Lesson Objectives:- Why the Sun shines
- How nuclear fusion in the Sun started
- Basic properties of the Sun
- The Sun's atmosphere
- The Sun's interior
Astronomers in the 19th century thought the Sun generated energy through gravitational contraction. As we have learned previously, a shrinking gas cloud heats up because the gravitational potential energy of the gas particles is converted into thermal energy as the gas moves inward.
Based on the Sun's mass, however, this could not explain how the Sun has shined for billions of years -- energy generation through gravitational contraction would only have lasted about 25 million years.
Then, in 1905, Albert Einstein published his theory of relativity, which told us that mass itself contains an enormous amount of potential energy. By the 1930s, scientists had discovered nuclear fusion and with that discovery, scientists finally understood how it was that the Sun was able to shine over billions of years.
In the early days of the solar system, the Sun was born from a collapsing cloud of nebular gas. Through gravitational contraction, as the gas contracted, the interior temperature increased. Eventually, it reached a temperature where the core was hot and dense enough for nuclear fusion to occur.
Since then, nuclear fusion has continued to steadily produce energy as a result of gravitational equilibrium and energy balance. Gravitational equilibrium means the outward push of the gas pressure in the Sun is equal to the inward pull of gravity. Energy balance means the rate at which fusion releases energy in the Sun's core is equal to the rate at which the Sun's surface radiates this energy into space.
Energy balance and gravitational equilibrium are important because if they were to fall out of balance, gravitational contraction would occur again, causing the Sun to shrink and raise its core temperature.
Think of it this way. Due to the Sun's size and mass, its gravity is extremely strong and is pulling everything into itself. But due to its extreme heat and energy being released from its core, that creates an outward pressure that counters gravity. As long as these forces balance out, the Sun's size and energy output will remain stable.
The Sun is a giant, glowing ball of hot gas, or plasma.
Spectroscopy tells us that the Sun is made almost entirely of hydrogen and helium. Angular size and distance can be used to calculate that it has a radius of almost 700,000 kilometers, or over 100 times the radius of the Earth.
Newton's version of Kepler's third law allows us to calculate the Sun's mass. It is 300,000 times greater than the Earth's mass and 1,000 times the mass of all of the planets in our solar system put together.
The outermost layer of the Sun's atmosphere is the corona, which is a low-density gas extending several million kilometers above the visible surface of the Sun. The corona has an extremely high temperature of around 1 million Kelvin, which is why this layer emits most of the Sun's X rays.
As you get closer to the Sun's visible surface, the middle layer of the atmosphere is the chromosphere, which is about 10,000 Kelvin and radiates most of the Sun's ultraviolet light.
The lowest layer of the atmosphere is the photosphere, which is the visible surface of the Sun. It has a much lower density than the Earth's atmosphere but has an average temperature of 5,800 Kelvin.
The photosphere is where sunspots occur, which are regions of intense magnetic fields.
Below the photosphere is the convection zone. Similar to the convection currents in Earth's mantle, the convection zone of the Sun is where hot gas rises upward and cooler gas descends. Convection is what causes the photosphere above it to seethe and churn.
Continuing deeper into the Sun, after the turbulent convection zone is the calmer radiation zone where the temperature is nearly 10 million Kelvin and energy moves outward through photons of light instead of gas convection.
If you keep moving inward, you reach the core, the source of the Sun's energy. There, nuclear fusion transforms hydrogen into helium. The temperature is about 15 million Kelvin and the pressure is 200 billion times that on Earth's surface. Energy produced in the core takes a long time to reach the surface -- probably a few hundred thousand years.