Size of the Sun

The Sun is a G-type main-sequence star (yellow dwarf) with a diameter of approximately 1,392,000 kilometers (about 865,000 miles). This makes it roughly 109 times wider than Earth. Its radius is around 696,000 km, and its volume is large enough to fit about 1.3 million Earths inside it. The Sun’s mass is about 1.989 × 10^30 kilograms, accounting for over 99.8% of the total mass in our solar system.

Energy Output

The Sun produces an enormous amount of energy through nuclear fusion in its core. Its total luminosity (energy radiated per second) is approximately 3.826 × 10^26 watts. This is equivalent to the energy released by about 92 billion megatons of TNT exploding every second. This energy is primarily in the form of electromagnetic radiation, including visible light, ultraviolet, and infrared, which takes about 8 minutes and 20 seconds to reach Earth. The energy sustains life on Earth by driving weather patterns, photosynthesis, and the water cycle.

Constant “Burning” (Nuclear Fusion Process)

The Sun doesn’t “burn” in the traditional chemical sense, like combustion with oxygen. Instead, it undergoes constant nuclear fusion in its core, where temperatures reach about 15 million degrees Celsius (27 million degrees Fahrenheit) and pressures are immense. Here, hydrogen nuclei (protons) fuse to form helium through the proton-proton chain reaction:

1. Two protons collide to form deuterium (a proton-neutron pair), releasing a positron and a neutrino.
2. Deuterium fuses with another proton to form helium-3.
3. Two helium-3 nuclei combine to form helium-4, releasing two protons.

This process converts about 620 million metric tons of hydrogen into 616 million metric tons of helium every second, with the mass difference (about 4 million tons) converted into energy according to Einstein’s equation E = mc² (where c is the speed of light). This “constant burning” has been ongoing for billions of years and is stable due to hydrostatic equilibrium, balancing gravitational collapse with outward radiation pressure.

Lifespan Calculation

The Sun’s lifespan refers to its main-sequence phase, during which it steadily fuses hydrogen into helium. To calculate this, we estimate the total energy available from fusion and divide by the rate at which it’s used (luminosity).

Step-by-Step Calculation:

1. Mass of the Sun (M): 1.989 × 10^30 kg.
2. Fraction of mass available for fusion (f): Only about 10% of the Sun’s mass (the core) can undergo fusion, and the efficiency of hydrogen-to-helium fusion is 0.7% (0.007) of the mass converted to energy. Thus, f ≈ 0.1 × 0.007 = 0.0007.
3. Speed of light (c): 3 × 10^8 m/s.
4. Total energy available (E): E = f × M × c² (using E = mc² for the converted mass).
5. Luminosity (L): 3.826 × 10^26 W (joules per second).
6. Lifetime in seconds (τ_seconds): τ_seconds = E / L.
7. Convert to years: Divide by the number of seconds in a year (≈ 3.15576 × 10^7, accounting for 365.25 days).

Using these values:

• E = 0.0007 × 1.989 × 10^30 × (3 × 10^8)^2 ≈ 1.252 × 10^44 J.
• τ_seconds ≈ 1.252 × 10^44 / 3.826 × 10^26 ≈ 3.272 × 10^17 seconds.
• τ_years ≈ 3.272 × 10^17 / 3.15576 × 10^7 ≈ 1.04 × 10^10 years (10.4 billion years).

The Sun is currently about 4.6 billion years old, so it has roughly 5.8 billion years left in its main-sequence phase. After that, it will expand into a red giant, eventually shedding its outer layers to become a white dwarf. This calculation assumes constant luminosity, though it actually increases slightly over time, shortening the remaining lifespan a bit.