The Sun is a G-type yellow dwarf star, about 4.6 billion years old and roughly halfway through its life. At 1.39 million km across, it could hold 1.3 million Earths.
Where the energy comes from
In the core, temperatures reach about 15 million °C and pressure is high enough for hydrogen nuclei to overcome electrostatic repulsion and fuse into helium. This proton-proton chain converts roughly 4 million tonnes of matter into energy every second, exactly as E = mc² describes.
That energy does not escape immediately. A photon born in the core takes tens to hundreds of thousands of years to work its way out through the dense radiative zone. The light you see today began its journey long before humans existed.
A layered structure
| Layer | Temperature | Character |
|---|---|---|
| Core | ~15,000,000 °C | Where fusion happens |
| Radiative zone | 7,000,000 - 2,000,000 °C | Energy carried by photons |
| Convective zone | ~2,000,000 - 5,500 °C | Plasma boiling like heated water |
| Photosphere | ~5,500 °C | The visible surface |
| Chromosphere | 4,500 - 20,000 °C | A thin reddish layer |
| Corona | 1,000,000 - 3,000,000 °C | The outermost halo |
A long-standing puzzle sits in the last two rows: the corona is hundreds of times hotter than the surface below it. Missions such as Parker Solar Probe were built to find out why, and current evidence favours magnetic waves plus countless small-scale reconnection events pumping energy upward.
The 11-year cycle
The Sun's magnetic field flips roughly every 11 years. At solar maximum, sunspot numbers surge and flares and coronal mass ejections become frequent.
When a CME is aimed at Earth, charged particles interact with the magnetosphere and produce aurorae. Strong events can disrupt satellites and GPS and even overload power grids - as in Québec in 1989, when six million people lost power for nine hours.
The future
In about five billion years the core will run out of hydrogen. The Sun will swell into a red giant, swallowing Mercury and Venus, then shed its outer layers as a planetary nebula, leaving a slowly cooling white dwarf behind.
References
- [1]Sun Fact Sheet — NASA NSSDC (2024)
- [2]Parker Solar Probe Mission Overview — NASA (2024)
- [3]Solar Dynamics Observatory — NASA (2024)
- [4]Solar Orbiter — ESA (2024)
- [5]Geomagnetic Storms — NOAA Space Weather Prediction Center (2024)
- [6]The Sun — NASA Science (2024)
Image: NASA/SDO (AIA) — Wikimedia Commons, Public domain.
