The Sun as a StarCambridge IGCSE Physics: Revision notes

The Sun is a medium-sized star, made up mostly of hydrogen and helium. It radiates most of its energy in the infrared, visible and ultraviolet regions of the electromagnetic spectrum.

The Sun is just one of many billions of stars that make up the Milky Way galaxy, and other stars in the Milky Way are far further from Earth than the Sun is.

Stars, including the Sun, are powered by nuclear reactions that release energy. In stable stars like the Sun, this energy comes from the fusion of hydrogen into helium in the core.

Fusion releases enormous amounts of energy because some mass is converted into the energy of radiation. This ongoing fusion is what makes the Sun (and all stable stars) shine.

Radiation from the Sun is the main source of energy for almost all of Earth's energy resources — the only exceptions are geothermal, nuclear and tidal energy, which do not rely on the Sun.

This means solar energy ultimately drives most weather systems, plant growth (via photosynthesis), and resources like wind, wave, and fossil fuels (which stored energy originally captured from sunlight by ancient organisms).

Nuclear fusion, as it occurs in the Sun, releases energy without the long-lived radioactive waste associated with nuclear fission. Because of this, research is being carried out to investigate how energy released by nuclear fusion could be used to generate electrical energy on a large scale here on Earth.

This remains an area of ongoing scientific research rather than a currently widespread commercial energy source.

A star such as the Sun follows this life cycle:

1. Nebula — an interstellar cloud of gas and dust (mainly hydrogen) begins to collapse under its own gravitational attraction
2. Protostar — the collapsing cloud heats up as it contracts
3. Main sequence star — the protostar becomes a stable star once the inward pull of gravity is balanced by the outward force from the high temperature generated by fusion in the core; the Sun is currently in this stage
4. Red giant — once hydrogen in the core runs low, the star expands
5. White dwarf — the outer layers are shed (forming a planetary nebula), leaving a small, dense white dwarf star at the centre

(This differs from the life cycle of much more massive stars, which end as red supergiants, then supernovae, leaving a neutron star or black hole.)