Module 6, Unit 1: Overview of the Sun

Star at the centre of the Solar System.

The Sun, the star at the center of our Solar System, is a fascinating celestial body that plays a crucial role in life on Earth. In this unit, we will explore the Sun's composition, structure, and the nuclear fusion process that powers it.

Composition of the Sun

The Sun is primarily composed of hydrogen (about 75% of its mass, or 92.1% of its volume), with helium making up most of the rest (about 25% of its mass and 7.8% of its volume). The remaining 2% is made up of heavier elements, including oxygen, carbon, neon, and iron, among others.

Structure of the Sun

The Sun can be divided into several layers, each with its unique properties:

1. Core: The Sun's core is its innermost layer, where the pressure and temperature (15 million degrees Celsius) are so immense that nuclear fusion occurs. This process converts hydrogen into helium and releases a tremendous amount of energy.

2. Radiative Zone: Surrounding the core, the radiative zone is where energy produced by nuclear fusion travels outward as electromagnetic radiation. In this zone, photons bounce around, taking about 170,000 years to travel through this layer.

3. Convective Zone: Above the radiative zone, the convective zone is where energy is transported by convection. Hot plasma rises towards the Sun's surface, cools down, and then sinks back down to heat up again.

4. Photosphere: The photosphere is the Sun's visible surface that we see from Earth. It's where the sunlight we see is emitted, and it has an average temperature of about 5,500 degrees Celsius.

5. Chromosphere: Above the photosphere, the chromosphere is a layer that emits a reddish glow as super-heated hydrogen burns off. But the red glow is usually invisible because of the overwhelming brightness of the photosphere.

6. Corona: The outermost layer of the Sun, visible during total solar eclipses, is the corona. It's extremely hot, with temperatures between 1 and 3 million degrees Celsius. The corona extends far out into space and transforms into solar wind.

Nuclear Fusion in the Sun

The Sun's energy comes from nuclear fusion in its core, a process where hydrogen nuclei combine to form helium. This process, known as the proton-proton chain, involves several steps and results in the conversion of mass into energy. This energy is then gradually released and makes its way to the Sun's surface, from where it radiates into space as sunlight.

The Sun has been burning for about 4.6 billion years, and it's expected to continue for another 5 billion years or so. As it exhausts its hydrogen supply, it will start burning helium, expand into a red giant, and eventually collapse into a white dwarf, marking the end of its life cycle.

In conclusion, the Sun is not just a bright spot in the sky. It's a complex, dynamic system with intricate processes happening at every moment. Its energy sustains life on Earth, drives our climate, and influences our space weather. Understanding the Sun helps us understand more about our Solar System and the universe beyond.