sun’s surface gets closer to us.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
The Sun’s Evolution
One can follow the history of the Sun on a Hertzsprung-Russell diagram. For most of the billions of years of the Sun’s history, it lies very close to the main sequence; but when the hydrogen in the core has been burned up, it starts to expand. It gets larger and larger—still produces lots of energy, but as it expands, it actually gets cooler at the surface, because all that energy is coming out of a larger surface area.
This process takes the Sun away from the main sequence; it takes it towards the upper right of the Hertzsprung-Russell diagram. In fact, as a result of hydrogen burning around the core, the core gets hotter and hotter and more and more energy is produced; all the remaining hydrogen in the core is consumed, and then it goes into a stage of helium burning in the core.
This is a major transition in the history of the Sun. The helium nuclei fuse form heavier nuclei. Most notably, three helium atoms combine together to form carbon-12. This is the process of helium burning. The consequence of this is that more and more energy is being produced by the Sun, it expands outward, and the Sun becomes a red giant star.
Red giant stars are the stars that are in the upper-right-hand corner of the Hertzsprung-Russell diagram which are helium-burning stars.
Learn more about the theory that the Big Bang was not an explosion but an expansion of space itself.
White Dwarf Sun
At that point, the Sun’s history changes.
When the helium burning stops, gravity starts taking over and the star begins to get smaller. As a star gets smaller, the gravitational potential energy is transferred into heat. Here, the energy input is not just nuclear reactions, it’s also the transfer of gravitational potential energy to heat.
When helium burning ends, when hydrogen burning ends, the Sun no longer will have nuclear fuel to keep it going, and it starts collapsing, and collapsing some more. The nuclear fires are out, but a huge amount of gravitational potential energy is converted into heat, and this creates the white dwarf star: incredibly hot, because of all that released gravitational potential energy, but virtually no nuclear reactions going on anymore.
So the white dwarf star shrinks down to something perhaps only the size of the Earth, and that star continues to shine, slowly getting cooler and cooler, radiating its life out into space.
The second law of thermodynamics applies to stars, and if you have a hot object in cold space, heat transfers from hot to cold. So what we see on the Hertzsprung-Russell diagram, what we see in our telescopes, are all these different kinds of stars that are typical parts of the Sun’s life cycle.
Common Questions about the Sun’s Life Cycle and Evolution
When the Sun formed, 4.5 billion years ago, it was much less brighter than it is today. It may have only been 70 percent of its current brightness.
In the final stages of the Sun’s life cycle, helium concentrations build up in the core, and hydrogen burning must therefore take place farther and farther out, in concentric layers around the core. Because the hydrogen burning is close to the Sun’s surface, it causes the Sun to expand, and eventually become a red giant star.
When helium burning ends, when hydrogen burning ends, the Sun no longer will have nuclear fuel to keep it going, and it starts collapsing. The nuclear fires would be out, but a huge amount of gravitational potential energy would be converted into heat, creating a white dwarf star.