6.2.1 The Sun as a star
- Syllabus
- 0625–2026–2027
- Topic
- 6.2.1
- Level
- —
Learning objectives
6.2.1.1Sun is a star of medium size• Know: the Sun is a star of medium size, consisting mostly of hydrogen and helium, and that it radiates most of its energy in the infrared, visible light and ultraviolet regions of the electromagnetic spectrum6.2.1.2Stars are powered by nuclear reactions• Know: stars are powered by nuclear reactions that release energy and that in stable stars the nuclear reactions involve the fusion of hydrogen into helium
Describe the Sun as a star
The Sun is a medium-sized star. It is the one star in our Solar System and the nearest star to Earth.
The Sun consists mostly of the elements hydrogen and helium. Other elements are present in much smaller amounts.
The Sun radiates most of its energy in three neighbouring regions of the electromagnetic spectrum: infrared, visible light and ultraviolet.
| Region | Position relative to visible light |
|---|---|
| infrared | longer wavelength than visible red light |
| visible light | detected by the human eye |
| ultraviolet | shorter wavelength than visible violet light |
The Sun does not emit only visible light, and most of its energy is not in radio waves, X-rays or gamma rays. 'Medium-sized star' does not mean the Sun is a planet or a galaxy.
Explain how fusion powers a stable star
Stars are powered by nuclear reactions in their cores. These reactions release energy that is eventually transferred from the star into space.
In a stable star, hydrogen nuclei join together to form helium nuclei. Joining small nuclei to make a larger nucleus is nuclear fusion.
Fusion releases energy. This energy moves through the star and is radiated from its surface as electromagnetic radiation.
| Process | What happens to nuclei | Role in a stable star |
|---|---|---|
| fusion | small nuclei join to form a larger nucleus | hydrogen forms helium and energy is released |
| fission | a large nucleus splits into smaller nuclei | not the process that powers a stable star |
Stellar fusion is not chemical burning: no oxygen is required. It is also not nuclear fission, and the direction is hydrogen to helium, not helium to hydrogen.