The Sun: Structure, Fusion and Atmosphere
Our Nearest Star
The Sun is a G-type main-sequence star with a diameter of about 1.4 million km — roughly 109 times Earth's diameter. Despite appearing as a calm yellow disc from Earth, it is a ferociously active ball of plasma with distinct internal layers, each transporting energy in different ways. Understanding the Sun's structure is fundamental to astronomy because virtually every star in the Universe operates on the same principles.
Observing the Sun Safely
The Sun can never be observed directly through an unfiltered telescope — the concentrated light and heat will instantly and permanently damage the eye. Safe methods include solar projection, where the telescope eyepiece projects an image of the Sun onto a white card, and using a dedicated H-alpha filter, which blocks all wavelengths except the red hydrogen emission line at 656 nm and reveals surface features invisible in white light. Full-aperture solar filters (Baader film) are also used and must be placed in front of the objective, never at the eyepiece.
The Sun has four internal regions from centre outwards: core (energy production by fusion), radiative zone (energy moves by radiation), convective zone (energy moves by convection), and photosphere (the visible surface at ~5,800 K).
The Sun's Internal Structure
The core occupies the inner 25% of the Sun's radius and reaches temperatures of about 15 million K and pressures millions of times greater than Earth's atmosphere. These extreme conditions are necessary for nuclear fusion to occur. Energy produced in the core takes an extraordinarily long time to escape — photons are absorbed and re-emitted by plasma particles so many times that the journey from core to surface takes around 100,000 years.
Surrounding the core is the radiative zone. Here the plasma is too dense for convection to operate efficiently. Energy travels in the form of electromagnetic radiation — photons zigzag outward in a process called a random walk, repeatedly absorbed and re-emitted by ions and electrons.
Above the radiative zone, the temperature drops enough for the plasma to become less opaque. In the convective zone, hot plasma rises towards the surface, cools by emitting energy, then sinks back down — exactly like water boiling in a saucepan. These rising and sinking columns produce the granular texture visible on the photosphere: bright granules are tops of rising columns, and darker intergranule lanes are descending cooler plasma.
The Photosphere and Atmosphere
The photosphere is the visible “surface” of the Sun at approximately 5,800 K. It is where sunlight we see is emitted. Above it lie two further layers of the solar atmosphere. The chromosphere extends to about 2,000 km above the photosphere and reaches temperatures of 20,000 K — paradoxically hotter than the photosphere below (the mechanism for this heating is still an active area of research). During a total solar eclipse, the chromosphere is briefly visible as a red arc.
Extending millions of kilometres into space is the corona. Temperatures here reach 1–3 million K. Because the corona emits strongly in X-ray wavelengths, it can only be fully imaged in X-rays or when the photosphere is blocked, either naturally (total solar eclipse) or artificially (using a coronagraph). The Sun looks completely different depending on the wavelength used to observe it: the photosphere dominates in visible light, the chromosphere in UV, and the corona in X-rays.
Counterintuitively, the Sun's temperature increases with altitude above the photosphere: photosphere ~5,800 K, chromosphere up to ~20,000 K, corona 1–3 million K. This coronal heating problem is a major unsolved question in solar physics.
Nuclear Fusion: The Proton-Proton Chain
The Sun's energy source is nuclear fusion. In the core, the dominant process is the proton-proton chain, in which four hydrogen nuclei (protons) are ultimately converted into one helium nucleus. The helium nucleus has slightly less mass than the four original protons — this small mass difference is converted into energy according to Einstein's relation between mass and energy. Each second, the Sun converts approximately 600 million tonnes of hydrogen into helium, releasing an enormous amount of energy that slowly works its way outward to the surface and into space.
The proton-proton chain is the primary fusion process for stars of the Sun's mass. Higher-mass stars instead use the CNO cycle (carbon-nitrogen-oxygen cycle), but both processes ultimately fuse four hydrogen nuclei into one helium nucleus. The Sun has enough hydrogen fuel to continue this process for approximately 5 billion more years.
Protons carry a positive charge and strongly repel each other (the electrostatic or Coulomb barrier). To fuse, protons must be brought close enough for the strong nuclear force to take over — a distance of about 10−15 m. This requires protons moving at extremely high speeds, which only occur at temperatures of around 15 million K and above. The outer layers of the Sun are far too cool and the pressure far too low for fusion to occur. Only the core maintains the necessary conditions.
Different layers of the Sun are best revealed in different parts of the spectrum. In visible light we see the photosphere and its granulation, sunspots and faculae. In ultraviolet we observe the chromosphere and transition region, and active regions become prominent. In X-rays the corona dominates, showing loops of hot plasma following magnetic field lines. In radio wavelengths the corona again dominates, and solar radio bursts can be detected during solar flares. This wavelength dependence is why solar observatories carry instruments across the full EM spectrum.