The Sun
The Sun is a typical star and our nearest example. Studying it reveals how all stars produce energy.
The Sun's basic properties
The Sun is about 1.39 million km across, about 109 times Earth's diameter, and has about 333,000 times Earth's mass. Its surface temperature is about 5,800 K, and its core reaches about 15 million K. It is about 4.6 billion years old, roughly halfway through its main-sequence life.
Nuclear fusion
In the Sun's core, hydrogen nuclei fuse into helium through the proton-proton chain. Four hydrogen nuclei become one helium nucleus with slightly less mass; the missing mass becomes energy according to \(E = mc^2\). The Sun converts about 4 million tonnes of mass into energy every second.
E = mc^2The Sun's interior
Energy produced in the core travels outward first through the radiative zone, where photons are absorbed and re-emitted countless times, taking tens of thousands of years or longer to cross. Then, in the outer convection zone, hot gas rises and cool gas sinks, carrying energy to the surface like boiling water.
The photosphere and atmosphere
The visible surface is the photosphere, showing granulation, the tops of convection cells. Above it lie the chromosphere and the corona, the Sun's outer atmosphere, visible during total solar eclipses. Strangely, the corona is over a million kelvin, far hotter than the photosphere; magnetic processes are thought to heat it.
Sunspots and the solar cycle
Sunspots are cooler, darker regions where strong magnetic fields inhibit convection. Their number rises and falls in an approximately 11-year solar cycle. Galileo observed sunspots in the 1600s; the period of very few sunspots from about 1645 to 1715 is called the Maunder Minimum.
Space weather
Solar flares (sudden bursts of energy) and coronal mass ejections (huge eruptions of charged particles) can disturb Earth's magnetic field, causing auroras, damaging satellites, disrupting radio and GPS, and in extreme cases damaging power grids. The 1859 Carrington Event was the most intense geomagnetic storm on record. A strong storm in May 2024 produced auroras seen far from the poles.
Solar neutrinos
Fusion produces neutrinos, nearly massless particles that escape the Sun's core almost unimpeded. Detecting them confirmed fusion is happening. Early experiments found fewer than predicted (the solar neutrino problem), resolved when physicists discovered that neutrinos change type as they travel, which requires them to have mass.
Worked example
Each second, the Sun converts about \(4 \times 10^9\) kg of mass into energy. Using \(E = mc^2 = 4 \times 10^9 \times (3 \times 10^8)^2 \approx 3.6 \times 10^{26}\) joules per second, which closely matches the Sun's measured luminosity of about \(3.8 \times 10^{26}\) watts.
Key takeaways
The Sun is 109 times Earth's diameter, 5,800 K at the surface and 15 million K at the core, and about 4.6 billion years old. Fusion of hydrogen into helium converts mass into energy via E = mc^2. Energy moves outward by radiation, then convection. The corona is mysteriously hot. Sunspots follow an 11-year cycle. Flares and coronal mass ejections cause space weather. Solar neutrinos confirmed fusion and showed neutrinos have mass.
Flashcard
About 109 times Earth's
Check your understanding
How does the Sun produce its energy?