Layers of the Sun – All These Worlds

Reference frame used on this page

Depths are measured as either “below the visible surface” or “above the photosphere,” depending on the layer

Thickness and boundary values are approximate because the Sun is a plasma with gradients, not a stack of rigid shells. Outer-atmosphere boundaries in particular vary by definition and by solar activity.

Solar size constant used

Nominal solar radius: 695,700 km (a standard nominal conversion constant used in astronomy)

Source: Nominal constants reference derived from IAU 2015 Resolution B3.

Overall composition (photosphere)

By mass (“by weight”): Hydrogen ≈ 73.46%, Helium ≈ 24.85% (with the remainder as heavier elements at much smaller fractions)
By number of atoms (common educational shorthand): Often stated as ~92% hydrogen and ~8% helium, reflecting number fractions rather than mass fractions

Sources: nominal radius reference (IAU-derived) and photospheric composition tables; NASA also publishes a number-fraction style summary. :contentReference[oaicite:0]{index=0}

At-a-glance: zones, temperatures, and where they sit

Numbers below are sourced (see citations after the table)

Layer / Zone	Where it is	Temperature (reported ranges)	Composition / state (fact-limited)
Core	Central region; reported thickness ~138,000 km	~15 million °C (core temperature)	Plasma dominated by H/He; hydrogen fusion to helium powers the Sun
Radiative zone	Between core and convection zone; extends to ~0.7 solar radius (fractional boundary)	Not a single value; energy transported outward primarily by radiation (photon diffusion)	Highly ionized plasma; photons repeatedly absorbed and re-emitted
Convection zone	Outermost interior; extends from ~200,000 km below the surface to the photosphere	~2,000,000 °C at the base; cooler upward toward the surface	At its base, temperatures are “cool” enough that heavier ions can hold onto some electrons, increasing opacity
Photosphere	Visible “surface”; ~400 km (250 miles) thick above the visible disk center	~6500 K (bottom) down to ~4000 K (top); commonly described as ~5,500 °C overall	Plasma where most visible light escapes; granulation is observed here
Chromosphere	~400 km to ~2100 km above the photosphere	~4000 K (bottom) to ~8000 K (top); other NASA communications describe rises to ~20,000 °C in parts	Ionization increases with height; dynamic region above the photosphere
Transition region	Very thin boundary between chromosphere and corona; ~100 km thick (one cited value)	Rapid rise from ~8000 K to ~500,000 K	Strong gradients in temperature and ionization
Corona	Outermost atmosphere; starts around ~2100 km above the photosphere (per one NASA definition)	≥ 500,000 K, up to a few million K; NASA also describes ~2 million °C / ~3.5 million °F values	Extremely hot, very low-density plasma; source region for solar wind

Sources: NASA Sun facts; NASA “Layers of the Sun” image article; NASA/MSFC solar interior; NASA “Layers of the Sun” blog post for terminology. :contentReference[oaicite:1]{index=1}

Core

Energy generation region

NASA describes the core as the hottest part of the Sun, where hydrogen is fused into helium. It reports a core temperature of about 27 million °F (15 million °C) and a core thickness of about 86,000 miles (138,000 km). :contentReference[oaicite:2]{index=2}

Temperature and scale (reported)

Temperature: ~15 million °C (reported core temperature)
Thickness: ~138,000 km

Chemical makeup and state (fact-limited)

What it is made of: Plasma dominated by hydrogen and helium; “hydrogen fused into helium” is the stated energy source
What changes with time: Helium is produced in the core by fusion (a compositional change in the core relative to the outer layers)

Radiative zone

Energy transport primarily by radiation (photon diffusion)

NASA describes energy moving outward from the core by radiation, with photons “bouncing around” in the radiative zone and taking about 170,000 years to reach the top of the convection zone. :contentReference[oaicite:3]{index=3}

A NASA heliophysics instructional PDF describes the radiative transport continuing until about 0.7 of the solar radius. :contentReference[oaicite:4]{index=4}

Where it sits (reported)

Position: Between the core and the convection zone; radiative transport continues until ~0.7 solar radius

Composition and physical state (what can be stated cleanly)

State: Hot, dense plasma; photons repeatedly interact with charged particles
Bulk chemistry: Still dominated by hydrogen and helium overall; heavy elements exist in trace fractions compared to H/He (photosphere composition provides the standard reference point)

Convection zone

Energy transport primarily by bulk motion of plasma

NASA describes the convection zone as the outermost layer of the solar interior and states it extends from a depth of about 200,000 km up to the visible surface. :contentReference[oaicite:5]{index=5}

Temperature and depth (reported)

Depth range: From ~200,000 km below the surface to the photosphere
Base temperature: ~2,000,000 °C at the base of the convection zone (reported)

Chemistry / ionization note (reported)

Opacity-relevant detail: At the base of the convection zone, heavier ions (e.g., C, N, O, Ca, Fe) can retain some electrons, increasing opacity and making radiation transport less effective

Photosphere

Visible surface (lowest layer of the atmosphere)

NASA describes the photosphere as the deepest layer we can observe directly and reports it extends from the visible surface to about 250 miles (400 km) above that, with temperatures from about 6500 K (bottom) to about 4000 K (top). :contentReference[oaicite:6]{index=6}

Thickness and temperature (reported)

Thickness: ~400 km above the visible surface reference point (one cited definition)
Temperature range: ~6500 K (bottom) to ~4000 K (top)

Chemical makeup (best grounded statement)

Bulk composition: Dominated by hydrogen and helium; published photospheric mass fractions are ~73.46% H and ~24.85% He by weight
What we can say without overreach: This is a plasma layer where most visible light escapes; fine structure includes granulation (convection patterns seen from above)

Chromosphere

Atmospheric layer above the photosphere

NASA reports the chromosphere spans from about 250 miles (400 km) to about 1300 miles (2100 km) above the photosphere, with temperatures from about 4000 K near the bottom to about 8000 K near the top. :contentReference[oaicite:7]{index=7}

NASA also describes the chromosphere more broadly as a region where temperature rises and can reach much higher values in some descriptions of the solar atmosphere. :contentReference[oaicite:8]{index=8}

Height and temperature (reported)

Height range: ~400 km to ~2100 km above the photosphere
Temperature range: ~4000 K (bottom) to ~8000 K (top)

Chemistry / state (fact-limited)

State: Hot, partially ionized plasma with ionization increasing with height
Bulk chemistry: Still hydrogen/helium dominated; spectral diagnostics (e.g., hydrogen lines) are central for observation, but this page avoids interpretive claims beyond the cited temperature/extent values

Transition region

Thin boundary where temperature rises rapidly toward coronal values

NASA describes the transition region as very narrow (about 60 miles / 100 km) and reports a temperature rise from about 8000 K to about 500,000 K across it. :contentReference[oaicite:9]{index=9}

Thickness and temperature jump (reported)

Thickness: ~100 km (one cited value)
Temperature change: ~8000 K → ~500,000 K (abrupt rise)

What “transition” means physically (fact-limited)

State change: A rapid change in temperature and ionization state over a small height range
Boundary role: Separates the chromosphere from the corona in one commonly used layered description

Corona

Upper atmosphere; source region for solar wind

NASA defines the corona as the outermost layer, starting about 1300 miles (2100 km) above the photosphere, and reports coronal temperatures of 500,000 K or more, up to a few million K. NASA’s Sun facts page also notes the corona can reach about 3.5 million °F (~2 million °C), emphasizing that it is hotter than the photosphere. :contentReference[oaicite:10]{index=10}

Height and temperature (reported)

Where it begins (one NASA definition): ~2100 km above the photosphere
Temperature range: ≥ 500,000 K; up to a few million K (NASA wording), with examples around ~2 million °C in NASA fact summaries

Composition and outflow (reported)

State: Extremely hot, very low-density plasma
Solar wind link: NASA describes that once material leaves the corona at supersonic speeds, it becomes the solar wind, forming the heliosphere

Source for corona-to-solar-wind relationship: NASA Sun facts. :contentReference[oaicite:11]{index=11}

Primary sources used

Direct links for verification

NASA “Our Sun: Facts” (core temperature, core thickness, photosphere temperature summary, corona temperature summary, structure order, and solar-wind statement). :contentReference[oaicite:12]{index=12}

NASA “Layers of the Sun” image article (photosphere/chromosphere/transition region/corona heights and temperatures, including the 100 km transition-region thickness). :contentReference[oaicite:13]{index=13}

NASA/MSFC SolarScience “The Solar Interior” (convection zone depth ~200,000 km, base temperature ~2,000,000 °C, and ion/electron opacity note). :contentReference[oaicite:14]{index=14}

NASA “Layers of the Sun” blog post (layer naming/terminology reinforcement). :contentReference[oaicite:15]{index=15}

Stanford Solar Center “Vital Statistics” (photospheric composition by weight: H 73.46%, He 24.85%). :contentReference[oaicite:16]{index=16}

NASA GSFC “Imagine the Universe!” Sun page (composition stated in number-fraction style: ~92% H, ~8% He). :contentReference[oaicite:17]{index=17}

Nominal solar radius reference (IAU-derived nominal constants summary PDF). :contentReference[oaicite:18]{index=18}

NASA heliophysics instructional PDF (radiative transport continuing until about 0.7 solar radius). :contentReference[oaicite:19]{index=19}