Lesson 38: The Sun, Our Star
In this lesson,
we will learn about how nuclear fusion causes the Sun to shine and how nuclear
fusion in the Sun started. We will then look at basic properties of the Sun
before moving on to a more detailed look at its atmosphere and 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.