Learning outcomes
- Identify the Sun as a medium-sized star.
- State its main composition.
- Identify the main electromagnetic regions in which it radiates energy.
- Explain that fusion of hydrogen into helium powers stable stars.
- Describe force balance in a stable star.
6.1 The Sun is a star
The Sun is not a unique type of object; it is a star, seen much brighter than other stars because it is vastly closer to Earth. It is described in the syllabus as a medium-sized star. Its enormous size compared with Earth does not make it unusually large among stars.
The Sun is a sphere of hot plasma rather than a solid or liquid body. Plasma is ionised matter containing freely moving charged particles. This state allows nuclear reactions and radiation transport to occur under extreme temperature and pressure.
6.2 Composition
The Sun consists mostly of hydrogen and helium. Hydrogen is the main fuel for nuclear fusion in the central region. Helium is both an original component and a product of hydrogen fusion.
Small quantities of heavier elements are present, but they make up only a minor fraction. Examination answers should use ‘mostly hydrogen and helium’ rather than stating that the Sun is burning ordinary gas in a chemical reaction.
6.3 Electromagnetic radiation from the Sun
The Sun emits a wide range of electromagnetic radiation. The syllabus highlights infrared, visible and ultraviolet as the regions containing most of its radiated energy. Infrared contributes heating, visible light enables vision and photosynthesis, and ultraviolet can cause biological damage.
Earth receives only a tiny fraction of the Sun’s total output because the energy spreads through space. The power received per unit area decreases with distance from the Sun.
6.4 Nuclear fusion
In stable stars, light hydrogen nuclei combine through a sequence of nuclear reactions, ultimately producing helium and releasing energy. This is fusion: the formation of a larger nucleus from smaller nuclei with energy released.
Fusion requires extremely high temperature so nuclei move rapidly enough to approach despite electrical repulsion between their positive charges. High pressure and density in the stellar core also increase the rate of collisions.
6.5 Stability of a star
Gravity pulls the star’s material inward. Energy released in the core maintains very high temperatures, producing an outward effect associated with gas and radiation pressure. A stable main-sequence star exists when inward gravitational attraction is balanced by this outward effect.
Balance does not mean that the star contains no motion or no energy transfer. It means the overall size remains approximately stable over a very long time because there is no unbalanced large-scale contraction or expansion.
Worked examples
Identifying the energy source
The Sun’s energy is not produced by combustion. It is released by nuclear fusion of hydrogen into helium in the core.
Explaining stable size
A stable star does not collapse because the inward pull of gravity is balanced by the outward effect caused by the very high central temperature.
Practical focus
Investigation or modelling activity
Analyse a simplified solar spectrum showing infrared, visible and ultraviolet regions. Identify which side has longer wavelength and which has higher frequency. Connect the spectrum to concepts from Volume 3 without assuming that visible light is the only radiation emitted.
Examination guidance
- Use “fusion”, not “fission”, for the energy source of stars.
- State hydrogen changes into helium.
- Do not describe the Sun as a burning solid object.
- Include both inward gravity and outward high-temperature effect when explaining stability.
- Name infrared, visible and ultraviolet when asked about main emitted regions.
Check your understanding
- What are the two main elements in the Sun?
- What nuclear process powers stable stars?
- Why is high temperature needed for fusion?
- What balances gravitational attraction in a stable star?
Answers
- Hydrogen and helium.
- Fusion of hydrogen into helium.
- It gives nuclei enough kinetic energy to approach closely despite electrostatic repulsion.
- An outward effect due to the very high temperature in the star’s centre.
