What is the Sun? Physical properties
The Sun is a star, the closest one to us. It is not on fire like wood. It is a ball of very hot plasma (gas so hot that atoms lose their electrons).
| Property | Value (rounded) |
|---|---|
| Diameter | 1.39 million km (109 × Earth) |
| Mass | 2 × 1030 kg (333 000 × Earth; 99.86% of the Solar System) |
| Average density | 1.41 g/cm³ (a bit more than water) |
| Distance from Earth | 150 million km = 1 astronomical unit (AU); light takes ~8 min 20 s |
| Surface temperature | ~5500 °C (5778 K) |
| Core temperature | ~15 million °C |
| Power (luminosity) | 3.8 × 1026 W |
| Make-up (by mass) | ~73% hydrogen, ~25% helium, ~2% heavier elements |
| Rotation | ~25 days at the equator, ~35 days near the poles (it is not solid) |
| Age | ~4.6 billion years; about halfway through its life |
The Sun is a medium-sized, yellow-white main-sequence star (spectral type G2). It looks huge and bright only because it is so close. We learn about its make-up from its spectrum: dark lines show which elements are present. Helium was first found in the Sun's spectrum in 1868, before it was found on Earth.
Safety: never look straight at the Sun, even through sunglasses. Use a pinhole projector or certified solar filters.
Structure of the Sun: the layers
The Sun has layers, like an onion. Inside (we cannot see these):
- Core: the inner quarter of the radius. About 15 million °C and very dense. Fusion happens only here.
- Radiative zone: energy moves outward as light (photons). Each photon is absorbed and sent out again in a random direction, so it zig-zags. The trip can take tens of thousands of years or more.
- Convective zone: the outer ~30%. Hot gas rises, cools, sinks and rises again, like boiling soup. We see the tops of these cells as granules on the surface.
The atmosphere (we can see these):
- Photosphere: the visible 'surface', ~5500 °C. Light escapes from here into space.
- Chromosphere: a thin reddish layer, seen during a total solar eclipse.
- Corona: the faint outer halo, over 1 million °C. Why it is so much hotter than the surface is still being studied; magnetic waves and tiny explosions are the main ideas.
Energy source: nuclear fusion and solar neutrinos
In the core, heat and pressure are so high that hydrogen nuclei (protons) crash together and stick. In a chain of steps called the proton–proton chain:
4 ¹H → ⁴He + 2 e⁺ + 2 neutrinos + energy
The helium nucleus is about 0.7% lighter than the 4 protons. That missing mass becomes energy by Einstein's E = mc². Because c² is huge, a tiny mass gives a huge energy.
The Sun turns about 4 million tonnes of mass into energy every second, yet it has enough hydrogen to shine for about 5 billion more years.
Solar neutrinos
Fusion also makes neutrinos: tiny particles with almost no mass and no charge. They hardly ever hit anything, so they fly straight out of the core and reach Earth in about 8 minutes. Billions pass through your thumbnail every second. Huge underground detectors (for example Super-Kamiokande in Japan and the Sudbury Neutrino Observatory in Canada) catch a few. Detecting them proved that fusion really powers the Sun. Early experiments found only about one-third of the expected neutrinos (the 'solar neutrino problem'). The answer: neutrinos change type on the way (neutrino oscillation), which showed they have a tiny mass.
Solar activity: sunspots, flares, prominences, CMEs
The Sun spins faster at its equator than at its poles, so its magnetic field gets twisted. Twisted fields cause:
- Sunspots: dark patches on the photosphere where strong magnetic fields block heat rising from below. They are ~3500–4500 °C, cooler than around them, so they look dark (on their own they would still glow brightly). They often come in pairs with opposite magnetic poles. Watching sunspots move showed that the Sun rotates.
- Solar flares: sudden flashes when twisted magnetic fields snap and reconnect. In minutes they release energy like billions of nuclear bombs, as X-rays, UV and fast particles.
- Prominences: huge loops of glowing gas held up by magnetic fields above the surface.
- Coronal mass ejections (CMEs): giant clouds of plasma thrown into space; they reach Earth in 1–3 days.
The 11-year solar cycle
The number of sunspots rises and falls about every 11 years (solar maximum and minimum). At each cycle the Sun's magnetic poles flip. Flares and CMEs are most common near maximum.
Effects of the Sun on Earth
Everyday effects: sunlight gives heat, drives weather, winds and the water cycle, and powers photosynthesis (the base of nearly all food chains). Solar panels and wind energy depend on it.
Solar wind is a steady stream of charged particles from the corona. Earth's magnetic field (magnetosphere) shields us and guides some particles to the poles.
Space weather from flares and CMEs can:
- light up the sky with auroras (northern and southern lights) when particles hit oxygen and nitrogen high in the air;
- block short-wave radio and disturb GPS;
- damage satellites and endanger astronauts;
- overload power lines and transformers (geomagnetic storms).
The ozone layer and atmosphere block most harmful UV and X-rays. Space agencies now watch the Sun all the time to give storm warnings.
Try it: project the Sun safely
Make a pinhole projector: poke a small clean hole in a card. Stand with your back to the Sun and let light pass through the hole onto a white sheet 1 m away. You will see a small round image of the Sun. Measure its diameter d and the distance L. The Sun's diameter ≈ (d ÷ L) × 150 million km. With d = 9.3 mm and L = 1 m you get about 1.4 million km. Never look at the Sun directly.
Key formulas and definitions
- Fusion: 4 ¹H → ⁴He + 2 e⁺ + 2 ν + energy
- E = mc² (energy = lost mass × speed of light squared, c = 3 × 10⁸ m/s)
- Light travel time t = distance ÷ c = 1.5 × 10¹¹ m ÷ 3 × 10⁸ m/s ≈ 500 s
- Pinhole: Sun's diameter ÷ Sun's distance = image diameter ÷ pinhole distance
- Solar cycle ≈ 11 years (magnetic cycle ≈ 22 years)
Worked examples
1. How long does sunlight take to reach Earth? (distance 1.5 × 10¹¹ m, c = 3 × 10⁸ m/s)
t = d ÷ c = 1.5 × 10¹¹ ÷ 3 × 10⁸ = 500 s ≈ 8 min 20 s. So we always see the Sun as it was about 8 minutes ago.
2. In one fusion chain, 4 protons have total mass 6.690 × 10⁻²⁷ kg and the helium nucleus has 6.644 × 10⁻²⁷ kg. Find the energy released.
Mass lost Δm = 0.046 × 10⁻²⁷ = 4.6 × 10⁻²⁹ kg. E = Δm c² = 4.6 × 10⁻²⁹ × (3 × 10⁸)² = 4.6 × 10⁻²⁹ × 9 × 10¹⁶ ≈ 4.1 × 10⁻¹² J. Tiny for one reaction, but the Sun does about 10³⁸ of them every second.
3. The Sun's power is 3.8 × 10²⁶ W. How much mass does it lose each second?
From E = mc², m = E ÷ c² = 3.8 × 10²⁶ ÷ 9 × 10¹⁶ ≈ 4.2 × 10⁹ kg per second, about 4 million tonnes every second.
4. Why do sunspots look dark if they are still about 4000 °C?
Brightness is compared with the surroundings. The photosphere around them is ~5500 °C and much brighter (a hotter surface gives off far more light). Next to it the cooler spot looks dark, like a torch bulb in front of a floodlight.
5. A big flare is seen at 10:00. Why might its X-rays affect radio at about 10:08, but the CME's particles only reach Earth 2 days later?
X-rays are light, travelling at 3 × 10⁸ m/s, so they arrive in about 8 minutes. The CME is a cloud of matter moving at roughly 500–2000 km/s, so 150 million km takes about 1–3 days.
Common mistakes
- Thinking the Sun is burning like a fire. Burning is a chemical reaction needing oxygen; the Sun shines by nuclear fusion.
- Thinking sunspots are cold or holes. They are only cooler than their surroundings, still thousands of degrees, caused by magnetic fields.
- Thinking light leaves the core in 8 minutes. Light takes ~8 minutes from the surface to Earth, but energy needs thousands of years to get out of the Sun's interior.
- Thinking the corona is the coolest layer because it is farthest out. It is over 1 million °C, much hotter than the photosphere.