You may already know the answer: the sun is powered by nuclear reactions. Specifically, the sun combines (light) hydrogen nuclei into (heavier) helium nuclei in a process called fusion. It also combines helium into heavier elements, but at a much slower rate.
In fact before the universe became transparent the universe was almost completely Hydrogen and Helium. Heavier elements, including us, relied on the nuclear processes in stars to produce the other elements that we are made of. Hydrogen and Helium are still the most abundant elements in the universe today. (Because of the Earth's atmosphere and Helium's inability to chemically bond, there is very little Helium here.)
We are discussing the Sun for three very different reasons.
- The Sun is probably the most familiar example of a plasma. The Sun has some great analogies with the plasma in the early universe we will be exploring. For example:
- The sun is about 8.5 light minutes away from Earth, so when we look at the Sun, we are seeing it as it was 8.5 minutes ago. (Don't look directly at the Sun!)
- Even though the Sun is "only" 8.5 light minutes away, it takes light millions of years to reach us from the center of the sun, because the light keeps scattering off the plasma.
- Because of all this scattering, we cannot see very far into the sun ....... at least not with light! So how can we know that nuclear process are powering the sun, or even anything at all, if we cannot see inside? You will have to wait for Friday's blog post to see how we can see through a plasma!
- Before people knew about nuclear power, they were still trying to understand where the sun's energy came from. Scientists in the 1800s correctly believed that the laws that applied to objects on Earth also applied to stars. Without knowing about nuclear power, these scientists got the wrong answer: they guessed that gravitational potential energy was being converted into heat.
We are going to address points (1) and (2) in Friday's blog post, but point (3) is worth looking at. Knowing the mass of the sun, and how much energy it is putting out, allows us to estimate how long the sun would last if the gravitational potential energy was powering it: about 30 million years. We know by a variety of other methods that the Earth (and the Sun) have been around longer than this.
Before discovering nuclear reactions, scientists of the 1800s had no chance of figuring out the process that keeps the Sun alight. It was a problem they worked on anyway, because they believed that even stars had to obey the same basic rules as elements here on Earth. This story shows one of the dangers of working in science: it is not enough to work hard; sometimes we just haven't come across the right experimental results to be able to correctly understand the phenomena we observe. But it also shows one of the greatest strengths in the scientific method, the need to revise and ensure all the data are consistent. Even if we don't have the tools to find the right explanation, we can rule out some of the compelling but wrong explanations to help those that come after us.
Even the incorrect explanation of gravitational potential energy powering the sun was not worthless. In order to get nuclear fusion, we need to do more than get Hydrogen together. We need to get high enough temperatures and pressures to "kick-start" the nuclear reactions. While the gravitational potential energy cannot power the Sun for the billions of years we know it has been burning, it is an essential part of starting the nuclear process. It also helps us understand that the reason that Jupiter and Saturn (also mostly Hydrogen and Helium) do not burn is because they are not massive enough. We gained a lot of knowledge from the scientists of the 1800s that were brave enough to pursue their curiosity to ask and attempt to answer really hard questions.
In Friday's post we will introduce neutrinos, and how they help us confirm that we have the correct description of the Sun's energy source.
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