Sun
The Sun is a G2V main-sequence star at the gravitational center of the Solar System whose core proton–proton thermonuclear fusion, tachocline hydromagnetic dynamo, and magnetized coronal outflow govern planetary climates and heliospheric plasma physics.
The Sun is the G2V main-sequence star at the gravitational center of the Solar System, accounting for 99.86% of the system's total mass and supplying the electromagnetic radiation and magnetized plasma outflow that govern planetary climates and space weather. Situated at a mean heliocentric distance of 1 astronomical unit (1 AU = 1.495978707 × 1011 m) from Earth, the Sun is a spheroidal body of ionized hydrogen and helium plasma with an equatorial radius of R☉ = 6.957 × 108 m (109.2 R⊕), a mass of M☉ = 1.9885 × 1030 kg (~332,946 M⊕), and a bolometric luminosity of L☉ = 3.828 × 1026 W. Its photospheric effective temperature of Teff = 5,772 K produces a continuum spectrum peaking near 502 nm and delivering a total solar irradiance of ~1,361 W m−2 at 1 AU.
Radiometric dating of calcium–aluminum-rich inclusions (CAIs) in carbonaceous chondrite meteorites establishes that the Sun formed ~4.567 billion years ago (4.567 Ga) from the gravitational collapse of a presolar molecular cloud core. Orbiting the Galactic Center at a radius of ~8.18 kiloparsecs (~26,700 light-years) at ~225 km s−1, the Sun completes one galactic revolution every 225 to 250 million years. By mass fraction, the contemporary solar photosphere consists of 73.81% hydrogen (X), 24.85% helium (Y), and 1.34% heavier elements (Z, primarily oxygen, carbon, neon, and iron).
Internal Radial Structure and Thermonuclear Fusion
Core Proton–Proton Chain and CNO Cycle
The solar interior comprises three concentric hydrodynamic zones—the thermonuclear core, the radiative zone, and the convective envelope—separated by the tachocline shear layer. Extending from the center to ~0.25 R☉ (~174,000 km), the solar core contains ~50% of the Sun's mass within 1.6% of its volume. Central pressure reaches 2.477 × 1016 Pa (247.7 billion bar), density peaks at 1.50 × 105 kg m−3, and temperature attains 1.57 × 107 K. Thermal agitation prevents electron degeneracy, allowing the ionized plasma to behave as an ideal gas in hydrostatic equilibrium.
Quantum-mechanical Gamow tunneling enables core protons to overcome their Coulomb barrier and fuse four hydrogen nuclei (1H) into one alpha particle (4He), two positrons (e+), and two electron neutrinos (νe), releasing 26.73 MeV (4.283 × 10−12 J) per reaction. To sustain L☉ = 3.828 × 1026 W, the core fuses ~6.0 × 1011 kg of hydrogen into 5.96 × 1011 kg of helium every second, converting 4.26 × 109 kg s−1 of rest mass into radiant energy via two pathways:
- Proton–Proton (pp) Chain (~99% of solar energy): Dominated by the pp-I branch (~84%), where two protons fuse into a deuteron (2H), proton capture forms 3He, and two 3He nuclei fuse into 4He plus two protons, alongside the secondary pp-II (~15%) and pp-III (~0.02%, 7Be and 8B) branches.
- CNO (Carbon–Nitrogen–Oxygen) Bi-Cycle (~1% of solar energy): Uses 12C, 13C, 14N, and 15O nuclei as fusion catalysts (T16 temperature sensitivity versus T4 for the pp chain), directly confirmed in 2020 when the Borexino detector measured the solar CNO neutrino flux.
| Solar Layer | Radial Extent (r / R☉) | Temperature Range (K) | Density Range (kg m−3) | Dominant Energy Transport and Physical Regime |
|---|---|---|---|---|
| Thermonuclear Core | 0.00 – 0.25 | 1.57 × 107 – 7.0 × 106 | 1.5 × 105 – 2.0 × 104 | Proton–proton chain (~99%) and CNO cycle (~1%) fusion; radiative diffusion |
| Radiative Zone | 0.25 – 0.71 | 7.0 × 106 – 2.1 × 106 | 2.0 × 104 – 200 | Solid-body rotation; Thomson scattering and free–free photon random walk |
| Tachocline | ~0.71 (thickness ~0.04) | ~2.1 × 106 | ~200 | Radial rotational shear layer; seat of the αΩ hydromagnetic solar dynamo |
| Convective Envelope | 0.71 – 1.00 | 2.1 × 106 – 5,772 | 200 – 2.0 × 10−4 | Latitudinal differential rotation; overturning thermal plume convection |
| Photosphere | 1.000 (thickness ~500 km) | 6,400 – 4,400 | 2.0 × 10−4 – 1.0 × 10−6 | H− continuum optical depth τ500 = 1 surface; granulation and sunspot umbrae |
| Chromosphere & Transition Region | 1.0007 – 1.003 | 4,400 – 1.0 × 106 | 1.0 × 10−6 – 1.0 × 10−11 | Magnetic canopy, spicules, Hα filaments, and steep non-radiative temperature rise |
| Solar Corona | > 1.003 – 10+ | 1.0 × 106 – 3.0 × 106 | < 1.0 × 10−11 | Low-beta magnetized plasma loops, coronal holes, and transonic solar wind acceleration |
Radiative Zone, Tachocline, and Convective Envelope
Surrounding the core from 0.25 R☉ to 0.713 R☉ is the radiative zone, where the sub-adiabatic temperature gradient inhibits thermal overturning. Core photons undergo Thomson and Compton scattering alongside bound-free absorption, executing a random walk with a sub-millimeter mean free path that requires ~105 to 1.7 × 105 years to reach the convection zone.
At r = 0.713 ± 0.003 R☉ (~2.1 × 106 K), bound electrons recombine onto heavy ions, increasing Rosseland opacity and triggering convective instability. Across the tachocline shear layer, rotation transitions from uniform solid-body rotation in the radiative zone (~27 days) to differential rotation in the convective envelope (0.713–1.00 R☉; 24.47 days at the equator vs. ~34 days near the poles), where buoyant plumes form supergranules (~30,000 km) and photospheric granules (~1,000 km, lifetimes 8–20 minutes).
Helioseismology, Solar Neutrinos, and the Hydromagnetic Dynamo
Helioseismic Oscillations and the Solar Neutrino Solution
Near-surface convection excites standing acoustic pressure waves (p-modes) with 5-minute periods (~3 mHz). Doppler inversions from SOHO/MDI, GONG, and SDO/HMI map internal sound speed, density, and rotation profiles to <0.1% uncertainty. Meanwhile, the Sudbury Neutrino Observatory (SNO) resolved the solar neutrino problem in 2001–2002 by measuring both charged-current (νe) and neutral-current (νe + νμ + ντ) interactions in heavy water, proving that core electron neutrinos undergo Mikheyev–Smirnov–Wolfenstein (MSW) flavor oscillations while matching Standard Solar Model production rates.
The αΩ Solar Dynamo and the 11-Year / 22-Year Magnetic Cycle
Rotational shear in the tachocline stretches poloidal magnetic fields into toroidal flux tubes via the Ω-effect. Magnetic buoyancy lifts these tubes to emerge as bipolar sunspots, while cyclonic turbulence twists toroidal fields back into a reversed poloidal field via the α-effect. This αΩ dynamo drives the 11-year Schwabe sunspot cycle and the 22-year Hale magnetic polarity cycle.
Solar Atmosphere, Coronal Heating, and Stellar Evolution
Photosphere, Chromosphere, and the Coronal Heating Problem
Across the ~500-km photosphere (where H− optical depth reaches τ500 = 1), temperature drops from ~6,400 K to ~4,400 K, producing limb darkening, Fraunhofer absorption lines, and magnetically cooled sunspot umbrae (1,500–4,000 G, ~3,800–4,200 K). Above the photosphere, the chromosphere (~500–2,100 km) warms to ~10,000 K before jumping across a ~100-km transition region to 1.0 × 106–3.0 × 106 K in the solar corona. Sustaining this hot corona—the coronal heating problem—is driven by Parker nanoflares (1016–1017 J magnetic reconnection impulses) and dissipation of Alfvén waves. Open-field coronal holes accelerate the supersonic solar wind to inflate the heliosphere, whereas active regions launch solar flares and coronal mass ejections toward Mercury, Venus, Earth, the Moon, and Mars.
Main-Sequence Brightening and Red-Giant Evolution
Core helium accumulation has increased solar luminosity by ~30% since Zero-Age Main Sequence ignition 4.567 Ga ago (L ≈ 0.70 L☉, the faint young Sun paradox). In ~5.4 billion years, core hydrogen exhaustion will expand the Sun into a red giant (~1 AU radius), engulfing Mercury and Venus, before triple-alpha helium fusion and planetary-nebula mass loss leave behind a ~0.54 M☉ carbon–oxygen white dwarf.
Frequently Asked Questions
Why is the Sun's outer corona hundreds of times hotter than its visible surface?
Sub-photospheric convection shakes and braids magnetic field lines rooted in the surface, launching Alfvén waves and tangled current sheets upward into the tenuous corona, where wave damping and nanoflare reconnection convert magnetic energy into thermal heat at 1–3 million K.
How does the Sun fuse hydrogen steadily without exploding?
The non-degenerate ideal-gas core acts as a self-regulating gravitational thermostat: any excess fusion energy expands and cools the core against gravity, reducing density and reaction rates back to hydrostatic equilibrium.
What color is the Sun when viewed from space?
Viewed outside Earth's atmosphere without Rayleigh scattering, the Sun appears white because its 5,772 K spectrum emits strongly across all visible wavelengths (400–700 nm).
References
- Bahcall, J. N., Serenelli, A. M., & Basu, S. (2005). "New solar opacities, abundances, helioseismology, and neutrino fluxes." The Astrophysical Journal, 621(1), L85–L88. https://doi.org/10.1086/428929
- Ahmad, Q. R., Allen, R. C., Andersen, T. C., et al. (SNO Collaboration). (2002). "Direct evidence for neutrino flavor transformation from neutral-current interactions in the Sudbury Neutrino Observatory." Physical Review Letters, 89(1), 011301. https://doi.org/10.1103/PhysRevLett.89.011301
- Agostini, M., Altenmüller, K., Appel, S., et al. (Borexino Collaboration). (2020). "Experimental evidence of neutrinos produced in the CNO fusion cycle in the Sun." Nature, 587(7835), 577–582. https://doi.org/10.1038/s41586-020-2934-0
- Charbonneau, P. (2020). "Dynamo models of the solar cycle." Living Reviews in Solar Physics, 17(1), 4. https://doi.org/10.1007/s41116-020-00025-6
- Asplund, M., Amarsi, A. M., & Grevesse, N. (2021). "The chemical makeup of the Sun: A 2020 vision." Astronomy & Astrophysics, 653, A141. https://doi.org/10.1051/0004-6361/202140445