The Sun

1. What is the Sun?

The Sun is a nearly perfect sphere of hot plasma at the center of our Solar System. It is the star that provides Earth and the other planets with light, heat, and energy necessary to sustain life.

2. Basic Facts

Type: G-type main-sequence star (G2V), often called a yellow dwarf
Age: About 4.6 billion years
Diameter: Approximately 1.39 million kilometers (about 109 times Earth's diameter)
Mass: About 1.989 × 10³⁰ kilograms (about 333,000 times Earth's mass)
Composition: About 74% hydrogen, 24% helium, and 2% heavier elements by mass
Distance from Earth: Approximately 149.6 million kilometers (1 Astronomical Unit, AU)
Surface Temperature: Around 5,500 °C (about 5,778 K)
Core Temperature: Around 15 million °C

3. Structure of the Sun

a) Core

The innermost part, extending about 20–25% of the Sun’s radius.
Site of nuclear fusion where hydrogen atoms fuse into helium, releasing huge amounts of energy.
Temperature: ~15 million °C
Pressure and density are extremely high.

b) Radiative Zone

Surrounds the core and extends up to about 70% of the Sun’s radius.
Energy generated in the core is transferred outward by radiation (photons).
Photons can take thousands to millions of years to pass through this zone due to constant absorption and re-emission.

c) Convective Zone

Outer 30% of the Sun’s radius.
Energy is transferred by convection — hot plasma rises, cools, and sinks.
This causes turbulent motions and granulation visible on the surface.

d) Photosphere

The visible "surface" of the Sun from which light is emitted.
Temperature: about 5,500 °C
Thickness: a few hundred kilometers
Features sunspots (cooler, darker regions caused by magnetic activity).

e) Chromosphere

Thin layer above the photosphere.
Temperature rises from about 4,500 °C to 20,000 °C.
Visible during solar eclipses as a reddish glow.

f) Corona

The Sun’s outer atmosphere extending millions of kilometers into space.
Temperature: 1–3 million °C (much hotter than the surface — a longstanding scientific mystery).
Visible during total solar eclipses as a pearly white halo.
Source of the solar wind (stream of charged particles flowing into space).

4. Energy Production

Nuclear fusion in the core converts hydrogen into helium via the proton-proton chain reaction, releasing energy in the form of gamma rays.
This energy slowly moves outward and eventually reaches the surface, escaping as visible light and other electromagnetic radiation.
The Sun converts about 600 million tons of hydrogen into helium every second!

5. Solar Activity

Sunspots: Cooler, darker regions caused by strong magnetic fields.
Solar flares: Sudden bursts of energy that release radiation and charged particles.
Coronal Mass Ejections (CMEs): Massive bursts of solar wind and magnetic fields ejected into space, which can affect Earth’s magnetosphere (causing auroras and sometimes disrupting satellites and power grids).
Solar activity follows roughly an 11-year cycle of increasing and decreasing activity.

6. Importance of the Sun

Source of life: Provides energy for photosynthesis, climate, and weather systems on Earth.
Gravitational anchor: Holds the Solar System together, keeping planets in orbit.
Space weather: Solar winds and flares influence the space environment around Earth and other planets.

7. Future of the Sun

The Sun is about halfway through its main-sequence phase.
In about 5 billion years, it will expand into a red giant, possibly engulfing Earth.
After shedding outer layers, it will end as a white dwarf, a dense stellar remnant cooling over billions of years.

8. Interesting Facts

The Sun accounts for about 99.86% of the total mass of the Solar System.
Light from the Sun takes about 8 minutes and 20 seconds to reach Earth.
The solar wind shapes the heliosphere, a protective bubble around the Solar System.