The Sun is a G2V main‑sequence star located 1 astronomical unit (au) from Earth, with standardized reference properties adopted by the International Astronomical Union (IAU) including a nominal radius of 695,700 km, luminosity of 3.828×10^26 W, and effective temperature of 5772 K. According to NASA, it contains about 99.8% of the Solar System’s mass and is the dominant source of energy for Earth. IAU Resolution B3; NASA Basics of Space Flight; NASA “It’s Surprisingly Hard to Go to the Sun”.

Classification and fundamental properties

The Sun is classified as a G‑type dwarf (G2 V), a spectral and luminosity class indicating a hydrogen‑fusing (main‑sequence) star with an effective surface temperature near 5770 K. NASA’s introductory material on the Solar System identifies the Sun’s spectral type and emphasizes that its mass dominates local dynamics. NASA Basics of Space Flight. The IAU’s 2015 nominal constants provide exact reference values widely used in astrophysics: radius 695,700 km, total solar irradiance (TSI) 1361 W m⁻² at 1 au, luminosity 3.828×10^26 W, and effective temperature 5772 K. IAU Resolution B3 (Prša et al. 2016, AJ); IAU Resolution B2 (Mamajek et al. 2015). NASA’s irradiance program refined the modern TSI consensus near 1361 W m⁻² and documents ~0.1% solar‑cycle variability. NASA Earth Science Solar Irradiance; NASA Solar Irradiance Data.

Formation and age

The age of the Sun is inferred from the chronology of the oldest Solar System solids. High‑precision U‑corrected Pb–Pb dating of calcium‑aluminum–rich inclusions (CAIs) yields 4567.30 ± 0.16 million years for initial condensation, with chondrule formation overlapping and extending ~3 Myr, indicating a rapid early evolution of the protoplanetary disk. Connelly et al., Science (2012). Recent work reviews CAI ages in the range ~4567.2–4568.7 Myr, consistent with a Sun and Solar System forming ~4.57 billion years ago. Astronomy & Astrophysics 2024 review; NASA overview of Sun’s youth.

Internal structure and energy generation

The solar interior comprises a central core, a radiative zone, and an outer convective zone, overlain by the photosphere, chromosphere, and corona. Core temperatures reach ~15 million °C, while the photosphere is about 5500 °C; coronal temperatures rise to millions of degrees. NASA Solar Temperatures; NASA Heliopedia. Energy is generated primarily by the proton–proton (pp) chain, which fuses hydrogen into helium and converts ~0.7% of mass to energy; the pp chain is the chief source of the Sun’s luminosity, with the CNO cycle contributing at the percent level. Britannica: proton–proton chain. Direct detection of CNO‑cycle neutrinos by the Borexino experiment confirms a ~1% contribution in the current Sun. Borexino collaboration, Nature 2020. Photospheric composition determinations based on 3D models indicate revised abundances of C, N, O and others that have implications for solar interior models and helioseismic inversions. Asplund et al., ARA&A 2009.

Rotation, helioseismology, and the magnetic cycle

Surface rotation is differential: the equator rotates in about 24–25 days while polar latitudes take roughly 35 days; the Sun’s mean synodic rotation seen from Earth is ~27 days. NASA MSFC: Solar Rotation; SOHO education page. Helioseismology—measurements of solar oscillations—constrains the internal rotation profile and structure, revealing a tachocline shear layer and near‑solid rotation in the radiative interior. Rev. Mod. Phys. 2002 review; ARA&A 2003 internal rotation review. The Sun’s magnetic activity follows an approximately 11‑year cycle traced by sunspot numbers and associated space‑weather phenomena; NOAA’s Space Weather Prediction Center provides operational and research predictions, with an updated experimental forecast product introduced in October 2023 and a revised cycle‑progression display adopted in February 2025. NOAA SWPC forecast update; NOAA Solar Cycle Progression; SWPC Testbed product. NASA’s public education materials summarize the cycle and its variability. NASA SDO “What will Solar Cycle 25 look like?”.

Atmosphere and activity: sunspots, flares, and CMEs

Sunspots mark intense magnetic concentrations and are often the sites of flares and coronal mass ejections (CMEs). Flares are classified by soft X‑ray output from A through X, with each class ten times the previous; the largest measured flare in 2003 saturated detectors near class X28. NASA solar flares overview; NASA “Solar Storms and Flares”. CMEs are massive ejections of magnetized plasma; typical speeds range from <250 km s⁻¹ to near 3000 km s⁻¹, with the fastest Earth‑directed CMEs reaching our planet in ~15–18 hours. NOAA SWPC: CMEs; NASA CME explainer. Historically, the 1859 Carrington Event produced global auroras and severe telegraph disruptions and is widely cited as the most intense recorded geomagnetic storm. Britannica: Geomagnetic storm of 1859.

Solar wind, heliosphere, and space weather

The Sun continuously drives the Solar Wind, a flow of magnetized plasma that forms the heliosphere, the Sun’s extended magnetic bubble that envelops the planets and modulates galactic cosmic rays. Typical solar‑wind speeds near Earth are ~400 km s⁻¹ for slow wind, with fast streams from coronal holes reaching 500–800 km s⁻¹. NOAA SWPC: Solar Wind; NASA SVS wind speeds. The heliosphere’s outer boundary, the heliopause, was directly crossed by Voyager 1 in 2012 and Voyager 2 in 2018, marking entry into interstellar space. NASA Voyager 2 interstellar entry; NASA heliosphere overview. When solar wind and CMEs interact with Earth’s magnetosphere, energy deposition produces Aurorae and geomagnetic storms; NASA and NOAA describe the precipitation physics and auroral morphology. NASA Auroras; NOAA SWPC Aurora.

Energy at Earth

The TSI at 1 au averages ~1361 W m⁻² and varies by ~0.1% over the solar cycle, a key boundary condition for Earth’s energy budget; NASA’s SORCE/TIM and TSIS‑1 missions underwrite present reference values. NASA Solar Irradiance Science. The IAU’s bolometric magnitude zero‑points were chosen to be consistent with the adopted nominal solar luminosity and TSI. IAU Resolution B2 (2015).

Exploration and observation

Continuous space‑based observations—from missions such as SOHO and SDO—monitor solar magnetic activity, irradiance, and helioseismology. NASA SDO overview. Parker Solar Probe is directly sampling the near‑Sun environment; NASA reported successive close approaches at ~7.26 million km from the surface in 2023–2024 and the mission’s record approach at ~3.8 million miles (≈6.1 million km) on December 24, 2024, reaching speeds near 430,000 mph. NASA blog, Oct. 3, 2024; Reuters report, Dec. 27, 2024. ESA’s Solar Orbiter complements these in‑situ and remote‑sensing studies. ESA “The Sun” portal.

Galactic context

The Sun resides in the Orion spur of the Milky Way and orbits the Galactic Center at a distance ~8.2 kpc, completing one revolution roughly every 225–250 million years. NASA’s educational materials give an orbital period near 250 million years, while Gaia and other measurements refine the Sun–Galactic Center distance and local rotation. NASA Basics of Space Flight; Gaia EDR3 analysis.

Units and constants used for cross‑reference

Astronomical distances in the inner Solar System are commonly expressed in astronomical units; since 2012 the IAU defines 1 au as exactly 149,597,870,700 m, facilitating unambiguous conversions among solar constants. JPL Glossary: au; IAU 2012 decision summarized by Britannica.

Related concepts and internal links

• –The Sun is the gravitational, radiative, and magnetic hub of the Solar System, shaping planetary climates and environments. NASA “It’s Surprisingly Hard to Go to the Sun”; NASA Basics of Space Flight.
• –Magnetically driven phenomena—Solar Wind, flares, and CMEs—produce Aurorae and geomagnetic storms; the 1859 Carrington Event remains a benchmark for extreme space weather. NOAA SWPC: Solar Wind; NASA Auroras; Britannica: Geomagnetic storm of 1859.
• –Near‑Sun exploration by Parker Solar Probe is constraining coronal heating and solar‑wind acceleration close to the source region. NASA blog, Oct. 3, 2024.
• –Interior structure and dynamics are probed by Helioseismology, while Nuclear fusion via the pp‑chain is established by solar‑neutrino measurements and stellar‑structure models. Rev. Mod. Phys. helioseismology review; Britannica: proton–proton chain; Borexino Nature 2020.