Astronomy 2e · The Sun: A Garden-Variety Star
The Structure and Composition of the Sun
On this page 9 sections
In 30 seconds
The Sun is the nearest star — close enough to resolve its layers, count its spots, and watch its storms — which makes it astronomy's best laboratory for understanding every other star. It is a sphere of hot plasma about 1.39 million km across (roughly 109 Earth diameters), with a mass of about 2 × 10³⁰ kg (about 333,000 times Earth's mass), holding more than 99.8% of all the mass in the solar system.
The Sun is built in layers. From the center outward: the core, Radiative zone Interior region where energy diffuses outward as photons Full entry →, and Convective zone Outer interior where hot plasma rises and cools Full entry →, then an atmosphere made of the Photosphere The thin layer of the Sun we see as its bright disk Full entry → (the bright "surface" we see), the Chromosphere A thin, pinkish layer just above the photosphere Full entry →, the transition region, and the Corona The Sun's very hot (1–3 million K), low-density outer atmosphere Full entry →, which feeds the Solar wind Stream of plasma escaping the corona Full entry → that fills the solar system.
Its composition is simple: roughly 73% hydrogen and 25% helium by mass, with everything else only about 2%. That mix and its temperature place the Sun on the main sequence as a G-type star — ordinary and "garden-variety," exactly why studying it teaches us about typical stars.
Why this matters
The Sun is the only star we can examine up close, so what we learn here becomes the template for interpreting every other star (Chapters 17–18) — a distant star's spectrum carries the same absorption-line evidence that reveals the Sun's composition. The 73/25/2 mix is also a cosmic clue: hydrogen and helium were forged in the Big Bang, while the heavier 2% were made inside earlier generations of stars (Chapters 28–29). Finally, the layered Sun is the machine behind solar activity and space weather (Topics 2–4) — the storms that disrupt satellites and power grids.
The college version
Core Concepts
Physical properties: an average star, writ large
- Luminosity ~3.8 × 10²⁶ W; photosphere ~5,800 K; core ~15 million K (commonly cited); spectral type G2 V — a main-sequence star of average temperature and size, the standard against which other stars are compared.
Composition: mostly hydrogen and helium
Astronomers know the Sun's composition without taking a sample, using spectroscopy: sunlight spread into a rainbow shows dark absorption lines (Fraunhofer lines Dark absorption lines in the solar spectrum Full entry →) at wavelengths where specific elements absorb. Matching them to laboratory spectra identifies the elements.
- Astronomers call every element heavier than helium a "metal", so the Sun's "metallicity" is only ~2%.
- History note: helium was discovered in the Sun's spectrum during the 1868 eclipse, decades before it was found on Earth.
The interior: three zones
- Core (0 to ~0.25 R☉): at ~15 million K and enormous pressure, hydrogen fuses into helium — the energy source that becomes sunlight (Chapter 16).
- Radiative zone (~0.25 to ~0.7 R☉): energy travels outward as photons absorbed and re-emitted again and again; the "random walk" is so slow that a photon may take ~100,000 years (commonly cited) to cross it.
- Convective zone (outer ~30% of the radius): the gas is cooler and more opaque, so energy travels by convection — hot plasma rises, radiates, cools, and sinks. The tops of these cells appear on the surface as granules.
The photosphere: the "surface" that is not a surface
The photosphere is the bright disk we see, but it is not solid — a thin layer of gas, only about 500 km thick, where the Sun becomes opaque to visible light; its temperature is about 5,800 K. Key features:
- Granulation Mottled pattern of convection cells on the photosphere Full entry →: a mottled pattern of convection cells ~1,000 km across that bubble up, cool, and sink in minutes — direct evidence of the convection below.
- Limb darkening The Sun's edge looks dimmer than its center Full entry →: the edge looks dimmer because we see higher, cooler gas — evidence of a temperature gradient in a gas layer, not a shiny surface.
Above the photosphere: chromosphere and corona
- Chromosphere (~2,000–3,000 km thick): a thin, pinkish layer at ~10,000–20,000 K, visible as a red ring (hydrogen's H-alpha emission) during a total eclipse; spicules — small jets of gas — shoot up through it.
- Transition region: a thin boundary where temperature jumps from ~20,000 K to over a million.
- Corona: the outer atmosphere at 1–3 million K but extremely low density, extending millions of kilometers and visible as a pearly halo at eclipses. Why it is hotter than the surface beneath it is the famous coronal heating problem (Topic 3).
- Solar wind: the corona's outer layers are so hot that plasma escapes the Sun's gravity, flowing outward at hundreds of km/s and filling the solar system (Topic 4).
The Sun rotates — but not like a solid
The Sun is a gas, so different latitudes spin at different rates (Differential rotation The equator spins faster than the poles (~25 vs ~35 days) Full entry →): the equator rotates in about 25 days, the poles in about 35 days (commonly cited). This uneven spin drags and twists the magnetic field — the engine of sunspots and the solar cycle (Topic 2).
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| The photosphere is a solid surface | A ~500 km layer of gas that has become opaque | The Sun is gas throughout; there is no "ground" |
| Core temperature and surface temperature are the same | They differ by ~2,500× | Core ~15 million K (fusion); photosphere ~5,800 K (emission) |
| The corona is cooler than the photosphere | It is far hotter | 1–3 million K vs 5,800 K — the coronal heating problem |
| The radiative zone is where hot gas rises | Convection | Energy diffuses as photons there; rising gas is the convective zone |
| The Sun burns like a campfire | Nuclear fusion | Combustion needs oxygen and lasts ~10⁴ years; fusion powers the Sun (Ch. 16) |
| Granules are sunspots | Convection cells | Granulation covers the surface; sunspots are magnetic, cooler regions |
Eli explains
The same idea, in plain words
Explain it like I’m 10
The Sun is a giant ball of glowing gas — mostly hydrogen — with layers like an onion. The middle is hot enough to make energy, the outer part boils like soup in a pot, and the bright "surface" we see is just a thin skin of gas where the Sun becomes see-through. It's not on fire and it's not solid — it's held together by its own gravity.
Worked example
You can't sample the Sun, yet its composition is one of the best-known quantities in astronomy. The chain of evidence:
- Spread the light. Pass sunlight through a prism: a continuous rainbow appears, crossed by dark lines — the Fraunhofer lines.
- Identify the absorbers. Comparing line positions with laboratory spectra names the elements: hydrogen dominates, then helium, plus sodium, iron, calcium, and others.
- Measure abundance. The strength of each element's lines, interpreted with a model of the Sun's atmosphere, gives ~73% hydrogen, ~25% helium, ~2% heavier elements by mass.
- Cross-check with physics. Chapter 16 shows the core converts hydrogen into helium; the observed 73/25 split is leftover fuel plus fusion product — consistent with the Sun's ~4.5-billion-year age.
Onion analogy. The Sun is layered like an onion, with energy made at the center and passed outward — but unlike an onion it is entirely gas; every "layer" is just a region where temperature, density, or opacity changes.
Key takeaways
- Composition by mass: ~73% H, ~25% He, ~2% heavier elements; known from absorption-line spectra.
- Radius ~109 R🜨; mass ~333,000 M🜨; contains >99.8% of the solar system's mass.
- Layer order: core → radiative zone → convective zone → photosphere → chromosphere → transition region → corona → solar wind.
- Photosphere ~5,800 K; core ~15 million K; corona 1–3 million K — hotter than the photosphere (heating problem).
- The photosphere is a ~500 km gas layer, not a solid surface; limb darkening and granulation prove it.
- Interior energy transport: radiation in the radiative zone, convection in the outer zone.
- Photons take ~100,000 years (order of magnitude) to diffuse out of the interior.
- Differential rotation: equator ~25 days, poles ~35 days — the seed of the solar cycle.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
List the Sun's layers from the center outward.
Show answer
Core → radiative zone → convective zone → photosphere → chromosphere → transition region → corona (→ solar wind beyond).
Roughly what fraction of the Sun's mass is hydrogen? Helium?
Show answer
About 73% hydrogen and 25% helium by mass; hydrogen dominates even more by number of atoms (~90%).
Why does the Sun's edge (limb) look darker than its center?
Show answer
Limb darkening: at the limb we see higher, cooler layers of the photosphere, which emit less light; the temperature gradient proves the photosphere is a gas layer, not a solid surface.
What single piece of evidence reveals the Sun's chemical composition, and how does it work?
Show answer
The dark absorption lines (Fraunhofer lines) in the Sun's spectrum; matching their wavelengths and strengths to laboratory spectra identifies elements and their abundances.
What is differential rotation, and why is it important for later topics?
Show answer
The equator rotates in ~25 days while the poles take ~35 days; the uneven spin drags and twists the Sun's magnetic field, which drives sunspots and the solar cycle (Topic 2).
Which is hotter — the photosphere or the corona? What is puzzling about that?
Show answer
The corona (1–3 million K) is far hotter than the photosphere (~5,800 K). How the corona is heated remains an unsolved problem — a clue that magnetic fields are involved.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Photosphere
- The thin layer of the Sun we see as its bright disk
- Chromosphere
- A thin, pinkish layer just above the photosphere
- Corona
- The Sun's very hot (1–3 million K), low-density outer atmosphere
- Radiative zone
- Interior region where energy diffuses outward as photons
- Convective zone
- Outer interior where hot plasma rises and cools
- Granulation
- Mottled pattern of convection cells on the photosphere
- Limb darkening
- The Sun's edge looks dimmer than its center
- Differential rotation
- The equator spins faster than the poles (~25 vs ~35 days)
- Fraunhofer lines
- Dark absorption lines in the solar spectrum
- Solar wind
- Stream of plasma escaping the corona
Sources & references
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